Astronomy & Astrophysics manuscript no. 7585 c© ESO 2018
October 27, 2018
The transverse proximity effect in spectral hardness on the line of
sight towards HE 2347−4342 ⋆
G. Worseck1, C. Fechner2,3, L. Wisotzki1, and A. Dall’Aglio1
1 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
3 Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
Received 2 April 2007 / Accepted 31 July 2007
ABSTRACT
We report the discovery of 14 quasars in the vicinity of HE 2347−4342, one of the two quasars whose intergalactic He ii forest has
been resolved with FUSE. By analysing the H i and the He ii opacity variations separately, no transverse proximity effect is detected
near three foreground quasars of HE 2347−4342: QSO J23503−4328 (z = 2.282, ϑ = 3.′59), QSO J23500−4319 (z = 2.302, ϑ = 8.′77)
and QSO J23495−4338 (z = 2.690, ϑ = 16.′28). This is primarily due to line contamination and overdensities probably created by
large-scale structure. By comparing the H i absorption and the corresponding He ii absorption, we estimated the fluctuating spectral
shape of the extragalactic UV radiation field along this line of sight. We find that the UV spectral shape near HE 2347−4342 and in
the projected vicinity of the three foreground quasars is statistically harder than expected from UV background models dominated by
quasars. In addition, we find three highly ionised metal line systems near the quasars. However, they do not yield further constraints
on the shape of the ionising field. We conclude that the foreground quasars show a transverse proximity effect that is detectable as
a local hardening of the UV radiation field, although the evidence is strongest for QSO J23495−4338. Thus, the relative spectral
hardness traces the proximity effect also in overdense regions prohibiting the traditional detection in the H i forest. Furthermore, we
emphasise that softening of quasar radiation by radiative transfer in the intergalactic medium is important to understand the observed
spectral shape variations. From the transverse proximity effect of QSO J23495−4338 we obtain a lower limit on the quasar lifetime
of ∼ 25 Myr.
Key words. quasars: general – quasars: absorption lines – intergalactic medium – diffuse radiation
1. Introduction
After reionisation the intergalactic medium (IGM) is kept highly
photoionised by the metagalactic UV radiation field generated
by the overall population of quasars and star-forming galax-
ies (e.g. Haardt & Madau 1996; Fardal et al. 1998; Bianchi et al.
2001; Sokasian et al. 2003). The intensity and spectral shape of
the UV background determines the ionisation state of the observ-
able elements in the IGM. In particular, the remaining fraction
of intergalactic neutral hydrogen and singly ionised helium is
responsible for the Lyα forest of H i and He ii.
On lines of sight passing near quasars the IGM will be sta-
tistically more ionised due to the local enhancement of the UV
flux that should result in a statistically higher IGM transmission
(’void’) in the QSO’s vicinity (Fardal & Shull 1993; Croft 2004;
McDonald et al. 2005). This so-called proximity effect has been
found with high statistical significance on lines of sight towards
luminous quasars (e.g. Bajtlik et al. 1988; Giallongo et al. 1996;
Scott et al. 2000). On the other hand, a transverse proximity ef-
fect created by foreground ionising sources nearby the line of
sight has not been clearly detected in the H i forest, except the
recent detection at z = 5.70 by Gallerani et al. (2007). While
two large H i voids have been claimed to be due to the transverse
Send offprint requests to: G. Worseck, e-mail: gworseck@aip.de
⋆ Based on observations collected at the European Southern
Observatory, Chile (Proposals 070.A-0425 and 074.A-0273). Data col-
lected under Proposals 068.A-0194, 070.A-0376 and 116.A-0106 was
obtained from the ESO Science Archive.
proximity effect by Dobrzycki & Bechtold (1991a, however
see Dobrzycki & Bechtold 1991b) and Srianand (1997), other
studies find at best marginal evidence (Fernández-Soto et al.
1995; Liske & Williger 2001), and most attempts resulted
in non-detections (Crotts 1989; Møller & Kjærgaard 1992;
Crotts & Fang 1998; Schirber et al. 2004; Croft 2004). This
has led to explanations involving the systematic effects of
anisotropic radiation, quasar variability (Schirber et al. 2004),
intrinsic overdensities (Loeb & Eisenstein 1995; Rollinde et al.
2005; Hennawi & Prochaska 2007; Guimarães et al. 2007) and
finite quasar lifetimes (Croft 2004).
Intergalactic He ii Lyα absorption (λrest = 303.7822 Å)
can be studied only towards the few quasars at z > 2 whose
far UV flux is not extinguished by intervening Lyman limit
systems (Picard & Jakobsen 1993; Jakobsen 1998). Of the six
quasars successfully observed so far, the lines of sight to-
wards HE 2347−4342 (z = 2.885) and HS 1700+6416 (z =
2.736) probe the post-reionisation era of He ii with an emerg-
ing He ii forest that has been resolved with FUSE (Kriss et al.
2001; Shull et al. 2004; Zheng et al. 2004; Fechner et al. 2006;
Fechner & Reimers 2007a).
In a highly ionised IGM a comparison of the H i with the
corresponding He ii absorption yields an estimate of the spectral
shape of the UV radiation field due to the different ionisation
thresholds of both species. The amount of He ii compared to H i
gives a measure of the spectral softness, generally expressed via
the column density ratio η = NHe ii/NH i. Typically, η <∼ 100 indi-
cates a hard radiation field generated by the surrounding quasar
http://arxiv.org/abs/0704.0187v2
2 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
population, whereas η >∼ 100 requires a significant contribu-
tion of star-forming galaxies or heavily softened quasar radiation
(e.g. Haardt & Madau 1996; Fardal et al. 1998; Haardt & Madau
2001).
The recent FUSE observations of the He ii Lyα forest re-
vealed large η fluctuations (1 <∼ η <∼ 1000) on small scales
of 0.001 <∼ ∆z <∼ 0.03 with a median η ≃ 80–100. Apart
from scatter due to the low-quality He ii data at S/N ∼ 5
(Fechner et al. 2006; Liu et al. 2006) and possible systematic er-
rors due to the generally assumed line broadening mechanism
(Fechner & Reimers 2007a), several physical reasons for these
η variations have been proposed. A combination of local den-
sity variations (Miralda-Escudé et al. 2000), radiative transfer
effects (Maselli & Ferrara 2005; Tittley & Meiksin 2006) and lo-
cal differences in the properties of quasars may be responsible
for the fluctuations. In particular, at any given point in the IGM
at z > 2 only a few quasars with a range of spectral indices
(Telfer et al. 2002; Scott et al. 2004) contribute to the UV back-
ground at hν ≥ 54.4 eV (Bolton et al. 2006).
Already low-resolution He ii spectra obtained with HST in-
dicate a fluctuating radiation field, which has been interpreted
as the onset of He ii reionisation in Strömgren spheres around
hard He ii photoionising sources along or near the line of
sight (Reimers et al. 1997; Heap et al. 2000; Smette et al. 2002).
Jakobsen et al. (2003) found a quasar coinciding with the promi-
nent He ii void at z = 3.05 towards Q 0302−003, thereby pre-
senting the first clear case of a transverse proximity effect. In
Worseck & Wisotzki (2006), hereafter Paper I, we revealed the
transverse proximity effect as a systematic increase in spectral
hardness around all four known foreground quasars along this
line of sight. This suggests that a hard radiation field is a sensi-
tive probe of the transverse proximity effect even if there is no
associated void in the H i forest, either because of the weakness
of the effect, or because of large-scale structure.
Along the line of sight towards HE 2347−4342 several He ii
voids have been claimed to be due to nearby unknown AGN
(Smette et al. 2002). Likewise, some forest regions with a de-
tected hard radiation field may correspond to proximity ef-
fect zones of putative foreground quasars (Fechner & Reimers
2007a). Here we report on results from a slitless spectroscopic
quasar survey in the vicinity of HE 2347−4342 and on spectral
shape fluctuations of the UV radiation field probably caused by
foreground quasars towards the sightline of HE 2347−4342. The
paper is structured as follows. Sect. 2 presents the observations
and the supplementary data employed for the paper. Although
we do not detect any transverse proximity effect in the H i for-
est (Sect. 3), the fluctuating UV spectral shape along the line
of sight indicates a hard radiation field in the projected vicinity
of the foreground quasars (Sect. 4). In Sect. 5 we study three
nearby metal line systems which could further constrain the ion-
ising field. We interpret the statistically significant excesses of
hard radiation as being due to the transverse proximity effect
(Sect. 6). We present our conclusions in Sect. 7. Throughout
the paper we adopt a flat cosmological model with Ωm = 0.3,
ΩΛ = 0.7 and H0 = 70 km s
−1 Mpc−1.
2. Observations and data reduction
2.1. Search for QSO candidates near HE 2347−4342
In October 2002 we observed a 25′ × 33′ field centered on
HE 2347−4342 (z = 2.885) with the ESO Wide Field Imager
(WFI, Baade et al. 1999) at the ESO/MPI 2.2 m Telescope (La
Silla) in its slitless spectroscopic mode (Wisotzki et al. 2001) as
part of a survey for faint quasars in the vicinity of established
high-redshift quasars. A short summary of the survey is given in
Paper I; a detailed description will follow in a separate paper.
A semi-automated search for emission line objects among
the slitless spectra of the ∼ 1400 detected objects in the field
resulted in 10 prime quasar candidates.
2.2. Spectroscopic follow-up
Follow-up spectroscopy of these 10 quasar candidates was ob-
tained with the Focal Reducer/Low Dispersion Spectrograph 2
(FORS2, Appenzeller et al. 1998) on ESO VLT UT1/Antu in
Visitor Mode on November 17 and 19, 2004 under variable see-
ing but clear conditions. The spectra were taken either with
the 300V grism or the 600B grism and a 1′′ slit kept at the
parallactic angle, resulting in a spectral resolution of ∼ 10 Å
FWHM and ∼ 4.5 Å FWHM, respectively. No order separa-
tion filter was employed, leading to possible order overlap at
λ > 6600 Å in the spectra taken with the 300V grism. Exposure
times were adjusted to yield S/N ∼ 20 in the quasar con-
tinuum. The spectra were calibrated in wavelength against the
FORS2 He/Ne/Ar/HgCd arc lamps and spectrophotometrically
calibrated against the HST standard stars Feige 110 and GD 108.
Data reduction was performed with standard IRAF tasks us-
ing the optimal extraction algorithm by Horne (1986). Figure 1
shows the spectra of the quasars together with 4 quasars from an-
other survey (Sect. 2.3). Table 1 summarises our spectroscopic
follow-up observations.
2.3. Additional quasars
We checked the ESO Science Archive for additional quasars in
the vicinity of HE 2347−4342 and found several unpublished
quasars from a deeper slitless spectroscopic survey using the
ESO VLT, the results of which (on the field of Q 0302−003) are
described in Jakobsen et al. (2003). We obtained their follow-up
spectra of quasars surrounding HE 2347−4342 from the archive
and publish them here in agreement with P. Jakobsen. In the
course of their survey FORS1 spectra of 10 candidates were
taken with the 300V grism crossed with the GG435 order separa-
tion filter and a 1′′ slit, calibrated against the standards LTT 7987
and GD 50. Seven of their candidates are actually quasars, of
which 3 were also found independently by our survey. The re-
maining 4 quasars are beyond our redshift-dependent magnitude
limit. The FORS1 spectra of the 4 additional quasars are dis-
played in Fig. 1 and listed separately in Table 1. According to
the quasar catalogue by Véron-Cetty & Véron (2006) there are
no other previously known quasars within a radius < 30′ around
HE 2347−4342.
2.4. Redshifts and magnitudes
Redshifts of the 14 quasars were determined by taking every de-
tectable emission line into account. Line peaks were measured
by eye and errors were estimated taking into account the S/N
of the lines, line asymmetries and the presence of absorption
systems. The quasar redshifts were derived by weighting the
measurements of detected lines. Since high-ionisation lines suf-
fer from systematic blueshifts with respect to the systemic red-
shift (Gaskell 1982; Tytler & Fan 1992; McIntosh et al. 1999),
a higher weight was given to low-ionisation lines. Obviously
blueshifted lines were discarded. Redshift errors were estimated
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 3
Fig. 1. VLT/FORS spectra of quasars in the vicinity of HE 2347−4342. The spectra are shown in black together with their 1σ noise
arrays (green lines). The small inserts show the corresponding discovery spectra from our slitless survey in the same units.
from the redshift differences of the remaining lines and their es-
timated errors.
The 14 discovered quasars lie in the broad redshift range
0.720 ≤ z ≤ 3.542. Fig. 2 shows their angular separations with
respect to HE 2347−4342. We find three background quasars
to HE 2347−4342 and we identify a pair of bright quasars at
z ≃ 1.763 separated by 7.′8. Three foreground quasars (labelled
A–C in Table 1 and Fig. 2) are located in the redshift range
to study the transverse proximity effect. Table 2 provides the
redshift measurements for the detected emission lines in their
spectra. The redshift of QSO J23503−4328 was based on Lyα
and C iv. The measurement of the Mg ii is uncertain because of
the decline of the resolving power of the 300V grism towards
the red, but yields a slightly higher redshift than the adopted
one. For QSO J23500−4319 we measured a consistent redshift
from the C iv and the C iii] line. The redshift measurement of
QSO J23495−4338 was difficult due to several metal absorp-
tion line systems of which only two Mg ii systems at z = 0.921
and z = 1.518 could be identified. In particular, Fe ii absorp-
tion from the z = 0.912 system hampered a redshift measure-
4 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Table 1. Quasars observed near the line of sight of HE 2347−4342. The first 10 listed quasars have been found in our survey, the
remaining 4 quasars result from the previously unpublished survey by P. Jakobsen. Quasar magnitudes are B and V magnitudes for
our survey and Jakobsen’s survey, respectively.
Object α (J2000) δ (J2000) z Magnitude Night Grism Exposure Airmass Seeing Abbr.
QSO J23510−4336 23h51m05.s50 −43◦36′57.′′2 0.720 ± 0.002 20.74 ± 0.27 19 Nov 2004 300V 1200 s 1.30 1.′′3
QSO J23507−4319 23h50m44.s97 −43◦19′26.′′0 0.850 ± 0.003 19.90 ± 0.07 17 Nov 2004 600B 360 s 1.28 0.′′7
QSO J23507−4326 23h50m45.s39 −43◦26′37.′′0 1.635 ± 0.003 21.05 ± 0.14 17 Nov 2004 300V 200 s 1.23 1.′′0
QSO J23509−4330 23h50m54.s80 −43◦30′42.′′2 1.762 ± 0.004 18.23 ± 0.03 17 Nov 2004 600B 300 s 1.08 0.′′7
QSO J23502−4334 23h50m16.s18 −43◦34′14.′′7 1.763 ± 0.003 18.95 ± 0.04 17 Nov 2004 300V 60 s 1.18 0.′′7
QSO J23503−4328 23h50m21.s55 −43◦28′43.′′7 2.282 ± 0.003 20.66 ± 0.11 17 Nov 2004 300V 400 s 1.20 0.′′7 A
QSO J23495−4338 23h49m34.s53 −43◦38′08.′′7 2.690 ± 0.006 20.21 ± 0.17 19 Nov 2004 300V 360 s 1.13 1.′′2 C
QSO J23511−4319 23h51m09.s44 −43◦19′41.′′6 3.020 ± 0.004 21.00 ± 0.14 17 Nov 2004 600B 1000 s 1.09 1.′′1
QSO J23514−4339 23h51m25.s54 −43◦39′02.′′9 3.240 ± 0.004 21.57 ± 0.29 17 Nov 2004 300V 1400 s 1.14 1.′′2
QSO J23503−4317 23h50m21.s94 −43◦17′30.′′0 3.542 ± 0.005 21.94 ± 0.62 19 Nov 2004 300V 1800 s 1.23 1.′′2
600B 1800 s 1.33 1.′′2
QSO J23515−4324 23h51m33.s05 −43◦24′45.′′2 1.278 ± 0.002 20.82 ± 0.14 06 Oct 2002 300V 900 s 1.24 0.′′7
QSO J23512−4332 23h51m15.s18 −43◦32′34.′′3 1.369 ± 0.001 21.52 ± 0.25 06 Oct 2002 300V 900 s 1.18 0.′′7
QSO J23508−4335 23h50m52.s91 −43◦35′06.′′8 1.778 ± 0.002 22.01 ± 0.32 06 Oct 2002 300V 900 s 1.11 0.′′9
QSO J23500−4319 23h50m00.s28 −43◦19′46.′′1 2.302 ± 0.002 22.61 ± 0.83 06 Oct 2002 300V 900 s 2.37 0.′′8 B
Table 2. Detected emission lines and redshifts of QSOs A–C.
Object Emission line λobs [Å] z
QSO J23503−4328 Lyα 3989 ± 4 2.281 ± 0.003
N v 4070 ± 8 2.282 ± 0.006
Si iv+O iv] 4585 ± 8 2.276 ± 0.006
C iv 5082 ± 4 2.281 ± 0.003
C iii] 6253 ± 7 2.276 ± 0.004
Mg ii 9196 ± 12 2.286 ± 0.004
2.282 ± 0.003
QSO J23500−4319 Si iv+O iv] 4613 ± 6 2.296 ± 0.004
C iv 5115 ± 3 2.302 ± 0.002
C iii] 6305 ± 2 2.303 ± 0.001
2.302 ± 0.002
QSO J23495−4338 Lyα 4513 ± 10 2.712 ± 0.008
O i+Si ii 4823 ± 10 2.694 ± 0.008
C ii 4930 ± 10 2.692 ± 0.007
Si iv+O iv] 5135 ± 15 2.669 ± 0.011
C iv 5691 ± 10 2.674 ± 0.006
C iii] 7028 ± 10 2.682 ± 0.005
2.690 ± 0.006
ment of the Lyα line. The C iv and the C iii] lines show unidenti-
fied absorption features. Thus, the redshift of QSO J23495−4338
is heavily weighted towards the very noisy low-ionisation lines
O i+Si ii and C ii. However, redshift uncertainties of the fore-
ground quasars do not significantly affect our results.
Apparent magnitudes were derived from target aquisition
images photometrically calibrated against the standard star fields
PG 2213−006 or Mark A (Landolt 1992). Unfortunately the
aquisition exposures of the faintest quasars were too short to de-
termine their magnitudes accurately. Magnitudes derived from
integration of the spectra are consistent with the photometric
ones after correcting for slit losses.
We note that QSO J23507−4326 is variable. This quasar
has been detected in both slitless surveys and had V ≃ 20.3
in October 2001, V ≃ 20.7 in October 2002 and V ≃ 21.0
in November 2004. We were able to discover this quasar
in its bright phase while missing the slightly fainter quasar
QSO J23515−4324 detected only in the survey by P. Jakobsen.
2.5. Optical spectra of HE 2347−4342
From the ESO Science Archive we retrieved the optical spectra
of HE 2347−4342 taken with UVES at VLT UT2/Kueyen in the
Large Programme “The Cosmic Evolution of the Intergalactic
Medium” (Bergeron et al. 2004). Data reduction was performed
Fig. 2. Distribution of separation angles ϑ vs. redshift z of the
quasars from Table 1 with respect to HE 2347−4342. Symbol
size indicates apparent optical magnitude.
using the UVES pipeline provided by ESO (Ballester et al.
2000). The vacuum-barycentric corrected co-added spectra yield
a S/N ∼ 100 in the Lyα forest at R ∼ 45000. The spectrum
was normalised in the covered wavelength range 3000 <∼ λ <∼
10000 Å using a cubic spline interpolation algorithm.
2.6. Far-UV spectra of HE 2347−4342
HE 2347−4342 is one of the two high-redshift quasars observed
successfully in the He ii Lyα forest below 303.7822 Å rest frame
wavelength with the Far Ultraviolet Spectroscopic Explorer
(FUSE) at a resolution of R ∼ 20000, although at a
S/N <∼ 5 (Kriss et al. 2001; Zheng et al. 2004). G. Kriss
and W. Zheng kindly provided the reduced FUSE spec-
trum of HE 2347−4342 described in Zheng et al. (2004). We
adopted their flux normalisation with a power law fλ =
3.3 × 10−15
λ/1000Å
)−2.4
erg cm−2 s−1 Å−1 reddened by the
Cardelli et al. (1989) extinction curve assuming E(B − V) =
0.014 (Schlegel et al. 1998).
3. The Lyα forest near the foreground quasars
Aiming to detect the transverse proximity effect as an underden-
sity (’void’) in the Lyα forest towards HE 2347−4342 we ex-
amined the forest regions in the projected vicinity of the three
foreground quasars labelled A–C in Table 1. The H i forest of
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 5
Fig. 3. The Lyα forest of HE 2347−4342 in the vicinity of the foreground quasars A–C from Table 1. The upper panels show the
normalised optical spectrum of HE 2347−4342 including Lyβ and metal lines (red) and the H i Lyα transmission obtained from the
line list by T.-S. Kim (black). The binned blue line shows the mean H i Lyα transmission in ∆z = 0.005 bins towards HE 2347−4342,
whereas the dashed green line indicates the expected mean transmission 〈T 〉exp. The lower panels display the corresponding He ii
transmission from the FUSE spectrum. [See the online edition of the Journal for a colour version of this figure.]
HE 2347−4342 has been analysed in several studies, e.g. by
Zheng et al. (2004) and Fechner & Reimers (2007a), hereafter
called Z04 and FR07, respectively. Since the line list from FR07
is limited to z > 2.29, T.-S. Kim (priv. comm.) kindly provided
an independent line list including the lower redshift Lyα forest
(z > 1.79). Both line lists agree very well in their overlapping
redshift range 2.29 < z < 2.89.
Figure 3 displays the H i and the He ii forest regions near
the foreground quasars A–C. The H i Lyα forest is contaminated
by metals. In particular at z < 2.332 there is severe contam-
ination due to the O vi absorption of the associated system of
HE 2347−4342 (Fechner et al. 2004). Because the strong O vi
absorption overlaps with the projected positions of QSO A and
QSO B it is very difficult to obtain a well-determined H i line
sample in this region. Furthermore, there is Lyβ absorption of
H i and He ii at z < 2.294. We also overplot in Fig. 3 the mean H i
Lyα transmission in ∆z = 0.005 bins obtained from T.-S. Kim’s
line list and the generally expected mean transmission over sev-
eral lines of sight 〈T 〉exp = e−τ
eff with τ
eff = 0.0032 (1 + z)
(Kim et al. 2002).
We do not detect a significant void near the three foreground
quasars, neither in the H i forest nor in the He ii forest. In the
vicinity of QSO A and QSO B, even a careful decontamina-
tion of the optical spectrum does not reveal a significant H i
underdensity. Instead, the transmission is fluctuating around the
mean. Due to the poor quality of the FUSE data in this region
(S/N <∼ 2) and the He ii Lyβ absorption from higher redshifts, a
simple search for He ii voids near QSO A and QSO B is impos-
sible. In the vicinity of QSO C the H i Lyα absorption is slightly
higher than on average. There is a small void at z ≃ 2.702 that
can be identified in the forests of both species. The probability of
chance occurrence of such small underdensities is high, so link-
ing this void to QSO C seems unjustified. However, note that the
He ii absorption in the vicinity of QSO C (z ∼ 2.69) is lower
than at z ∼ 2.71 in spite of the same H i absorption. This points
to fluctuations in the spectral shape of the ionising radiation near
the quasar (Sect. 4.3).
Given the luminosities and distances of our foreground
quasars to the sightline of HE 2347−4342, could we expect to
detect the transverse proximity effect as voids in the H i forest?
As in Paper I, we modelled the impact of the foreground quasars
on the line of sight towards HE 2347−4342 with the parameter
ω (z) =
fνLL , j
4πJν (z)
1 + z′j
)−α j+1
1 + z j
αJν + 3
α j + 3
z j, 0
z j, z
which is the ratio between the summed photoionisation rates of
n quasars at redshifts z j with rest frame Lyman limit fluxes fνLL , j,
penetrating the absorber at redshift z and the overall UV back-
ground with Lyman limit intensity Jν. dL(z j, 0) is the luminosity
distance of QSO j, and dL(z j, z) is its luminosity distance as seen
at the absorber; the redshift of the quasar as seen at the absorber
is z′j (Liske 2000). A value ω ≫ 1 predicts a highly significant
proximity effect.
We assumed a constant UV background at 1 ryd of Jν =
7×10−22 erg cm−2 s−1 Hz−1 sr−1 (Scott et al. 2000) with a power-
law shape Jν ∝ ν
−αJν and αJν = 1.8. The quasar Lyman limit
fluxes were estimated from the spectra by fitting a power law
fν ∝ ν
−α to the quasar continuum redward of the Lyα emis-
sion line, excluding the emission lines. The spectra were scaled
to yield the measured photometric magnitudes. Table 3 lists the
resulting spectral indices, the H i Lyman limit fluxes, and the
transverse distances.
The combined effects of QSOs A and B result in a peak
ωmax ≃ 0.89, while QSO C yields ωmax ≃ 0.11. So we expect
only a weak signature of the transverse proximity effect that can
be easily diluted by small-scale transmission fluctuations around
〈T 〉exp. Thus, the apparent lack of a transverse proximity effect
in the H i forest is no surprise.
We can also roughly estimate the amplitude of the proximity
effect in the He ii forest. Extrapolating the power laws (QSOs
and background) above 4 ryd at η = 50 (Haardt & Madau 1996,
hereafter HM96) we get ωmax ≃ 20 near QSO A and ωmax ≃ 2
near QSO C. A softer background would result in higher values
of ω, whereas absorption of ionising photons in the He ii forest
would decrease ω. However, due to the arising He ii Lyβ forest
and the low S/N in the FUSE data near QSOs A and B, even
high ω values do not necessarily result in a visible He ii void.
6 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Table 3. Rest frame Lyman limit fluxes of foreground QSOs. A
power law fν ∝ ν
−α is fitted to the QSO continua and fνLL is the
extrapolated H i Lyman limit flux in the QSO rest frame. d⊥(z)
denotes the transverse proper distance to the line of sight towards
HE 2347−4342.
QSO Abbr. z α fνLL [µJy] d⊥(z) [Mpc]
QSO J23503−4328 A 2.282 0.21 16 1.76
QSO J23500−4319 B 2.302 0.84 1 4.33
QSO J23495−4338 C 2.690 0.24 29 7.75
In the direct vicinity of QSO C the He ii data is not saturated,
but shows no clear void structure either. We will show in the
following sections that the spectral shape of the radiation field is
a more sensitive indicator of the transverse proximity effect than
the detection of voids in the forests.
4. The fluctuating shape of the UV radiation field
towards HE 2347−4342
4.1. Diagnostics
If both hydrogen and helium are highly ionised in the IGM
with roughly primordial abundances, the column density ra-
tio η = NHe ii/NH i indicates the softness of the UV radiation
field impinging on the absorbers. Theoretically, η can be de-
rived numerically via photoionisation models of the IGM with
an adopted population of ionising sources. At the redshifts of
interest, 50 <∼ η <∼ 100 is predicted for a UV background gen-
erated by quasars (HM96; Fardal et al. 1998), whereas higher
values indicate a contribution of star-forming galaxies (e.g.
Haardt & Madau 2001, hereafter HM01).
The He ii forest has been resolved with FUSE towards
HE 2347−4342 and HS 1700+6416, allowing a direct estimation
of η by fitting the absorption lines (Kriss et al. 2001; Zheng et al.
2004; Fechner et al. 2006, FR07). Due to the low S/N and the
strong line blending in the He ii forest the He ii lines have to be
fitted with absorber redshifts and Doppler parameters fixed from
the fitting of the H i data of much higher quality. Generally, pure
non-thermal line broadening (bHe ii = bH i) is assumed (how-
ever, see FR07 and Sect. 6 below). The He ii forest towards
HE 2347−4342 was fitted independently by Z04 and FR07. In
the following, we rely on the line fitting results from FR07,
which at any rate are consistent with those obtained by Z04 in
the redshift ranges near the quasars.
All current studies indicate that η is strongly fluctuating on
very small scales in the range 1 <∼ η <∼ 1000. The median column
density ratio towards HE 2347−4342 is η ≃ 62 (Z04), whereas
Fechner et al. (2006) find a higher value of η ≃ 85 towards
HS 1700+6416. Both studies find evidence for an evolution of
η towards smaller values at lower redshifts. However, only part
of the scatter in η is due to redshift evolution and statistical er-
rors, so the spectral shape of the UV radiation field has to fluc-
tuate (FR07). Although the analyses of both available lines of
sight give consistent results, cosmic variance may bias the de-
rived median η and its evolution. This is of particular interest for
our study, since we want to reveal local excesses of low η near
the quasars with respect to the median (Sect. 4.3). Clearly, more
lines of sight with He ii absorption would be required to yield
tighter constraints on the redshift evolution of η.
The detailed results of visual line fitting may be subjective
and may depend on the used fitting software. In particular, am-
biguities in the decomposition of blended H i lines can affect the
derived η values (Fechner & Reimers 2007b). Therefore we also
analyse the UV spectral shape variations using the ratio of the
effective optical depths
τeff,He ii
τeff,H i
. (2)
As introduced in Paper I, this parameter is a resolution-
independent estimator of the spectral shape of the UV radiation
field with small (high) R values indicating hard (soft) radiation
on a certain redshift scale ∆z. Shull et al. (2004) followed a sim-
ilar approach by taking η ≃ 4τHe ii/τH i for a restricted τ range
on scales of ∆z = 1.6× 10−4 and ∆z = 6.6× 10−4. However, this
scaling relation between τ and η is only valid at the centre of
an absorption line (Miralda-Escudé 1993). The column density
ratio is defined per absorption line and not as a continuous quan-
tity, whereas R can be defined on any scale. While R and η are
correlated (see below), there is no simple conversion between
R and η and the correlation will depend on the adopted redshift
scale of R.
4.2. Fluctuations in R and η along the line of sight
We obtained R(z) by binning both normalised Lyα forest spec-
tra of H i and He ii into aligned redshift bins of ∆z = 0.005
in the range 2.3325 < z < 2.8975 and computed R =
ln 〈THe ii〉/ ln 〈TH i〉 with the mean transmission 〈THe ii〉 and
〈TH i〉. The choice of the redshift binning scale was motivated by
the typical scale of η fluctuations 0.001 <∼ ∆z <∼ 0.03 (Kriss et al.
2001, FR07). We adopted the binning procedure by Telfer et al.
(2002) in order to deal with original flux bins that only partly
overlap with the new bins. The errors were computed accord-
ingly. Due to the high absorption and the low S/N of the He ii
data we occasionally encountered unphysical values 〈THe ii〉 ≤ 0.
These were replaced by their errors, yielding lower limits on R.
We mostly neglected the usually small metal contamination in
the computation of 〈TH i〉 in the Lyα forest because the errors in
R are dominated by the low S/N and the more uncertain contin-
uum level of the He ii spectrum. The FUSE data in the redshift
bins at z = 2.375, 2.380, 2.730, 2.735, 2.845 and 2.850 are con-
taminated by galactic H2 absorption, so no R measurement on
the full scale of ∆z = 0.005 can be performed there.
At 2.29 <∼ zLyα <∼ 2.33 the H i Lyα forest is severely con-
taminated by O vi from the associated system of HE 2347−4342
(Fechner et al. 2004). Furthermore, the Lyβ forest of both
species emerges at zLyα < 2.294. Because this excess absorp-
tion would bias the direct estimation of R in the spectra, we tried
to decontaminate the forests at z < 2.332. 〈TH i〉 was computed
from the H i Lyα forest reconstructed from the line list by T.-
S. Kim (Sect. 3). The corresponding 〈THe ii〉 was obtained after
dividing the FUSE data by the simulated Lyβ absorption of the
lines at higher redshift. Since the decontamination depends on
the validity of the He ii line parameters as well as on the com-
pleteness of the H i line list in the complex region contaminated
by O vi, the derived R values at z < 2.332 have to be regarded as
rough estimates.
The resulting R(z) is shown in the upper panel of Fig. 4. The
optical depth ratio strongly fluctuates around its median value
R ≃ 4.8 obtained for uncontaminated redshift bins, indicating
spectral fluctuations in the UV radiation field. We also show in
Fig. 4 the median η(z) on the same redshift bins based on the
line fitting results in FR07. Also the median η strongly fluctuates
with a slight trend of an increase with redshift (Z04). Clearly,
the data is inconsistent with a spatially uniform UV background,
but the median η ≃ 70 of the line sample is consistent with
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 7
Fig. 4. The fluctuating spectral shape of the UV
background towards HE 2347−4342. The up-
per panel shows the ratio of effective optical
depths R vs. redshift z in ∆z = 0.005 bins.
Data points at z < 2.332 (crosses) have been
decontaminated from O vi and Lyβ absorp-
tion (see text). Foreground quasars are marked
with letters and vertical dotted lines as well
as HE 2347−4342 (star symbol). The green
dashed line indicates the median R ≃ 4.8 ob-
tained at z > 2.332 in uncontaminated bins. The
lower panel shows the median η from FR07 in
the same redshift bins. The red dashed line in-
dicates the median η ≃ 70 of the line sample.
quasar-dominated models of the UV background. A comparison
of R(z) and η(z) reveals that both quantities are correlated. The
Spearman rank order correlation coefficient is rS = 0.67 with a
probability of no correlation PS = 6 × 10
There is a scatter in the relation between R and η, which is
due to noise in the He ii data and due to the fact that R is a spec-
tral softness indicator that is smoothed in redshift. Therefore, in
addition to the UV spectral shape, R will depend on the den-
sity fluctuations of the Lyα forest on the adopted scale. In or-
der to estimate the scatter in R due to these density fluctua-
tions, we simulated H i and He ii Lyα forest spectra. We gen-
erated 100 H i forests with the same overall redshift evolution of
eff,H i = 0.0032 (1 + z)
3.37 (Kim et al. 2002) based on the empir-
ical line distribution functions in redshift z, column density NH i
and Doppler parameter bH i (e.g. Kim et al. 2001). We modelled
each forest as a composition of lines with Voigt profiles using the
approximation by Tepper-Garcı́a (2006). The spectral resolution
(R ∼ 42000) and quality (S/N ∼ 100) closely matches the opti-
cal data of HE 2347−4342. The corresponding He ii forests were
generated at FUSE resolution with a S/N = 4 for four constant
values of η = 10, 20, 50 and 100. We assumed pure non-thermal
broadening of the lines. Then we computed R at 2 ≤ z ≤ 3 on our
adopted scale ∆z = 0.005, yielding 20000 R measurements for
each considered η. For convenience we took out the general red-
shift dependence of τeff,H i via dividing by the expected effective
optical depth τ
eff,H i, so
τeff,H i
eff,H i
is a measure of H i overdensity (D > 1) or underdensity (D < 1).
In Fig. 5 we show the relation R(D) obtained from the Monte
Carlo simulations and compare it to the distribution observed
towards HE 2347−4342. The simulated R(D) can be fitted rea-
sonably with a 3rd order polynomial in logarithmic space, yield-
ing a general decrease of R with D for every η. The root-mean-
square scatter increases from 0.13 dex for η = 10 to 0.18 dex for
η = 100. At D >∼ 3 the R(D) distribution flattens due to saturation
of high-column density absorbers on the flat part of the curve of
growth. The flattening causes substantial overlap between the
simulated R distributions at D >∼ 5, making R increasingly in-
sensitive to the underlying η. However, at D <∼ 3 hard radiation
Fig. 5. Dependence of R on D = τeff,H i/τ
eff,H i for different sim-
ulated values of η. The black lines indicate the polynomial fits
to the simulated distributions in logarithmic space. Red filled
circles represent the measured R(D) towards HE 2347−4342 in
uncontaminated bins at z > 2.332. The horizontal dotted line de-
notes R = 2. [See the online edition of the Journal for a colour
version of this figure.]
and soft radiation can be reasonably well distinguished. We also
overplot the measured R(D) towards HE 2347−4342 in Fig. 5.
The observed distribution is inconsistent with a constant η, but
the majority of values falls into the modelled η range. While
many high R values indicate η > 100, values with R <∼ 2 corre-
spond to η <∼ 20 at D <∼ 3. Thus, the very low R values always in-
dicate a hard radiation field up to moderate overdensities. As we
will see in the next section, the saturation effect probably does
not play a role in relating a hard radiation field to the nearby
quasars.
4.3. The UV radiation field near the quasars
We now investigate in greater detail the spectral shape of the UV
radiation field near the four quasars with available data on R and
η: the background quasar HE 2347−4342 and the foreground
QSOs A–C. Due to the small number of comparison values de-
rived from only two lines of sight, we will adopt η = 100 as
a characteristic value for the overall UV background at z > 2.6
8 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
(HE 2347−4342, QSO C) and a value of η = 50 at z ∼ 2.3 (QSOs
A and B). The former value is close to the median η = 102
obtained by Fechner et al. (2006) at 2.58 < z < 2.75 towards
HS 1700+6416, whereas the latter η value accounts for the prob-
able evolution of η with redshift. Furthermore, we will compare
the η values in the vicinity of the quasars to models of the UV
background.
4.3.1. HE 2347−4342
A close inspection of Fig. 4 reveals a strongly fluctuating radi-
ation field near HE 2347−4342 with some very small, but also
high R values. Also the column density ratio shows large fluc-
tuations (1 <∼ η <∼ 1000) with six η <∼ 10 absorbers out of the
20 absorbers at z > 2.86. These strong variations of the spec-
tral shape are likely due to radiative transfer effects in the asso-
ciated absorption system causing an apparent lack of the prox-
imity effect of HE 2347−4342 (Reimers et al. 1997). The high
He ii column densities of the associated system may soften the
quasar radiation with increasing distance and Fig. 4 supports this
interpretation. Due to the probable strong softening of the hard
quasar radiation on small scales, the relative spectral hardness
near HE 2347−4342 is only revealed by individual low η val-
ues instead of robust median values. However, also the highly
ionised metal species of the associated system (Fechner et al.
2004) favour the presence of hard QSO radiation. Thus we
conclude that despite the lack of a radiation-induced void near
HE 2347−4342, its impact onto the IGM can be detected via the
relative spectral hardness of the UV radiation field. The three
R < 2 values near HE 2347−4342 have D < 3, so they are prob-
ably not affected by saturation.
4.3.2. QSOs A and B
If our decontamination of the Lyα forests near the two z ∼ 2.3
QSOs A and B is correct, R should reflect UV spectral shape
variations also in that region. Indeed, the redshift bin at z =
2.280 next to QSO A (z = 2.282) is a local R minimum with
R ≃ 1.5. At z = 2.270 we find R ≃ 0.8. At the redshift of QSO B
(z = 2.302) the radiation field is quite soft, but we note a low
R ∼ 1 at z = 2.310. We obtain D < 3 for the four R <∼ 2 values
near QSO A and QSO B, so saturation is not relevant, and the
low R values correspond to low η values.
The measured η values in this redshift region are presented
in Fig. 6. The error bars are only indicative, since blended line
components are not independent and the He ii column densities
are derived with constraints from the H i forest. Lower limits on η
result from features detected in He ii but not in H i. Due to ambi-
guities in the line profile decomposition at the H i detection limit
and the present low quality of the He ii data it is hard to judge the
reality of most of these added components (Fechner & Reimers
2007b). Nevertheless, since η for adjacent lines may be not inde-
pendent due to line blending, we must include the lower limits
in the analysis. At z < 2.294 the fitting of He ii lines becomes
unreliable due to the arising Lyβ forest. Therefore, no direct es-
timates of η can be obtained in the immediate vicinity of QSO A.
Furthermore, the H i line sample may be incomplete or the line
parameters may be not well constrained due to blending with the
O vi of the associated system of HE 2347−4342.
Considering these caveats, the median η ≃ 19 obtained for
the values at z < 2.332 shown in Fig. 6 is only an estimate.
Nevertheless, this is much lower than the typical values η ∼ 50
found at z ∼ 2.3 towards HS 1700+6416 (Fechner et al. 2006).
Fig. 6. Column density ratio η vs. redshift z in the vicinity of
QSO A and QSO B. The long (short) dashed line indicates the
median η ≃ 19 in this redshift range (η = 50 for a UV back-
ground generated by quasars). At z < 2.294 the He ii Lyβ forest
sets in.
Moreover, it is also lower than at slightly higher redshifts to-
wards HE 2347−4342. For instance, the median η increases to
η = 79 in the redshift range 2.35 ≤ z ≤ 2.40. This is inconsis-
tent with the smooth redshift evolution of η on large scales in-
ferred by Z04 and Fechner et al. (2006) for both available sight-
lines. Thus, we infer an excess of hard radiation in the vicinity of
QSO A and QSO B. The most extreme η values are located in the
projected vicinity of QSO B, with 6 lines reaching η < 1. If es-
timated correctly, these low η values require local hard sources
and cannot be generated by the diffuse UV background. Both
foreground quasars could be responsible for the hard radiation
field because of similar light travel times to the probably affected
absorbers (tA ≃ 2tB).
4.3.3. QSO C
Since metal contamination of the H i forest is small in the pro-
jected vicinity of QSO C (Fig. 3), the UV spectral shape is bet-
ter constrained here than near QSO A and QSO B. From Fig. 4
we note a local R minimum (R ≃ 1.3) that exactly coincides
with the redshift of QSO C (z = 2.690). At higher redshifts
R rises, possibly indicating a softer ionising field. However, at
2.63 <∼ z <∼ 2.695 the optical depth ratio is continuously below
the median with R < 2 in five redshift bins. Due to the H i over-
densities near QSO C, all R < 2 values have D > 1, but only
the bin at z = 2.635 has D ≃ 4, so the remaining ones may still
indicate low column density ratios η.
Figure 7 displays the η values from FR07 in the redshift
range 2.63 < z < 2.73 in the projected vicinity of QSO C.
For comparison, we also indicate η = 100 that is consistent
with the median η = 102 towards HS 1700+6416 in this red-
shift range (Fechner et al. 2006). While the data generally shows
strong fluctuations around the median over the whole covered
redshift range (Z04; Fechner et al. 2006), there is an apparent
excess of small η values near QSO C indicating a predominantly
hard radiation field. From the data, a median η ≃ 46 is obtained
at 2.63 < z < 2.73 including the lower limits on η. The median η
near QSO C is lower than the median η towards HS 1700+6416
by a factor of two and also slightly lower than the η obtained for
spatially uniform UV backgrounds generated by quasars. The
relative agreement of the median η near QSO C and hard ver-
sions of quasar UV background models may result from the soft-
ening of the quasar radiation by the IGM at the large proper dis-
tances d >∼ 7.75 Mpc considered here (Table 3). This will be
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 9
Fig. 7. Column density ratio η vs. redshift z
in the vicinity of QSO C. The short dashed
line denotes η = 100 that is consistent with
the median η ≃ 102 obtained in the range
2.58 < z < 2.75 towards HS 1700+6416
(Fechner et al. 2006). The long dashed line in-
dicates the median η ≃ 46 obtained for the
shown η values (2.63 < z < 2.73).
further explored in Sect. 6.2. The larger contrast between the
median η near QSO C and the median η towards HS 1700+6416
yields stronger evidence for a local hardening of the UV radia-
tion near QSO C. However, this comparison value derived from
the single additional line of sight tracing this redshift range may
be biased itself.
Near QSO C the column density ratio still fluctuates and is
not homogeneously low as naively expected. We also note an
apparent offset of the low η region near QSO C towards lower
redshift due to fewer absorbers with low η at z > 2.69. While
some of the fluctuations can be explained by uncertainties to
recover η reliably from the present data, the very low η ≤ 10
values (≃ 24% of the data in Fig. 7) are likely intrinsically low.
These η values are in conflict with a homogeneous diffuse UV
background, and are likely affected by a local hard source. In
Sect. 6.4 we will estimate the error budget of η by Monte Carlo
simulations.
If QSO C creates a fluctuation in the spectral shape of the
UV background, the distance between the quasar and the line of
sight implies a light travel time of t = 25 Myr. The low (high)
redshift end of the region shown in Fig. 7 corresponds to a light
travel time of 64 Myr (44 Myr). Since these light travel times are
comparable, we argue that it is important to consider not only
the immediate projected vicinity of QSO C to be affected by the
proximity effect (see also Fig. 10 below).
In summary, both spectral shape indicators R and η indicate
a predominantly hard UV radiation field near all four known
quasars in this field. Many η values in the projected vicinity
of the quasars indicate a harder radiation than expected even
for model UV backgrounds of quasars alone. This points to a
transverse proximity effect detectable via the relative spectral
hardness. However, there are other locations along the line of
sight with an inferred hard radiation field, but without an asso-
ciated quasar, most notably the regions at z ∼ 2.48 and z ∼ 2.53
(Fig. 4). Before discussing these in detail (Sect. 6.3), we search
for additional evidence for hard radiation near the foreground
quasars by analysing nearby metal line systems.
5. Constraints from metal line systems
Observed metal line systems provide an additional tool to con-
strain the spectral shape of the ionising radiation. Since pho-
toionisation modelling depends on several free parameters, ap-
propriate systems should preferably show many different ionic
species. Fechner et al. (2004) analysed the associated metal line
system of HE 2347−4342 and found evidence for a hard quasar
spectral energy distribution at the absorbers with highest veloc-
ities that are probably closest to the quasar. Their large He ii
column densities probably shield the other absorbers which are
better modelled with a softer radiation field. The results by
Table 4. Measured column densities of the metal line system at
z = 2.2753. Several components of H i remain unresolved.
# v [km s−1] H i C iv Nv
1 −106.2 13.25 ± 0.59 12.70 ± 0.27
2 −94.3
13.634 ± 0.005
13.26 ± 0.57 12.40 ± 0.42
3 −45.8 12.76 ± 0.39 12.43 ± 0.14
4 −32.0
13.319 ± 0.015
12.80 ± 0.40 12.88 ± 0.06
5 0.0 13.232 ± 0.019 13.17 ± 0.03 12.94 ± 0.02
6 44.8 12.604 ± 0.020 12.51 ± 0.09 12.21 ± 0.05
7 91.5 13.042 ± 0.007 12.79 ± 0.05 11.98 ± 0.11
Fechner et al. (2004) are consistent with the more direct hard-
ness estimators R and η near HE 2347−4342 (Sect. 4.3.1).
In the spectrum of HE 2347−4342 an intervening metal line
system is detected at z = 2.7119 which is close to the redshift
of QSO C (∆z = 0.022) at a proper distance of d ≃ 10.0 Mpc.
At z = 2.2753 there is another system showing multiple com-
ponents of C iv and N v as well as only weak H i absorption
(NH i < 10
13.7 cm−2). The presence of N v and weak H i features
with associated metal absorption are characteristic of intrinsic
absorption systems exposed to hard radiation. Due to the small
proper distance to QSO A (d ≃ 3.1 Mpc) this system is proba-
bly illuminated by the radiation of the close-by quasar. A third
suitable metal line system at z = 2.3132 is closer to QSO B
(d ≃ 6.1 Mpc) than to QSO A (d ≃ 12.0 Mpc). But since
QSO A is much brighter than QSO B (Table 3), the metal line
system at z = 2.3132 might be affected by both quasars. Due
to their small relative velocities with respect to the quasars of
< 3000 km s−1 the systems are likely associated to the quasars
(e.g. Weymann et al. 1981).
In order to construct CLOUDY models (Ferland et al. 1998,
version 05.07) we assumed a single-phase medium, i.e. all ob-
served ions arise from the same gas phase, as well as a solar
abundance pattern (Asplund et al. 2005) at a constant metallicity
throughout the system. Furthermore, we assumed pure photoion-
isation and neglected a possible contribution of collisional ion-
isation. The absorbers were modelled as distinct, plane-parallel
slabs of constant density testing different ionising spectra.
5.1. The system at z = 2.275 near QSO A
The system at z = 2.2753 shows seven components of C iv
and N v along with unsaturated features of H i (Fig. 8). The ab-
sorber densities are constrained by the C iv/N v ratio. For the
HM01 background scaled to yield log Jb = −21.15 at the H i
Lyman limit (Scott et al. 2000), we derive densities in the range
10−4.38 to 10−3.35 cm−3 at a metallicity of ∼ 0.6 solar. The es-
10 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Fig. 8. Metal line system at z = 2.2753 towards HE 2347−4342.
The displayed profiles assume the rescaled HM01 background.
Zero velocity corresponds to z = 2.2753.
timated absorber sizes are ∼ 5 kpc or even smaller, where the
sizes are computed according to NH = nH l with the absorbing
path length l. With an additional contribution by QSO A, mod-
elled as a power law with α = 0.21 and H i Lyman limit intensity
log Jq = −21.9 at the location of the absorber, we obtain an even
higher metallicity of ∼ 11 times solar. Densities in the range
10−2.95 to 10−2.01 cm−3 are found leading to very small absorbers
of . 10 pc.
Both models lead to unusually high metallicities and very
small absorber sizes. However, Schaye et al. (2007) recently re-
ported on a large population of compact high-metallicity ab-
sorbers. Using the HM01 background they found typical sizes of
∼ 100 pc and densities of 10−3.5 cm−3 for absorbers with nearly
solar or even super-solar metallicities. In fact, this system is part
of the sample by Schaye et al. (2007).
Since the system exhibits only a few different species, it is
impossible to discriminate between the soft and the hard radi-
ation model. In principle, both models lead to a consistent de-
scription of the observed metal lines. The soft HM01 UV back-
ground yields η ∼ 170 for the modelled absorbers, whereas the
model including the hard radiation of QSO A leads to η ∼ 10.
Recall that the He ii forest cannot be used to measure η directly
due to blending with Lyβ features and very low S/N.
5.2. The systems near QSO B and QSO C
The systems at z = 2.3132 and z = 2.7119 are located near
QSO B and QSO C, respectively. Only few ions are observed and
some of them may even be blended. Therefore, no significant
conclusions based on CLOUDY models can be drawn. Using the
column density estimates we find that the system at z = 2.7119
close to QSO C exhibiting C iv and O vi can be described con-
sistently with a HM01+QSO background. Models assuming a
quasar flux of log Jq & −22.5 seen by the absorber yield η . 40,
consistent with the direct measurements from the He ii forest.
However, the metal transitions alone do not provide strong con-
straints.
The system at z = 2.3132 shows C iv in six components
along with Si iv and Si iii. The Lyman series of this system suf-
fers from severe blending preventing a reliable estimation of the
H i column density. Therefore metallicities and absorber sizes
cannot be estimated. Adopting our column density estimates we
infer that this system can be reasonably modelled with or with-
out a specific quasar contribution.
6. Discussion
6.1. The transverse proximity effect in spectral hardness
Fourteen quasars have been found in the vicinity of
HE 2347−4342 of which three are located in the usable part of
the H i Lyα forest towards HE 2347−4342. No H i underdensity
is detected near these foreground quasars even when correcting
for contamination by the O vi absorption from the associated
system of HE 2347−4342. An estimate of the predicted effect
confirms that even if existing, the classical proximity effect is
probably too weak to be detected on this line of sight due to the
high UV background at 1 ryd and small-scale variance in the H i
transmission (Sect. 3).
However, the analysis of the spectral shape of the UV radia-
tion field near the foreground quasars yields a markedly different
result. The spectral shape is fluctuating, but it is predominantly
hard near HE 2347−4342 and the known foreground quasars.
Close to QSO C, both estimators R and η are consistent with a
significantly harder radiation field than on average. There is a
sharp R minimum located precisely at the redshift of the quasar,
but embedded in a broader region of low R values statistically
consistent with a hard radiation field of η <∼ 10 (Fig. 4). The
column density ratio η is also lower than on average and indi-
cates a harder radiation field than obtained for quasar-dominated
models of the UV background (Fig. 7). Because of line blend-
ing, only one of the three metal line systems detected near the
foreground quasars can be used to estimate the shape of the
ionising field. The metal line system at z = 2.275 can be de-
scribed reasonably by the HM01 background with or without a
local ionising component by QSO A. The He ii forest does not
provide independent constraints for this absorber. Line blending
prevents an unambiguous detection of O vi at z = 2.712, leaving
the shape of the ionising field poorly constrained without tak-
ing into account the He ii forest. Thus, the systems show highly
ionised metal species, but our attempts to identify a local quasar
radiation component towards them remain inconclusive.
The most probable sources for the hard radiation field at
z ∼ 2.30 and z ∼ 2.69 towards HE 2347−4342 are the nearby
foreground quasars. In particular, the absorbers with η <∼ 10 have
to be located in the vicinity of an AGN, since the filtering of
quasar radiation over large distances results in η >∼ 50. Also star-
forming galaxies close to the line of sight cannot yield the low
η values, since they are unable to produce significant numbers
of photons at hν > 54.4 eV (Leitherer et al. 1999; Smith et al.
2002; Schaerer 2003). We conclude that there is evidence for a
transverse proximity effect of QSO C detectable via the relative
spectral hardness. There are also indications that QSO A and
QSO B show the same effect, although contamination adds un-
certainty to the spectral shape variations in their projected vicin-
Given these incidences of a hard radiation field near the
quasars, how do these results relate to those of Paper I, in which
we investigated the line of sight towards Q 0302−003? Both
lines of sight show He ii absorption and on both lines of sight
we find evidence for a predominantly hard radiation field near
the quasars in the background and the foreground. However, the
decrease of η near quasars towards Q 0302−003 appears to be
much smoother than towards HE 2347−4342.
There are several reasons for the lack of small-scale spec-
tral shape variations on the line of sight to Q 0302−003. First,
the low-resolution STIS spectrum of Q 0302−003 does not re-
solve the He ii lines and limits the visible scale of fluctuations
to ∆z & 0.006 (Paper I). Much smaller scales can be probed
in the resolved He ii forest of HE 2347−4342, but the fitting of
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 11
blended noisy He ii features may result in artifical η variations.
We will discuss the uncertainties of η below (Sect. 6.4). Second,
Q 0302−003 (z = 3.285) probes higher redshifts, where the He ii
fraction in the IGM is significantly higher and the inferred radi-
ation field is very soft (η ∼ 350 in the Gunn-Peterson trough).
Therefore, the impact of a hard source on the spectral shape is
likely to be more pronounced than at lower redshifts after the
end of He ii reionisation, where η of the UV background gradu-
ally decreases.
6.2. The decrease of η near QSO C
We now investigate quantitatively whether the foreground
quasars are capable of creating a hardness fluctuation on the
sightline towards HE 2347−4342. Unfortunately, since only one
quasar is located in an uncontaminated region of the Lyα forests,
we can present sufficient evidence only for QSO C. For the other
two quasars the data is too sparse and contamination adds uncer-
tainty to the derived η, but in principle QSO A should also show
a strong effect, because its Lyman limit flux penetrating the line
of sight is ∼ 8 times higher than the one of QSO C.
Heap et al. (2000) and Smette et al. (2002) presented simple
models of the decrease of η in front of a quasar taking into ac-
count the absorption of ionising photons by the IGM. In a highly
photoionised IGM with helium mass fraction Y ≃ 0.24 and tem-
perature T ≃ 2 × 104 K we have
4 (1 − Y)
αHe ii
ΓHe ii
≃ 0.42
ΓHe ii
, (4)
where Γi and αi are the photoionisation rate and the radiative
recombination coefficient for species i (Fardal et al. 1998). The
photoionisation rate is Γi = Γi,b + Γi,q with a contribution of the
background and the quasar. The contribution of the quasar to the
photoionisation rate of species i at the jth absorber in front of it
(z j > z j+1) is
Γi,q(z j) =
σi fν,i
h(1 + zq)
1 + zq
1 + z j
)−α+1 ( dL(zq, 0)
dL(zq, z j)
x−α−4exp
Ni,kσix
1 + zk
1 + z j
dx, (5)
with the photoionisation cross section at the Lyman limit σi, the
observed Lyman limit flux fν,i and x = ν/νi with the Lyman limit
frequency νi. Extrapolating the power law continuum flux to the
He ii Lyman limit yields fν,He ii = fν,H i4
−α. With the spectral in-
dex α from Table 3 we obtain ηmin ≃ 2.3 for QSO C.
We simulated η(z) for a set of 1000 Monte Carlo Lyα for-
est spectra generated with the procedure discussed in Sect. 4.2.
We assumed ΓH i,b = 1.75 × 10
−12 s−1 corresponding to the UV
background from Sect. 3 and ηb = 100, which agrees with the
median η towards HS 1700+6416 in the redshift range under
consideration (Fechner et al. 2006). The intervening absorbers
successively block the quasar flux. Especially, every absorber
with log NH i > 15.8 will truncate the quasar flux at hν > 4 ryd
due to a He ii Lyman limit system, leading to an abrupt softening
of the radiation field.
Figure 9 presents the simulated decrease of the median η
approaching QSO C assuming a constant quasar luminosity,
isotropic radiation and an infinite quasar lifetime together with
the upper and lower percentiles of the η distribution obtained
in bins of proper distance ∆d = 2 Mpc. The spread in the
simulated η is due to line-of-sight differences in the absorber
Fig. 9. Column density ratio η vs. proper distance d. The black
line shows the modelled decrease of the median η approaching
QSO C with respect to the ambient ηb = 100 (dashed line).
Green lines mark the upper and lower quartiles of the simu-
lated η distribution in bins of ∆d = 2 Mpc. QSO C is located at
7.75 Mpc. Filled circles show the median η from FR07 in con-
centric rings of ∆d = 2 Mpc around the quasar. Error bars are the
quartile distances to the median. The arrow marks the metal line
system at z = 2.7122 at d = 12.03 Mpc. [See the online edition
of the Journal for a colour version of this figure.]
properties. Since we consider the transverse proximity effect,
we are limited to a proper distance d >∼ 7.75 Mpc (Table 3).
The model agrees reasonably with the median η of the data ob-
tained in concentric rings around the quasar. As expected, in-
dividual η values strongly deviate from this simple model due
to the assumptions of the quasar properties (constant luminosity
and spectral index, isotropic radiation) and due to the unknown
real distribution of absorbers in transverse direction. Recently,
Hennawi & Prochaska (2007) found evidence for excess small-
scale clustering of high-column density systems in transverse
direction to quasar sightlines. In Sect. 4.3.3 we found indica-
tions that the η distribution around QSO C is not symmetric,
which could be due to such anisotropic shielding. However, this
does not imply an intrinsic anisotropy due to the unknown mat-
ter distribution around the quasar and the large uncertainties in
individual η values. Moreover, the line-of-sight variance at a
constant η = 100 is too small to explain the large observed
spread of the η values. Clearly, a self-consistent explanation
of the small-scale η fluctuations would require hydrodynami-
cal simulations of cosmological radiative transfer in order to in-
vestigate possible shielding effects and the statistical distribu-
tion of η values near quasars. While there is recent progress in
case of the UV background (Sokasian et al. 2003; Croft 2004;
Maselli & Ferrara 2005; Bolton et al. 2006), a proper treatment
of three-dimensional radiative transfer in the IGM around a
quasar is still in its infancy. However, our simplified approach
suggests that QSO C is capable of changing the spectral shape
of the UV radiation field by the right order of magnitude to ex-
plain the low η values in its vicinity. Also a variation in the sizes
and the centres of the bins chosen for Fig. 9 does not drastically
change the indicated excess of low η at d <∼ 14 Mpc. Figure 9
also shows very clearly that the sphere of influence for the trans-
verse proximity effect is not limited to the immediate vicinity of
the quasar.
Figure 10 shows a two-dimensional cut in comoving space
near QSO C in the plane spanned by both lines of sight. The
minimum separation of both lines of sight corresponds to a light
travel time of ≃ 25.2 Myr, but the lifetime of QSO C could be
>∼ 40 Myr due to the low η values at larger distances. The fluctua-
12 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Fig. 10. Transverse comoving separation ∆r⊥ vs. line-of-sight
comoving separation ∆r‖ with respect to QSO C. Black (green)
points denote absorbers with η < 100 (η ≥ 100) on the line of
sight towards HE 2347−4342 (curved line) with indicated red-
shifts. The blue arrow points to the metal line system at z =
2.712. The half circles show the distance travelled by light emit-
ted at the indicated times prior to our observation. The minimum
light travel time between the two lines of sight is 25.19 Myr. [See
the online edition of the Journal for a colour version of this fig-
ure.]
tions of the UV spectral shape could be explained by shadowing
of the hard QSO radiation by unknown intervening structures
between both lines of sight.
6.3. Other regions with an inferred hard UV radiation field
In Fig. 4 we note two additional regions at z ∼ 2.48 and z ∼ 2.53
where R is prominently small and where there is no nearby
quasar. Also the fitted η(z) shows very low values apparently
unrelated to a known foreground quasar. Figure 11 displays the
redshift distribution of the η ≤ 10 subsample. The low η values
are clustered with two peaks near the foreground quasars, but
also at z ∼ 2.40, z ∼ 2.48 and z ∼ 2.53. At the first glance the ex-
istence of such regions seems to undermine the relation between
the foreground quasars and a low η in their vicinity. However,
there are several plausible explanations for the remaining low η
values:
1. Unknown quasars: We can conclude from Paper I that the
quasars responsible for hardness fluctuations may be very
faint (like Q 0302-D113 in Paper I) or may reside at large
distances (Q 0301−005 in Paper I). QSO C is located near
the edge of our survey area centred on HE 2347−4342, so
other quasars capable of influencing the UV spectral shape
might be located outside the field of view. Moreover, in or-
der to sample the full quasar luminosity domain (MB ≤ −23)
at z ∼ 2.5 our survey is still too shallow by ∼ 1 mag-
nitude. Therefore, a larger and/or deeper survey around
HE 2347−4342 is desirable.
2. Quasar lifetime: Assuming that quasars are long-lived and
radiate isotropically, every statistically significant low η fluc-
tuation should be due to a nearby quasar. On the other hand,
short-lived quasars will not be correlated with a hard radia-
tion field due to the light travel time from the quasar to the
background line of sight. Quasar lifetimes are poorly con-
strained by observations to 1 <∼ tq <∼ 100 Myr (Martini 2004).
This could be short enough to create relic light echoes from
extinct quasars. The comoving space density of quasars with
Fig. 11. Redshift distribution of low-η absorbers. The open
(hashed) histogram shows all (NH i ≤ 10
14 cm−2) absorbers with
η ≤ 10. Letters and dotted lines mark foreground quasars. The
horizontal dashed line denotes the estimated average number of
absorbers scattered from η = 80 to η ≤ 10 (≃ 0.64 per bin).
MB < −23 at z ≃ 2.5 is ≃ 3.7×10
−6 Mpc−3 (Wolf et al. 2003)
resulting in an average proper separation of ∼ 18.5 Mpc be-
tween two lines of sight. This translates into a light travel
time of ∼ 60 Myr which is of the same order as the quasar
lifetime. So it is quite possible that some quasars have al-
ready turned off, but their hard radiation is still present.
3. Obscured quasars: Anisotropic emission of type I quasars
may lead to redshift offsets between regions with an inferred
hard radiation field and quasars close to the line of sight. In
the extreme case the putative quasar radiates in transverse di-
rection, but is obscured on our line of sight (type II quasar).
The space density of type II AGN at z > 2 is very uncer-
tain due to the challenging optical follow-up that limits the
survey completeness (e.g. Barger et al. 2003; Szokoly et al.
2004; Krumpe et al. 2007). Thus, the fraction of obscured
AGN at high redshift is highly debated (Akylas et al. 2006;
Treister & Urry 2006), but may well equal that of type I
AGN in the luminosity range of interest (Ueda et al. 2003).
We believe that a combination of the above effects is respon-
sible for the loose correlation between low η values and active
quasars. In particular, at z ∼ 2.4 we infer a hard radiation field in
a H i void (FR07), which may have been created by a luminous
quasar that is unlikely to be missed by our survey (V <∼ 22).
In Fig. 11 we also indicate the error level due to inaccurate
line fitting and noise in the He ii data (dashed line) obtained from
simulated data (see below). The low number of η values scattered
from a simulated η = 80 to η ≤ 10 implies that the overdensities
of such small η values are statistically significant. Constraining
the sample to lines with log(NH i) < 14 due to a possible bias
caused by thermal broadening does not remove the significant
clusters of lines with small η.
6.4. Uncertainties in the spectral hardness
Our findings are likely to be affected by random errors and possi-
bly also by systematic errors mostly related to the He ii data. The
poor quality of the FUSE spectrum of HE 2347−4342 (S/N <∼ 5)
contributes to the fluctuations in η even if the η value was con-
stant (Fechner et al. 2006, see also below). The optical depth
ratio R should be less affected by noise, since it is an average
over a broader redshift range ∆z = 0.005. The low S/N and the
generally high absorption at η ≫ 1 provide uncertainty for the
continuum determination in the He ii spectrum. The extrapolated
reddened power law is certainly an approximation.
Although the η fitting results from FR07 are broadly consis-
tent with those of Z04 and agree well in the regions near the fore-
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 13
ground quasars, there are substantial differences in some redshift
ranges. This is probably due to the combined effects of low He ii
data quality, different data analysis software and ambiguities in
the deblending of lines. At present, η cannot be reliably deter-
mined at individual absorbers unless metal transitions provide
further constraints.
In order to assess the random scatter in η due to the low S/N
He ii data and ambiguities in the line deblending of both species,
we again used Monte Carlo simulations. Ten H i Lyα forest spec-
tra were generated in the range 2 < z < 3 via the Monte Carlo
procedure outlined in Sect. 4.2. The resolution R ∼ 42000 and
S/N = 100 closely resembles the optical data of HE 2347−4342.
We also generated the corresponding He ii forests at FUSE res-
olution and S/N = 4. We assumed pure non-thermal line broad-
ening and η = 80. Voigt profiles were automatically fitted to
the H i spectra using AUTOVP 1 (Davé et al. 1997). The He ii
spectra were then automatically fitted with redshifts z and non-
thermal Doppler parameters bH i fixed from the fitted H i line
lists, yielding 7565 simulated η values. On average the recov-
ered η is slightly higher than the simulated one (median η ≃ 89)
with a large spread (0 < η <∼ 8000), but only 285 lines have
η ≤ 10. Thus, we estimate a probability P ≃ 3.8% that η is
scattered randomly from η = 80 to η ≤ 10 if the assumption of
non-thermal broadening is correct. Note that this probability is
likely an upper limit due to the fact that only H i Lyα was used
to obtain the line parameters, which results in large error bars
for saturated lines on the flat part of the curve of growth. In the
real data, these errors were avoided by fitting unsaturated higher
orders of the Lyman series wherever possible.
In the line sample by FR07, 94 out of the 526 absorbers have
η ≤ 10, whereas our simulation implies that only ∼ 20 are ex-
pected to be randomly scattered to η ≤ 10 if η was constant.
Thus, the major part of the scatter of η in the data is due to real
fluctuations in the UV spectral shape. The majority of the low
η ≤ 10 values is inconsistent with η ≥ 80, so they indicate a
hard radiation field in spite of the low S/N in the He ii data. Yet,
due to the large intrinsic scatter obtained from the simulations,
individual η values hardly trace the variations of the UV spectral
shape. Local spatial averages should be more reliable (FR07).
Since the transverse proximity effect zones always extend over
some redshift range, this requirement is fulfilled and on average
we reveal a harder radiation field than expected.
Concerning the high tail of the simulated distribution at
η = 80, ∼ 15% of the lines are returned with η >∼ 200. This
may indicate that a fraction of the observed high η values is still
consistent with a substantially harder radiation field, underlining
that single η values poorly constrain the spectral shape.
Possibly, some η values are systematically too low due to
the assumption of non-thermal broadening (bHe ii = bH i) when
fitting the He ii forest. FR07 found that this leads to underes-
timated η values at NH i >∼ 10
13 cm−2 if the lines are in fact
thermally broadened (bHe ii = 0.5bH i). Non-thermal broaden-
ing is caused by turbulent gas motions or the differential Hubble
flow, with the latter affecting in particular the low-column den-
sity forest. Thermal broadening becomes important in collapsed
structures at high column densities. In simulations of the Lyα
forest, non-thermal broadening has been found to dominate
(Zhang et al. 1995, 1998; Hernquist et al. 1996; Weinberg et al.
1997; Bolton et al. 2006; Liu et al. 2006). This has been con-
firmed observationally for the low-column density forest (Z04;
Rauch et al. 2005). On the other hand, eight out of eleven ab-
sorbers with NH i > 10
14 cm−2 in the vicinity of QSO C have
1 http://ursa.as.arizona.edu/˜rad/autovp.tar
η ≤ 10 (Fig. 7). Although the column density ratio of these
absorbers could be underestimated due to an unknown contri-
bution of thermal broadening, the statistical evidence for a hard
radiation field is based on the vast majority of low-column den-
sity lines. The median η obtained in this region does not in-
crease significantly after excluding the suspected lines (∼ 53
vs. ∼ 40). This is still much lower than the median η ∼ 100
towards HS 1700+6416 in this redshift range (Fechner et al.
2006). Therefore, it is unlikely that our results are biased due
to the assumed line broadening.
7. Conclusions
Traditionally, the transverse proximity effect of a quasar has
been claimed to be detectable as a radiation-induced void in the
H i Lyα forest. But due to several systematic effects like quasar
variability, finite quasar lifetime, intrinsic overdensities around
quasars, or anisotropic radiation, most searches yielded negative
results (e.g. Schirber et al. 2004; Croft 2004).
In this paper, we have analysed the fluctuating spectral shape
of the UV background in the projected vicinity of the three
foreground quasars QSO J23503−4328, QSO J23500−4319 and
QSO J23495−4338 (dubbed QSO A, B and C) on the line of
sight towards HE 2347−4342 (z = 2.885). By comparing the
H i absorption and the corresponding He ii absorption, we have
presented evidence for a statistical excess of hard UV radiation
near the foreground quasars. However, due to contamination of
the forests near QSO A (z = 2.282) and QSO B (z = 2.302),
the evidence is strongest for QSO C (z = 2.690). We interpret
these indicators for an excess of hard radiation near the fore-
ground quasars as a manifestation of the transverse proximity
effect. A simple model indicates that the foreground quasars are
capable of generating the observed hard radiation over the ob-
served distances of several Mpc. Furthermore, we tried to model
the ionising radiation field of three metal line systems close to
the foreground quasars. Two of those are strongly affected by
line blending and do not allow for reliable photoionisation mod-
els. The remaining system can be modelled reasonably with or
without a contribution by a local quasar. Future larger samples of
highly ionised unblended metal systems near foreground quasars
may provide evidence for local hardness fluctuations.
In Worseck & Wisotzki (2006) we revealed the transverse
proximity effect as a systematic local hardness fluctuation
around four foreground quasars near Q 0302−003 and pointed
out that the relative UV spectral hardness is a sensitive phys-
ical indicator of the proximity effect over distances of several
Mpc. In this study we are able to confirm this on a second line
of sight. Evidently, small-scale transmission fluctuations in the
H i forest can dilute the small predicted signature of the effect.
However, the hard spectral shape of the UV radiation field still
indicates the transverse proximity effect despite the H i density
fluctuations. Thus, we confirm our previous result that the spec-
tral hardness breaks the density degeneracy that affects the tradi-
tional searches for the proximity effect. Moreover, the predicted
transverse proximity effect of the quasars in the H i forest is weak
due to the high UV background at 1 ryd. Still the UV spectral
shape is able to discriminate local UV sources independent of
the amplitude of the UV background.
Bolton et al. (2006) find that the large UV spectral shape
fluctuations in the IGM are likely due to the small number of
quasars contributing to the He ii ionisation rate at any given
point, whereas the H i ionisation rate is rather homogeneous
due to the probable contribution of star-forming galaxies (e.g.
Bianchi et al. 2001; Sokasian et al. 2003; Shapley et al. 2006).
14 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Our findings confirm the picture that AGN create the hard part of
the intergalactic UV radiation field. If the quasar is active long
enough, its hard radiation field can be observed penetrating a
background line of sight. It is also likely that light echoes from
already extinguished quasars are responsible for some locations
of hard radiation without an associated quasar. The transverse
proximity effect of QSO C implies a minimum quasar lifetime
of ∼ 25 Myr (probably even ∼ 40 Myr), providing additional
constraints to more indirect estimates (e.g. Martini 2004, and
references therein).
However, the UV radiation field near the foreground quasars
is not homogeneously hard as naively expected, but still shows
fluctuations. Apart from substantial measurement uncertainties,
the unknown density structure around the quasar could shield the
ionising radiation in some directions, maybe even preferentially
in transverse direction to the line of sight (Hennawi & Prochaska
2007). Thus, radiative transfer effects may become important
to explain a fluctuating UV spectral shape in the presence of
a nearby quasar. Large-scale simulations of cosmological radia-
tive transfer with discrete ionising sources are required to adress
these issues in detail.
Moreover, the He ii forest has been resolved so far only to-
wards two quasars at a very low S/N <∼ 5. While the low data
quality primarily creates uncertainties in the spectral shape on
small spatial scales, large scales could be affected by cosmic
variance. Thus, the general redshift evolution of the UV spec-
tral shape is not well known and estimates obtained from single
lines of sight may well be biased by local sources.
Acknowledgements. We thank the staff of the ESO observatories La Silla and
Paranal for their professional assistance in obtaining the optical data discussed in
this paper. We are grateful to Peter Jakobsen for agreeing to publish the quasars
from his survey. We thank Gerard Kriss and Wei Zheng for providing the re-
duced FUSE spectrum of HE 2347−4342. Tae-Sun Kim kindly supplied an addi-
tional line list of HE 2347−4342. GW and ADA acknowledge support by a HWP
grant from the state of Brandenburg, Germany. CF is supported by the Deutsche
Forschungsgemeinschaft under RE 353/49-1. We thank the anonymous referee
for helpful comments.
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G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 15
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List of Objects
‘HE 2347−4342’ on page 1
‘HS 1700+6416’ on page 1
‘Q 0302−003’ on page 2
‘QSO J23510−4336’ on page 4
‘QSO J23507−4319’ on page 4
‘QSO J23507−4326’ on page 4
‘QSO J23509−4330’ on page 4
‘QSO J23502−4334’ on page 4
‘QSO J23503−4328’ on page 4
‘QSO J23495−4338’ on page 4
‘QSO J23511−4319’ on page 4
‘QSO J23514−4339’ on page 4
‘QSO J23503−4317’ on page 4
‘QSO J23515−4324’ on page 4
‘QSO J23512−4332’ on page 4
‘QSO J23508−4335’ on page 4
‘QSO J23500−4319’ on page 4
‘Q 0302-D113’ on page 12
‘Q 0301−005’ on page 12
	Introduction
	Observations and data reduction
	Search for QSO candidates near HE 2347-4342
	Spectroscopic follow-up
	Additional quasars
	Redshifts and magnitudes
	Optical spectra of HE 2347-4342
	Far-UV spectra of HE 2347-4342
	The Ly forest near the foreground quasars
	The fluctuating shape of the UV radiation field towards HE 2347-4342
	Diagnostics
	Fluctuations in R and  along the line of sight
	The UV radiation field near the quasars
	HE 2347-4342
	QSOs A and B
	QSO C
	Constraints from metal line systems
	The system at z=2.275 near QSO A
	The systems near QSO B and QSO C
	Discussion
	The transverse proximity effect in spectral hardness
	The decrease of  near QSO C
	Other regions with an inferred hard UV radiation field
	Uncertainties in the spectral hardness
	Conclusions

  We report the discovery of 14 quasars in the vicinity of HE2347-4342, one of
the two quasars whose intergalactic HeII forest has been resolved with FUSE. By
analysing the HI and the HeII opacity variations separately, no transverse
proximity effect is detected near three foreground quasars of HE2347-4342:
QSOJ23503-4328 (z=2.282, $\vartheta=3.59$ arcmin), QSOJ23500-4319 (z=2.302,
$\vartheta=8.77$ arcmin) and QSOJ23495-4338 (z=2.690, $\vartheta=16.28$
arcmin). This is primarily due to line contamination and overdensities probably
created by large-scale structure. By comparing the HI absorption and the
corresponding HeII absorption, we estimated the fluctuating spectral shape of
the extragalactic UV radiation field along this line of sight. We find that the
UV spectral shape near HE2347-4342 and in the projected vicinity of the three
foreground quasars is statistically harder than expected from UV background
models dominated by quasars. In addition, we find three highly ionised metal
line systems near the quasars. However, they do not yield further constraints
on the shape of the ionising field. We conclude that the foreground quasars
show a transverse proximity effect that is detectable as a local hardening of
the UV radiation field, although the evidence is strongest for QSOJ23495-4338.
Thus, the relative spectral hardness traces the proximity effect also in
overdense regions prohibiting the traditional detection in the HI forest.
Furthermore, we emphasise that softening of quasar radiation by radiative
transfer in the intergalactic medium is important to understand the observed
spectral shape variations. From the transverse proximity effect of
QSOJ23495-4338 we obtain a lower limit on the quasar lifetime of ~25 Myr.

Introduction
After reionisation the intergalactic medium (IGM) is kept highly
photoionised by the metagalactic UV radiation field generated
by the overall population of quasars and star-forming galax-
ies (e.g. Haardt & Madau 1996; Fardal et al. 1998; Bianchi et al.
2001; Sokasian et al. 2003). The intensity and spectral shape of
the UV background determines the ionisation state of the observ-
able elements in the IGM. In particular, the remaining fraction
of intergalactic neutral hydrogen and singly ionised helium is
responsible for the Lyα forest of H i and He ii.
On lines of sight passing near quasars the IGM will be sta-
tistically more ionised due to the local enhancement of the UV
flux that should result in a statistically higher IGM transmission
(’void’) in the QSO’s vicinity (Fardal & Shull 1993; Croft 2004;
McDonald et al. 2005). This so-called proximity effect has been
found with high statistical significance on lines of sight towards
luminous quasars (e.g. Bajtlik et al. 1988; Giallongo et al. 1996;
Scott et al. 2000). On the other hand, a transverse proximity ef-
fect created by foreground ionising sources nearby the line of
sight has not been clearly detected in the H i forest, except the
recent detection at z = 5.70 by Gallerani et al. (2007). While
two large H i voids have been claimed to be due to the transverse
Send offprint requests to: G. Worseck, e-mail: gworseck@aip.de
⋆ Based on observations collected at the European Southern
Observatory, Chile (Proposals 070.A-0425 and 074.A-0273). Data col-
lected under Proposals 068.A-0194, 070.A-0376 and 116.A-0106 was
obtained from the ESO Science Archive.
proximity effect by Dobrzycki & Bechtold (1991a, however
see Dobrzycki & Bechtold 1991b) and Srianand (1997), other
studies find at best marginal evidence (Fernández-Soto et al.
1995; Liske & Williger 2001), and most attempts resulted
in non-detections (Crotts 1989; Møller & Kjærgaard 1992;
Crotts & Fang 1998; Schirber et al. 2004; Croft 2004). This
has led to explanations involving the systematic effects of
anisotropic radiation, quasar variability (Schirber et al. 2004),
intrinsic overdensities (Loeb & Eisenstein 1995; Rollinde et al.
2005; Hennawi & Prochaska 2007; Guimarães et al. 2007) and
finite quasar lifetimes (Croft 2004).
Intergalactic He ii Lyα absorption (λrest = 303.7822 Å)
can be studied only towards the few quasars at z > 2 whose
far UV flux is not extinguished by intervening Lyman limit
systems (Picard & Jakobsen 1993; Jakobsen 1998). Of the six
quasars successfully observed so far, the lines of sight to-
wards HE 2347−4342 (z = 2.885) and HS 1700+6416 (z =
2.736) probe the post-reionisation era of He ii with an emerg-
ing He ii forest that has been resolved with FUSE (Kriss et al.
2001; Shull et al. 2004; Zheng et al. 2004; Fechner et al. 2006;
Fechner & Reimers 2007a).
In a highly ionised IGM a comparison of the H i with the
corresponding He ii absorption yields an estimate of the spectral
shape of the UV radiation field due to the different ionisation
thresholds of both species. The amount of He ii compared to H i
gives a measure of the spectral softness, generally expressed via
the column density ratio η = NHe ii/NH i. Typically, η <∼ 100 indi-
cates a hard radiation field generated by the surrounding quasar
http://arxiv.org/abs/0704.0187v2
2 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
population, whereas η >∼ 100 requires a significant contribu-
tion of star-forming galaxies or heavily softened quasar radiation
(e.g. Haardt & Madau 1996; Fardal et al. 1998; Haardt & Madau
2001).
The recent FUSE observations of the He ii Lyα forest re-
vealed large η fluctuations (1 <∼ η <∼ 1000) on small scales
of 0.001 <∼ ∆z <∼ 0.03 with a median η ≃ 80–100. Apart
from scatter due to the low-quality He ii data at S/N ∼ 5
(Fechner et al. 2006; Liu et al. 2006) and possible systematic er-
rors due to the generally assumed line broadening mechanism
(Fechner & Reimers 2007a), several physical reasons for these
η variations have been proposed. A combination of local den-
sity variations (Miralda-Escudé et al. 2000), radiative transfer
effects (Maselli & Ferrara 2005; Tittley & Meiksin 2006) and lo-
cal differences in the properties of quasars may be responsible
for the fluctuations. In particular, at any given point in the IGM
at z > 2 only a few quasars with a range of spectral indices
(Telfer et al. 2002; Scott et al. 2004) contribute to the UV back-
ground at hν ≥ 54.4 eV (Bolton et al. 2006).
Already low-resolution He ii spectra obtained with HST in-
dicate a fluctuating radiation field, which has been interpreted
as the onset of He ii reionisation in Strömgren spheres around
hard He ii photoionising sources along or near the line of
sight (Reimers et al. 1997; Heap et al. 2000; Smette et al. 2002).
Jakobsen et al. (2003) found a quasar coinciding with the promi-
nent He ii void at z = 3.05 towards Q 0302−003, thereby pre-
senting the first clear case of a transverse proximity effect. In
Worseck & Wisotzki (2006), hereafter Paper I, we revealed the
transverse proximity effect as a systematic increase in spectral
hardness around all four known foreground quasars along this
line of sight. This suggests that a hard radiation field is a sensi-
tive probe of the transverse proximity effect even if there is no
associated void in the H i forest, either because of the weakness
of the effect, or because of large-scale structure.
Along the line of sight towards HE 2347−4342 several He ii
voids have been claimed to be due to nearby unknown AGN
(Smette et al. 2002). Likewise, some forest regions with a de-
tected hard radiation field may correspond to proximity ef-
fect zones of putative foreground quasars (Fechner & Reimers
2007a). Here we report on results from a slitless spectroscopic
quasar survey in the vicinity of HE 2347−4342 and on spectral
shape fluctuations of the UV radiation field probably caused by
foreground quasars towards the sightline of HE 2347−4342. The
paper is structured as follows. Sect. 2 presents the observations
and the supplementary data employed for the paper. Although
we do not detect any transverse proximity effect in the H i for-
est (Sect. 3), the fluctuating UV spectral shape along the line
of sight indicates a hard radiation field in the projected vicinity
of the foreground quasars (Sect. 4). In Sect. 5 we study three
nearby metal line systems which could further constrain the ion-
ising field. We interpret the statistically significant excesses of
hard radiation as being due to the transverse proximity effect
(Sect. 6). We present our conclusions in Sect. 7. Throughout
the paper we adopt a flat cosmological model with Ωm = 0.3,
ΩΛ = 0.7 and H0 = 70 km s
−1 Mpc−1.
2. Observations and data reduction
2.1. Search for QSO candidates near HE 2347−4342
In October 2002 we observed a 25′ × 33′ field centered on
HE 2347−4342 (z = 2.885) with the ESO Wide Field Imager
(WFI, Baade et al. 1999) at the ESO/MPI 2.2 m Telescope (La
Silla) in its slitless spectroscopic mode (Wisotzki et al. 2001) as
part of a survey for faint quasars in the vicinity of established
high-redshift quasars. A short summary of the survey is given in
Paper I; a detailed description will follow in a separate paper.
A semi-automated search for emission line objects among
the slitless spectra of the ∼ 1400 detected objects in the field
resulted in 10 prime quasar candidates.
2.2. Spectroscopic follow-up
Follow-up spectroscopy of these 10 quasar candidates was ob-
tained with the Focal Reducer/Low Dispersion Spectrograph 2
(FORS2, Appenzeller et al. 1998) on ESO VLT UT1/Antu in
Visitor Mode on November 17 and 19, 2004 under variable see-
ing but clear conditions. The spectra were taken either with
the 300V grism or the 600B grism and a 1′′ slit kept at the
parallactic angle, resulting in a spectral resolution of ∼ 10 Å
FWHM and ∼ 4.5 Å FWHM, respectively. No order separa-
tion filter was employed, leading to possible order overlap at
λ > 6600 Å in the spectra taken with the 300V grism. Exposure
times were adjusted to yield S/N ∼ 20 in the quasar con-
tinuum. The spectra were calibrated in wavelength against the
FORS2 He/Ne/Ar/HgCd arc lamps and spectrophotometrically
calibrated against the HST standard stars Feige 110 and GD 108.
Data reduction was performed with standard IRAF tasks us-
ing the optimal extraction algorithm by Horne (1986). Figure 1
shows the spectra of the quasars together with 4 quasars from an-
other survey (Sect. 2.3). Table 1 summarises our spectroscopic
follow-up observations.
2.3. Additional quasars
We checked the ESO Science Archive for additional quasars in
the vicinity of HE 2347−4342 and found several unpublished
quasars from a deeper slitless spectroscopic survey using the
ESO VLT, the results of which (on the field of Q 0302−003) are
described in Jakobsen et al. (2003). We obtained their follow-up
spectra of quasars surrounding HE 2347−4342 from the archive
and publish them here in agreement with P. Jakobsen. In the
course of their survey FORS1 spectra of 10 candidates were
taken with the 300V grism crossed with the GG435 order separa-
tion filter and a 1′′ slit, calibrated against the standards LTT 7987
and GD 50. Seven of their candidates are actually quasars, of
which 3 were also found independently by our survey. The re-
maining 4 quasars are beyond our redshift-dependent magnitude
limit. The FORS1 spectra of the 4 additional quasars are dis-
played in Fig. 1 and listed separately in Table 1. According to
the quasar catalogue by Véron-Cetty & Véron (2006) there are
no other previously known quasars within a radius < 30′ around
HE 2347−4342.
2.4. Redshifts and magnitudes
Redshifts of the 14 quasars were determined by taking every de-
tectable emission line into account. Line peaks were measured
by eye and errors were estimated taking into account the S/N
of the lines, line asymmetries and the presence of absorption
systems. The quasar redshifts were derived by weighting the
measurements of detected lines. Since high-ionisation lines suf-
fer from systematic blueshifts with respect to the systemic red-
shift (Gaskell 1982; Tytler & Fan 1992; McIntosh et al. 1999),
a higher weight was given to low-ionisation lines. Obviously
blueshifted lines were discarded. Redshift errors were estimated
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 3
Fig. 1. VLT/FORS spectra of quasars in the vicinity of HE 2347−4342. The spectra are shown in black together with their 1σ noise
arrays (green lines). The small inserts show the corresponding discovery spectra from our slitless survey in the same units.
from the redshift differences of the remaining lines and their es-
timated errors.
The 14 discovered quasars lie in the broad redshift range
0.720 ≤ z ≤ 3.542. Fig. 2 shows their angular separations with
respect to HE 2347−4342. We find three background quasars
to HE 2347−4342 and we identify a pair of bright quasars at
z ≃ 1.763 separated by 7.′8. Three foreground quasars (labelled
A–C in Table 1 and Fig. 2) are located in the redshift range
to study the transverse proximity effect. Table 2 provides the
redshift measurements for the detected emission lines in their
spectra. The redshift of QSO J23503−4328 was based on Lyα
and C iv. The measurement of the Mg ii is uncertain because of
the decline of the resolving power of the 300V grism towards
the red, but yields a slightly higher redshift than the adopted
one. For QSO J23500−4319 we measured a consistent redshift
from the C iv and the C iii] line. The redshift measurement of
QSO J23495−4338 was difficult due to several metal absorp-
tion line systems of which only two Mg ii systems at z = 0.921
and z = 1.518 could be identified. In particular, Fe ii absorp-
tion from the z = 0.912 system hampered a redshift measure-
4 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Table 1. Quasars observed near the line of sight of HE 2347−4342. The first 10 listed quasars have been found in our survey, the
remaining 4 quasars result from the previously unpublished survey by P. Jakobsen. Quasar magnitudes are B and V magnitudes for
our survey and Jakobsen’s survey, respectively.
Object α (J2000) δ (J2000) z Magnitude Night Grism Exposure Airmass Seeing Abbr.
QSO J23510−4336 23h51m05.s50 −43◦36′57.′′2 0.720 ± 0.002 20.74 ± 0.27 19 Nov 2004 300V 1200 s 1.30 1.′′3
QSO J23507−4319 23h50m44.s97 −43◦19′26.′′0 0.850 ± 0.003 19.90 ± 0.07 17 Nov 2004 600B 360 s 1.28 0.′′7
QSO J23507−4326 23h50m45.s39 −43◦26′37.′′0 1.635 ± 0.003 21.05 ± 0.14 17 Nov 2004 300V 200 s 1.23 1.′′0
QSO J23509−4330 23h50m54.s80 −43◦30′42.′′2 1.762 ± 0.004 18.23 ± 0.03 17 Nov 2004 600B 300 s 1.08 0.′′7
QSO J23502−4334 23h50m16.s18 −43◦34′14.′′7 1.763 ± 0.003 18.95 ± 0.04 17 Nov 2004 300V 60 s 1.18 0.′′7
QSO J23503−4328 23h50m21.s55 −43◦28′43.′′7 2.282 ± 0.003 20.66 ± 0.11 17 Nov 2004 300V 400 s 1.20 0.′′7 A
QSO J23495−4338 23h49m34.s53 −43◦38′08.′′7 2.690 ± 0.006 20.21 ± 0.17 19 Nov 2004 300V 360 s 1.13 1.′′2 C
QSO J23511−4319 23h51m09.s44 −43◦19′41.′′6 3.020 ± 0.004 21.00 ± 0.14 17 Nov 2004 600B 1000 s 1.09 1.′′1
QSO J23514−4339 23h51m25.s54 −43◦39′02.′′9 3.240 ± 0.004 21.57 ± 0.29 17 Nov 2004 300V 1400 s 1.14 1.′′2
QSO J23503−4317 23h50m21.s94 −43◦17′30.′′0 3.542 ± 0.005 21.94 ± 0.62 19 Nov 2004 300V 1800 s 1.23 1.′′2
600B 1800 s 1.33 1.′′2
QSO J23515−4324 23h51m33.s05 −43◦24′45.′′2 1.278 ± 0.002 20.82 ± 0.14 06 Oct 2002 300V 900 s 1.24 0.′′7
QSO J23512−4332 23h51m15.s18 −43◦32′34.′′3 1.369 ± 0.001 21.52 ± 0.25 06 Oct 2002 300V 900 s 1.18 0.′′7
QSO J23508−4335 23h50m52.s91 −43◦35′06.′′8 1.778 ± 0.002 22.01 ± 0.32 06 Oct 2002 300V 900 s 1.11 0.′′9
QSO J23500−4319 23h50m00.s28 −43◦19′46.′′1 2.302 ± 0.002 22.61 ± 0.83 06 Oct 2002 300V 900 s 2.37 0.′′8 B
Table 2. Detected emission lines and redshifts of QSOs A–C.
Object Emission line λobs [Å] z
QSO J23503−4328 Lyα 3989 ± 4 2.281 ± 0.003
N v 4070 ± 8 2.282 ± 0.006
Si iv+O iv] 4585 ± 8 2.276 ± 0.006
C iv 5082 ± 4 2.281 ± 0.003
C iii] 6253 ± 7 2.276 ± 0.004
Mg ii 9196 ± 12 2.286 ± 0.004
2.282 ± 0.003
QSO J23500−4319 Si iv+O iv] 4613 ± 6 2.296 ± 0.004
C iv 5115 ± 3 2.302 ± 0.002
C iii] 6305 ± 2 2.303 ± 0.001
2.302 ± 0.002
QSO J23495−4338 Lyα 4513 ± 10 2.712 ± 0.008
O i+Si ii 4823 ± 10 2.694 ± 0.008
C ii 4930 ± 10 2.692 ± 0.007
Si iv+O iv] 5135 ± 15 2.669 ± 0.011
C iv 5691 ± 10 2.674 ± 0.006
C iii] 7028 ± 10 2.682 ± 0.005
2.690 ± 0.006
ment of the Lyα line. The C iv and the C iii] lines show unidenti-
fied absorption features. Thus, the redshift of QSO J23495−4338
is heavily weighted towards the very noisy low-ionisation lines
O i+Si ii and C ii. However, redshift uncertainties of the fore-
ground quasars do not significantly affect our results.
Apparent magnitudes were derived from target aquisition
images photometrically calibrated against the standard star fields
PG 2213−006 or Mark A (Landolt 1992). Unfortunately the
aquisition exposures of the faintest quasars were too short to de-
termine their magnitudes accurately. Magnitudes derived from
integration of the spectra are consistent with the photometric
ones after correcting for slit losses.
We note that QSO J23507−4326 is variable. This quasar
has been detected in both slitless surveys and had V ≃ 20.3
in October 2001, V ≃ 20.7 in October 2002 and V ≃ 21.0
in November 2004. We were able to discover this quasar
in its bright phase while missing the slightly fainter quasar
QSO J23515−4324 detected only in the survey by P. Jakobsen.
2.5. Optical spectra of HE 2347−4342
From the ESO Science Archive we retrieved the optical spectra
of HE 2347−4342 taken with UVES at VLT UT2/Kueyen in the
Large Programme “The Cosmic Evolution of the Intergalactic
Medium” (Bergeron et al. 2004). Data reduction was performed
Fig. 2. Distribution of separation angles ϑ vs. redshift z of the
quasars from Table 1 with respect to HE 2347−4342. Symbol
size indicates apparent optical magnitude.
using the UVES pipeline provided by ESO (Ballester et al.
2000). The vacuum-barycentric corrected co-added spectra yield
a S/N ∼ 100 in the Lyα forest at R ∼ 45000. The spectrum
was normalised in the covered wavelength range 3000 <∼ λ <∼
10000 Å using a cubic spline interpolation algorithm.
2.6. Far-UV spectra of HE 2347−4342
HE 2347−4342 is one of the two high-redshift quasars observed
successfully in the He ii Lyα forest below 303.7822 Å rest frame
wavelength with the Far Ultraviolet Spectroscopic Explorer
(FUSE) at a resolution of R ∼ 20000, although at a
S/N <∼ 5 (Kriss et al. 2001; Zheng et al. 2004). G. Kriss
and W. Zheng kindly provided the reduced FUSE spec-
trum of HE 2347−4342 described in Zheng et al. (2004). We
adopted their flux normalisation with a power law fλ =
3.3 × 10−15
λ/1000Å
)−2.4
erg cm−2 s−1 Å−1 reddened by the
Cardelli et al. (1989) extinction curve assuming E(B − V) =
0.014 (Schlegel et al. 1998).
3. The Lyα forest near the foreground quasars
Aiming to detect the transverse proximity effect as an underden-
sity (’void’) in the Lyα forest towards HE 2347−4342 we ex-
amined the forest regions in the projected vicinity of the three
foreground quasars labelled A–C in Table 1. The H i forest of
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 5
Fig. 3. The Lyα forest of HE 2347−4342 in the vicinity of the foreground quasars A–C from Table 1. The upper panels show the
normalised optical spectrum of HE 2347−4342 including Lyβ and metal lines (red) and the H i Lyα transmission obtained from the
line list by T.-S. Kim (black). The binned blue line shows the mean H i Lyα transmission in ∆z = 0.005 bins towards HE 2347−4342,
whereas the dashed green line indicates the expected mean transmission 〈T 〉exp. The lower panels display the corresponding He ii
transmission from the FUSE spectrum. [See the online edition of the Journal for a colour version of this figure.]
HE 2347−4342 has been analysed in several studies, e.g. by
Zheng et al. (2004) and Fechner & Reimers (2007a), hereafter
called Z04 and FR07, respectively. Since the line list from FR07
is limited to z > 2.29, T.-S. Kim (priv. comm.) kindly provided
an independent line list including the lower redshift Lyα forest
(z > 1.79). Both line lists agree very well in their overlapping
redshift range 2.29 < z < 2.89.
Figure 3 displays the H i and the He ii forest regions near
the foreground quasars A–C. The H i Lyα forest is contaminated
by metals. In particular at z < 2.332 there is severe contam-
ination due to the O vi absorption of the associated system of
HE 2347−4342 (Fechner et al. 2004). Because the strong O vi
absorption overlaps with the projected positions of QSO A and
QSO B it is very difficult to obtain a well-determined H i line
sample in this region. Furthermore, there is Lyβ absorption of
H i and He ii at z < 2.294. We also overplot in Fig. 3 the mean H i
Lyα transmission in ∆z = 0.005 bins obtained from T.-S. Kim’s
line list and the generally expected mean transmission over sev-
eral lines of sight 〈T 〉exp = e−τ
eff with τ
eff = 0.0032 (1 + z)
(Kim et al. 2002).
We do not detect a significant void near the three foreground
quasars, neither in the H i forest nor in the He ii forest. In the
vicinity of QSO A and QSO B, even a careful decontamina-
tion of the optical spectrum does not reveal a significant H i
underdensity. Instead, the transmission is fluctuating around the
mean. Due to the poor quality of the FUSE data in this region
(S/N <∼ 2) and the He ii Lyβ absorption from higher redshifts, a
simple search for He ii voids near QSO A and QSO B is impos-
sible. In the vicinity of QSO C the H i Lyα absorption is slightly
higher than on average. There is a small void at z ≃ 2.702 that
can be identified in the forests of both species. The probability of
chance occurrence of such small underdensities is high, so link-
ing this void to QSO C seems unjustified. However, note that the
He ii absorption in the vicinity of QSO C (z ∼ 2.69) is lower
than at z ∼ 2.71 in spite of the same H i absorption. This points
to fluctuations in the spectral shape of the ionising radiation near
the quasar (Sect. 4.3).
Given the luminosities and distances of our foreground
quasars to the sightline of HE 2347−4342, could we expect to
detect the transverse proximity effect as voids in the H i forest?
As in Paper I, we modelled the impact of the foreground quasars
on the line of sight towards HE 2347−4342 with the parameter
ω (z) =
fνLL , j
4πJν (z)
1 + z′j
)−α j+1
1 + z j
αJν + 3
α j + 3
z j, 0
z j, z
which is the ratio between the summed photoionisation rates of
n quasars at redshifts z j with rest frame Lyman limit fluxes fνLL , j,
penetrating the absorber at redshift z and the overall UV back-
ground with Lyman limit intensity Jν. dL(z j, 0) is the luminosity
distance of QSO j, and dL(z j, z) is its luminosity distance as seen
at the absorber; the redshift of the quasar as seen at the absorber
is z′j (Liske 2000). A value ω ≫ 1 predicts a highly significant
proximity effect.
We assumed a constant UV background at 1 ryd of Jν =
7×10−22 erg cm−2 s−1 Hz−1 sr−1 (Scott et al. 2000) with a power-
law shape Jν ∝ ν
−αJν and αJν = 1.8. The quasar Lyman limit
fluxes were estimated from the spectra by fitting a power law
fν ∝ ν
−α to the quasar continuum redward of the Lyα emis-
sion line, excluding the emission lines. The spectra were scaled
to yield the measured photometric magnitudes. Table 3 lists the
resulting spectral indices, the H i Lyman limit fluxes, and the
transverse distances.
The combined effects of QSOs A and B result in a peak
ωmax ≃ 0.89, while QSO C yields ωmax ≃ 0.11. So we expect
only a weak signature of the transverse proximity effect that can
be easily diluted by small-scale transmission fluctuations around
〈T 〉exp. Thus, the apparent lack of a transverse proximity effect
in the H i forest is no surprise.
We can also roughly estimate the amplitude of the proximity
effect in the He ii forest. Extrapolating the power laws (QSOs
and background) above 4 ryd at η = 50 (Haardt & Madau 1996,
hereafter HM96) we get ωmax ≃ 20 near QSO A and ωmax ≃ 2
near QSO C. A softer background would result in higher values
of ω, whereas absorption of ionising photons in the He ii forest
would decrease ω. However, due to the arising He ii Lyβ forest
and the low S/N in the FUSE data near QSOs A and B, even
high ω values do not necessarily result in a visible He ii void.
6 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Table 3. Rest frame Lyman limit fluxes of foreground QSOs. A
power law fν ∝ ν
−α is fitted to the QSO continua and fνLL is the
extrapolated H i Lyman limit flux in the QSO rest frame. d⊥(z)
denotes the transverse proper distance to the line of sight towards
HE 2347−4342.
QSO Abbr. z α fνLL [µJy] d⊥(z) [Mpc]
QSO J23503−4328 A 2.282 0.21 16 1.76
QSO J23500−4319 B 2.302 0.84 1 4.33
QSO J23495−4338 C 2.690 0.24 29 7.75
In the direct vicinity of QSO C the He ii data is not saturated,
but shows no clear void structure either. We will show in the
following sections that the spectral shape of the radiation field is
a more sensitive indicator of the transverse proximity effect than
the detection of voids in the forests.
4. The fluctuating shape of the UV radiation field
towards HE 2347−4342
4.1. Diagnostics
If both hydrogen and helium are highly ionised in the IGM
with roughly primordial abundances, the column density ra-
tio η = NHe ii/NH i indicates the softness of the UV radiation
field impinging on the absorbers. Theoretically, η can be de-
rived numerically via photoionisation models of the IGM with
an adopted population of ionising sources. At the redshifts of
interest, 50 <∼ η <∼ 100 is predicted for a UV background gen-
erated by quasars (HM96; Fardal et al. 1998), whereas higher
values indicate a contribution of star-forming galaxies (e.g.
Haardt & Madau 2001, hereafter HM01).
The He ii forest has been resolved with FUSE towards
HE 2347−4342 and HS 1700+6416, allowing a direct estimation
of η by fitting the absorption lines (Kriss et al. 2001; Zheng et al.
2004; Fechner et al. 2006, FR07). Due to the low S/N and the
strong line blending in the He ii forest the He ii lines have to be
fitted with absorber redshifts and Doppler parameters fixed from
the fitting of the H i data of much higher quality. Generally, pure
non-thermal line broadening (bHe ii = bH i) is assumed (how-
ever, see FR07 and Sect. 6 below). The He ii forest towards
HE 2347−4342 was fitted independently by Z04 and FR07. In
the following, we rely on the line fitting results from FR07,
which at any rate are consistent with those obtained by Z04 in
the redshift ranges near the quasars.
All current studies indicate that η is strongly fluctuating on
very small scales in the range 1 <∼ η <∼ 1000. The median column
density ratio towards HE 2347−4342 is η ≃ 62 (Z04), whereas
Fechner et al. (2006) find a higher value of η ≃ 85 towards
HS 1700+6416. Both studies find evidence for an evolution of
η towards smaller values at lower redshifts. However, only part
of the scatter in η is due to redshift evolution and statistical er-
rors, so the spectral shape of the UV radiation field has to fluc-
tuate (FR07). Although the analyses of both available lines of
sight give consistent results, cosmic variance may bias the de-
rived median η and its evolution. This is of particular interest for
our study, since we want to reveal local excesses of low η near
the quasars with respect to the median (Sect. 4.3). Clearly, more
lines of sight with He ii absorption would be required to yield
tighter constraints on the redshift evolution of η.
The detailed results of visual line fitting may be subjective
and may depend on the used fitting software. In particular, am-
biguities in the decomposition of blended H i lines can affect the
derived η values (Fechner & Reimers 2007b). Therefore we also
analyse the UV spectral shape variations using the ratio of the
effective optical depths
τeff,He ii
τeff,H i
. (2)
As introduced in Paper I, this parameter is a resolution-
independent estimator of the spectral shape of the UV radiation
field with small (high) R values indicating hard (soft) radiation
on a certain redshift scale ∆z. Shull et al. (2004) followed a sim-
ilar approach by taking η ≃ 4τHe ii/τH i for a restricted τ range
on scales of ∆z = 1.6× 10−4 and ∆z = 6.6× 10−4. However, this
scaling relation between τ and η is only valid at the centre of
an absorption line (Miralda-Escudé 1993). The column density
ratio is defined per absorption line and not as a continuous quan-
tity, whereas R can be defined on any scale. While R and η are
correlated (see below), there is no simple conversion between
R and η and the correlation will depend on the adopted redshift
scale of R.
4.2. Fluctuations in R and η along the line of sight
We obtained R(z) by binning both normalised Lyα forest spec-
tra of H i and He ii into aligned redshift bins of ∆z = 0.005
in the range 2.3325 < z < 2.8975 and computed R =
ln 〈THe ii〉/ ln 〈TH i〉 with the mean transmission 〈THe ii〉 and
〈TH i〉. The choice of the redshift binning scale was motivated by
the typical scale of η fluctuations 0.001 <∼ ∆z <∼ 0.03 (Kriss et al.
2001, FR07). We adopted the binning procedure by Telfer et al.
(2002) in order to deal with original flux bins that only partly
overlap with the new bins. The errors were computed accord-
ingly. Due to the high absorption and the low S/N of the He ii
data we occasionally encountered unphysical values 〈THe ii〉 ≤ 0.
These were replaced by their errors, yielding lower limits on R.
We mostly neglected the usually small metal contamination in
the computation of 〈TH i〉 in the Lyα forest because the errors in
R are dominated by the low S/N and the more uncertain contin-
uum level of the He ii spectrum. The FUSE data in the redshift
bins at z = 2.375, 2.380, 2.730, 2.735, 2.845 and 2.850 are con-
taminated by galactic H2 absorption, so no R measurement on
the full scale of ∆z = 0.005 can be performed there.
At 2.29 <∼ zLyα <∼ 2.33 the H i Lyα forest is severely con-
taminated by O vi from the associated system of HE 2347−4342
(Fechner et al. 2004). Furthermore, the Lyβ forest of both
species emerges at zLyα < 2.294. Because this excess absorp-
tion would bias the direct estimation of R in the spectra, we tried
to decontaminate the forests at z < 2.332. 〈TH i〉 was computed
from the H i Lyα forest reconstructed from the line list by T.-
S. Kim (Sect. 3). The corresponding 〈THe ii〉 was obtained after
dividing the FUSE data by the simulated Lyβ absorption of the
lines at higher redshift. Since the decontamination depends on
the validity of the He ii line parameters as well as on the com-
pleteness of the H i line list in the complex region contaminated
by O vi, the derived R values at z < 2.332 have to be regarded as
rough estimates.
The resulting R(z) is shown in the upper panel of Fig. 4. The
optical depth ratio strongly fluctuates around its median value
R ≃ 4.8 obtained for uncontaminated redshift bins, indicating
spectral fluctuations in the UV radiation field. We also show in
Fig. 4 the median η(z) on the same redshift bins based on the
line fitting results in FR07. Also the median η strongly fluctuates
with a slight trend of an increase with redshift (Z04). Clearly,
the data is inconsistent with a spatially uniform UV background,
but the median η ≃ 70 of the line sample is consistent with
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 7
Fig. 4. The fluctuating spectral shape of the UV
background towards HE 2347−4342. The up-
per panel shows the ratio of effective optical
depths R vs. redshift z in ∆z = 0.005 bins.
Data points at z < 2.332 (crosses) have been
decontaminated from O vi and Lyβ absorp-
tion (see text). Foreground quasars are marked
with letters and vertical dotted lines as well
as HE 2347−4342 (star symbol). The green
dashed line indicates the median R ≃ 4.8 ob-
tained at z > 2.332 in uncontaminated bins. The
lower panel shows the median η from FR07 in
the same redshift bins. The red dashed line in-
dicates the median η ≃ 70 of the line sample.
quasar-dominated models of the UV background. A comparison
of R(z) and η(z) reveals that both quantities are correlated. The
Spearman rank order correlation coefficient is rS = 0.67 with a
probability of no correlation PS = 6 × 10
There is a scatter in the relation between R and η, which is
due to noise in the He ii data and due to the fact that R is a spec-
tral softness indicator that is smoothed in redshift. Therefore, in
addition to the UV spectral shape, R will depend on the den-
sity fluctuations of the Lyα forest on the adopted scale. In or-
der to estimate the scatter in R due to these density fluctua-
tions, we simulated H i and He ii Lyα forest spectra. We gen-
erated 100 H i forests with the same overall redshift evolution of
eff,H i = 0.0032 (1 + z)
3.37 (Kim et al. 2002) based on the empir-
ical line distribution functions in redshift z, column density NH i
and Doppler parameter bH i (e.g. Kim et al. 2001). We modelled
each forest as a composition of lines with Voigt profiles using the
approximation by Tepper-Garcı́a (2006). The spectral resolution
(R ∼ 42000) and quality (S/N ∼ 100) closely matches the opti-
cal data of HE 2347−4342. The corresponding He ii forests were
generated at FUSE resolution with a S/N = 4 for four constant
values of η = 10, 20, 50 and 100. We assumed pure non-thermal
broadening of the lines. Then we computed R at 2 ≤ z ≤ 3 on our
adopted scale ∆z = 0.005, yielding 20000 R measurements for
each considered η. For convenience we took out the general red-
shift dependence of τeff,H i via dividing by the expected effective
optical depth τ
eff,H i, so
τeff,H i
eff,H i
is a measure of H i overdensity (D > 1) or underdensity (D < 1).
In Fig. 5 we show the relation R(D) obtained from the Monte
Carlo simulations and compare it to the distribution observed
towards HE 2347−4342. The simulated R(D) can be fitted rea-
sonably with a 3rd order polynomial in logarithmic space, yield-
ing a general decrease of R with D for every η. The root-mean-
square scatter increases from 0.13 dex for η = 10 to 0.18 dex for
η = 100. At D >∼ 3 the R(D) distribution flattens due to saturation
of high-column density absorbers on the flat part of the curve of
growth. The flattening causes substantial overlap between the
simulated R distributions at D >∼ 5, making R increasingly in-
sensitive to the underlying η. However, at D <∼ 3 hard radiation
Fig. 5. Dependence of R on D = τeff,H i/τ
eff,H i for different sim-
ulated values of η. The black lines indicate the polynomial fits
to the simulated distributions in logarithmic space. Red filled
circles represent the measured R(D) towards HE 2347−4342 in
uncontaminated bins at z > 2.332. The horizontal dotted line de-
notes R = 2. [See the online edition of the Journal for a colour
version of this figure.]
and soft radiation can be reasonably well distinguished. We also
overplot the measured R(D) towards HE 2347−4342 in Fig. 5.
The observed distribution is inconsistent with a constant η, but
the majority of values falls into the modelled η range. While
many high R values indicate η > 100, values with R <∼ 2 corre-
spond to η <∼ 20 at D <∼ 3. Thus, the very low R values always in-
dicate a hard radiation field up to moderate overdensities. As we
will see in the next section, the saturation effect probably does
not play a role in relating a hard radiation field to the nearby
quasars.
4.3. The UV radiation field near the quasars
We now investigate in greater detail the spectral shape of the UV
radiation field near the four quasars with available data on R and
η: the background quasar HE 2347−4342 and the foreground
QSOs A–C. Due to the small number of comparison values de-
rived from only two lines of sight, we will adopt η = 100 as
a characteristic value for the overall UV background at z > 2.6
8 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
(HE 2347−4342, QSO C) and a value of η = 50 at z ∼ 2.3 (QSOs
A and B). The former value is close to the median η = 102
obtained by Fechner et al. (2006) at 2.58 < z < 2.75 towards
HS 1700+6416, whereas the latter η value accounts for the prob-
able evolution of η with redshift. Furthermore, we will compare
the η values in the vicinity of the quasars to models of the UV
background.
4.3.1. HE 2347−4342
A close inspection of Fig. 4 reveals a strongly fluctuating radi-
ation field near HE 2347−4342 with some very small, but also
high R values. Also the column density ratio shows large fluc-
tuations (1 <∼ η <∼ 1000) with six η <∼ 10 absorbers out of the
20 absorbers at z > 2.86. These strong variations of the spec-
tral shape are likely due to radiative transfer effects in the asso-
ciated absorption system causing an apparent lack of the prox-
imity effect of HE 2347−4342 (Reimers et al. 1997). The high
He ii column densities of the associated system may soften the
quasar radiation with increasing distance and Fig. 4 supports this
interpretation. Due to the probable strong softening of the hard
quasar radiation on small scales, the relative spectral hardness
near HE 2347−4342 is only revealed by individual low η val-
ues instead of robust median values. However, also the highly
ionised metal species of the associated system (Fechner et al.
2004) favour the presence of hard QSO radiation. Thus we
conclude that despite the lack of a radiation-induced void near
HE 2347−4342, its impact onto the IGM can be detected via the
relative spectral hardness of the UV radiation field. The three
R < 2 values near HE 2347−4342 have D < 3, so they are prob-
ably not affected by saturation.
4.3.2. QSOs A and B
If our decontamination of the Lyα forests near the two z ∼ 2.3
QSOs A and B is correct, R should reflect UV spectral shape
variations also in that region. Indeed, the redshift bin at z =
2.280 next to QSO A (z = 2.282) is a local R minimum with
R ≃ 1.5. At z = 2.270 we find R ≃ 0.8. At the redshift of QSO B
(z = 2.302) the radiation field is quite soft, but we note a low
R ∼ 1 at z = 2.310. We obtain D < 3 for the four R <∼ 2 values
near QSO A and QSO B, so saturation is not relevant, and the
low R values correspond to low η values.
The measured η values in this redshift region are presented
in Fig. 6. The error bars are only indicative, since blended line
components are not independent and the He ii column densities
are derived with constraints from the H i forest. Lower limits on η
result from features detected in He ii but not in H i. Due to ambi-
guities in the line profile decomposition at the H i detection limit
and the present low quality of the He ii data it is hard to judge the
reality of most of these added components (Fechner & Reimers
2007b). Nevertheless, since η for adjacent lines may be not inde-
pendent due to line blending, we must include the lower limits
in the analysis. At z < 2.294 the fitting of He ii lines becomes
unreliable due to the arising Lyβ forest. Therefore, no direct es-
timates of η can be obtained in the immediate vicinity of QSO A.
Furthermore, the H i line sample may be incomplete or the line
parameters may be not well constrained due to blending with the
O vi of the associated system of HE 2347−4342.
Considering these caveats, the median η ≃ 19 obtained for
the values at z < 2.332 shown in Fig. 6 is only an estimate.
Nevertheless, this is much lower than the typical values η ∼ 50
found at z ∼ 2.3 towards HS 1700+6416 (Fechner et al. 2006).
Fig. 6. Column density ratio η vs. redshift z in the vicinity of
QSO A and QSO B. The long (short) dashed line indicates the
median η ≃ 19 in this redshift range (η = 50 for a UV back-
ground generated by quasars). At z < 2.294 the He ii Lyβ forest
sets in.
Moreover, it is also lower than at slightly higher redshifts to-
wards HE 2347−4342. For instance, the median η increases to
η = 79 in the redshift range 2.35 ≤ z ≤ 2.40. This is inconsis-
tent with the smooth redshift evolution of η on large scales in-
ferred by Z04 and Fechner et al. (2006) for both available sight-
lines. Thus, we infer an excess of hard radiation in the vicinity of
QSO A and QSO B. The most extreme η values are located in the
projected vicinity of QSO B, with 6 lines reaching η < 1. If es-
timated correctly, these low η values require local hard sources
and cannot be generated by the diffuse UV background. Both
foreground quasars could be responsible for the hard radiation
field because of similar light travel times to the probably affected
absorbers (tA ≃ 2tB).
4.3.3. QSO C
Since metal contamination of the H i forest is small in the pro-
jected vicinity of QSO C (Fig. 3), the UV spectral shape is bet-
ter constrained here than near QSO A and QSO B. From Fig. 4
we note a local R minimum (R ≃ 1.3) that exactly coincides
with the redshift of QSO C (z = 2.690). At higher redshifts
R rises, possibly indicating a softer ionising field. However, at
2.63 <∼ z <∼ 2.695 the optical depth ratio is continuously below
the median with R < 2 in five redshift bins. Due to the H i over-
densities near QSO C, all R < 2 values have D > 1, but only
the bin at z = 2.635 has D ≃ 4, so the remaining ones may still
indicate low column density ratios η.
Figure 7 displays the η values from FR07 in the redshift
range 2.63 < z < 2.73 in the projected vicinity of QSO C.
For comparison, we also indicate η = 100 that is consistent
with the median η = 102 towards HS 1700+6416 in this red-
shift range (Fechner et al. 2006). While the data generally shows
strong fluctuations around the median over the whole covered
redshift range (Z04; Fechner et al. 2006), there is an apparent
excess of small η values near QSO C indicating a predominantly
hard radiation field. From the data, a median η ≃ 46 is obtained
at 2.63 < z < 2.73 including the lower limits on η. The median η
near QSO C is lower than the median η towards HS 1700+6416
by a factor of two and also slightly lower than the η obtained for
spatially uniform UV backgrounds generated by quasars. The
relative agreement of the median η near QSO C and hard ver-
sions of quasar UV background models may result from the soft-
ening of the quasar radiation by the IGM at the large proper dis-
tances d >∼ 7.75 Mpc considered here (Table 3). This will be
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 9
Fig. 7. Column density ratio η vs. redshift z
in the vicinity of QSO C. The short dashed
line denotes η = 100 that is consistent with
the median η ≃ 102 obtained in the range
2.58 < z < 2.75 towards HS 1700+6416
(Fechner et al. 2006). The long dashed line in-
dicates the median η ≃ 46 obtained for the
shown η values (2.63 < z < 2.73).
further explored in Sect. 6.2. The larger contrast between the
median η near QSO C and the median η towards HS 1700+6416
yields stronger evidence for a local hardening of the UV radia-
tion near QSO C. However, this comparison value derived from
the single additional line of sight tracing this redshift range may
be biased itself.
Near QSO C the column density ratio still fluctuates and is
not homogeneously low as naively expected. We also note an
apparent offset of the low η region near QSO C towards lower
redshift due to fewer absorbers with low η at z > 2.69. While
some of the fluctuations can be explained by uncertainties to
recover η reliably from the present data, the very low η ≤ 10
values (≃ 24% of the data in Fig. 7) are likely intrinsically low.
These η values are in conflict with a homogeneous diffuse UV
background, and are likely affected by a local hard source. In
Sect. 6.4 we will estimate the error budget of η by Monte Carlo
simulations.
If QSO C creates a fluctuation in the spectral shape of the
UV background, the distance between the quasar and the line of
sight implies a light travel time of t = 25 Myr. The low (high)
redshift end of the region shown in Fig. 7 corresponds to a light
travel time of 64 Myr (44 Myr). Since these light travel times are
comparable, we argue that it is important to consider not only
the immediate projected vicinity of QSO C to be affected by the
proximity effect (see also Fig. 10 below).
In summary, both spectral shape indicators R and η indicate
a predominantly hard UV radiation field near all four known
quasars in this field. Many η values in the projected vicinity
of the quasars indicate a harder radiation than expected even
for model UV backgrounds of quasars alone. This points to a
transverse proximity effect detectable via the relative spectral
hardness. However, there are other locations along the line of
sight with an inferred hard radiation field, but without an asso-
ciated quasar, most notably the regions at z ∼ 2.48 and z ∼ 2.53
(Fig. 4). Before discussing these in detail (Sect. 6.3), we search
for additional evidence for hard radiation near the foreground
quasars by analysing nearby metal line systems.
5. Constraints from metal line systems
Observed metal line systems provide an additional tool to con-
strain the spectral shape of the ionising radiation. Since pho-
toionisation modelling depends on several free parameters, ap-
propriate systems should preferably show many different ionic
species. Fechner et al. (2004) analysed the associated metal line
system of HE 2347−4342 and found evidence for a hard quasar
spectral energy distribution at the absorbers with highest veloc-
ities that are probably closest to the quasar. Their large He ii
column densities probably shield the other absorbers which are
better modelled with a softer radiation field. The results by
Table 4. Measured column densities of the metal line system at
z = 2.2753. Several components of H i remain unresolved.
# v [km s−1] H i C iv Nv
1 −106.2 13.25 ± 0.59 12.70 ± 0.27
2 −94.3
13.634 ± 0.005
13.26 ± 0.57 12.40 ± 0.42
3 −45.8 12.76 ± 0.39 12.43 ± 0.14
4 −32.0
13.319 ± 0.015
12.80 ± 0.40 12.88 ± 0.06
5 0.0 13.232 ± 0.019 13.17 ± 0.03 12.94 ± 0.02
6 44.8 12.604 ± 0.020 12.51 ± 0.09 12.21 ± 0.05
7 91.5 13.042 ± 0.007 12.79 ± 0.05 11.98 ± 0.11
Fechner et al. (2004) are consistent with the more direct hard-
ness estimators R and η near HE 2347−4342 (Sect. 4.3.1).
In the spectrum of HE 2347−4342 an intervening metal line
system is detected at z = 2.7119 which is close to the redshift
of QSO C (∆z = 0.022) at a proper distance of d ≃ 10.0 Mpc.
At z = 2.2753 there is another system showing multiple com-
ponents of C iv and N v as well as only weak H i absorption
(NH i < 10
13.7 cm−2). The presence of N v and weak H i features
with associated metal absorption are characteristic of intrinsic
absorption systems exposed to hard radiation. Due to the small
proper distance to QSO A (d ≃ 3.1 Mpc) this system is proba-
bly illuminated by the radiation of the close-by quasar. A third
suitable metal line system at z = 2.3132 is closer to QSO B
(d ≃ 6.1 Mpc) than to QSO A (d ≃ 12.0 Mpc). But since
QSO A is much brighter than QSO B (Table 3), the metal line
system at z = 2.3132 might be affected by both quasars. Due
to their small relative velocities with respect to the quasars of
< 3000 km s−1 the systems are likely associated to the quasars
(e.g. Weymann et al. 1981).
In order to construct CLOUDY models (Ferland et al. 1998,
version 05.07) we assumed a single-phase medium, i.e. all ob-
served ions arise from the same gas phase, as well as a solar
abundance pattern (Asplund et al. 2005) at a constant metallicity
throughout the system. Furthermore, we assumed pure photoion-
isation and neglected a possible contribution of collisional ion-
isation. The absorbers were modelled as distinct, plane-parallel
slabs of constant density testing different ionising spectra.
5.1. The system at z = 2.275 near QSO A
The system at z = 2.2753 shows seven components of C iv
and N v along with unsaturated features of H i (Fig. 8). The ab-
sorber densities are constrained by the C iv/N v ratio. For the
HM01 background scaled to yield log Jb = −21.15 at the H i
Lyman limit (Scott et al. 2000), we derive densities in the range
10−4.38 to 10−3.35 cm−3 at a metallicity of ∼ 0.6 solar. The es-
10 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Fig. 8. Metal line system at z = 2.2753 towards HE 2347−4342.
The displayed profiles assume the rescaled HM01 background.
Zero velocity corresponds to z = 2.2753.
timated absorber sizes are ∼ 5 kpc or even smaller, where the
sizes are computed according to NH = nH l with the absorbing
path length l. With an additional contribution by QSO A, mod-
elled as a power law with α = 0.21 and H i Lyman limit intensity
log Jq = −21.9 at the location of the absorber, we obtain an even
higher metallicity of ∼ 11 times solar. Densities in the range
10−2.95 to 10−2.01 cm−3 are found leading to very small absorbers
of . 10 pc.
Both models lead to unusually high metallicities and very
small absorber sizes. However, Schaye et al. (2007) recently re-
ported on a large population of compact high-metallicity ab-
sorbers. Using the HM01 background they found typical sizes of
∼ 100 pc and densities of 10−3.5 cm−3 for absorbers with nearly
solar or even super-solar metallicities. In fact, this system is part
of the sample by Schaye et al. (2007).
Since the system exhibits only a few different species, it is
impossible to discriminate between the soft and the hard radi-
ation model. In principle, both models lead to a consistent de-
scription of the observed metal lines. The soft HM01 UV back-
ground yields η ∼ 170 for the modelled absorbers, whereas the
model including the hard radiation of QSO A leads to η ∼ 10.
Recall that the He ii forest cannot be used to measure η directly
due to blending with Lyβ features and very low S/N.
5.2. The systems near QSO B and QSO C
The systems at z = 2.3132 and z = 2.7119 are located near
QSO B and QSO C, respectively. Only few ions are observed and
some of them may even be blended. Therefore, no significant
conclusions based on CLOUDY models can be drawn. Using the
column density estimates we find that the system at z = 2.7119
close to QSO C exhibiting C iv and O vi can be described con-
sistently with a HM01+QSO background. Models assuming a
quasar flux of log Jq & −22.5 seen by the absorber yield η . 40,
consistent with the direct measurements from the He ii forest.
However, the metal transitions alone do not provide strong con-
straints.
The system at z = 2.3132 shows C iv in six components
along with Si iv and Si iii. The Lyman series of this system suf-
fers from severe blending preventing a reliable estimation of the
H i column density. Therefore metallicities and absorber sizes
cannot be estimated. Adopting our column density estimates we
infer that this system can be reasonably modelled with or with-
out a specific quasar contribution.
6. Discussion
6.1. The transverse proximity effect in spectral hardness
Fourteen quasars have been found in the vicinity of
HE 2347−4342 of which three are located in the usable part of
the H i Lyα forest towards HE 2347−4342. No H i underdensity
is detected near these foreground quasars even when correcting
for contamination by the O vi absorption from the associated
system of HE 2347−4342. An estimate of the predicted effect
confirms that even if existing, the classical proximity effect is
probably too weak to be detected on this line of sight due to the
high UV background at 1 ryd and small-scale variance in the H i
transmission (Sect. 3).
However, the analysis of the spectral shape of the UV radia-
tion field near the foreground quasars yields a markedly different
result. The spectral shape is fluctuating, but it is predominantly
hard near HE 2347−4342 and the known foreground quasars.
Close to QSO C, both estimators R and η are consistent with a
significantly harder radiation field than on average. There is a
sharp R minimum located precisely at the redshift of the quasar,
but embedded in a broader region of low R values statistically
consistent with a hard radiation field of η <∼ 10 (Fig. 4). The
column density ratio η is also lower than on average and indi-
cates a harder radiation field than obtained for quasar-dominated
models of the UV background (Fig. 7). Because of line blend-
ing, only one of the three metal line systems detected near the
foreground quasars can be used to estimate the shape of the
ionising field. The metal line system at z = 2.275 can be de-
scribed reasonably by the HM01 background with or without a
local ionising component by QSO A. The He ii forest does not
provide independent constraints for this absorber. Line blending
prevents an unambiguous detection of O vi at z = 2.712, leaving
the shape of the ionising field poorly constrained without tak-
ing into account the He ii forest. Thus, the systems show highly
ionised metal species, but our attempts to identify a local quasar
radiation component towards them remain inconclusive.
The most probable sources for the hard radiation field at
z ∼ 2.30 and z ∼ 2.69 towards HE 2347−4342 are the nearby
foreground quasars. In particular, the absorbers with η <∼ 10 have
to be located in the vicinity of an AGN, since the filtering of
quasar radiation over large distances results in η >∼ 50. Also star-
forming galaxies close to the line of sight cannot yield the low
η values, since they are unable to produce significant numbers
of photons at hν > 54.4 eV (Leitherer et al. 1999; Smith et al.
2002; Schaerer 2003). We conclude that there is evidence for a
transverse proximity effect of QSO C detectable via the relative
spectral hardness. There are also indications that QSO A and
QSO B show the same effect, although contamination adds un-
certainty to the spectral shape variations in their projected vicin-
Given these incidences of a hard radiation field near the
quasars, how do these results relate to those of Paper I, in which
we investigated the line of sight towards Q 0302−003? Both
lines of sight show He ii absorption and on both lines of sight
we find evidence for a predominantly hard radiation field near
the quasars in the background and the foreground. However, the
decrease of η near quasars towards Q 0302−003 appears to be
much smoother than towards HE 2347−4342.
There are several reasons for the lack of small-scale spec-
tral shape variations on the line of sight to Q 0302−003. First,
the low-resolution STIS spectrum of Q 0302−003 does not re-
solve the He ii lines and limits the visible scale of fluctuations
to ∆z & 0.006 (Paper I). Much smaller scales can be probed
in the resolved He ii forest of HE 2347−4342, but the fitting of
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 11
blended noisy He ii features may result in artifical η variations.
We will discuss the uncertainties of η below (Sect. 6.4). Second,
Q 0302−003 (z = 3.285) probes higher redshifts, where the He ii
fraction in the IGM is significantly higher and the inferred radi-
ation field is very soft (η ∼ 350 in the Gunn-Peterson trough).
Therefore, the impact of a hard source on the spectral shape is
likely to be more pronounced than at lower redshifts after the
end of He ii reionisation, where η of the UV background gradu-
ally decreases.
6.2. The decrease of η near QSO C
We now investigate quantitatively whether the foreground
quasars are capable of creating a hardness fluctuation on the
sightline towards HE 2347−4342. Unfortunately, since only one
quasar is located in an uncontaminated region of the Lyα forests,
we can present sufficient evidence only for QSO C. For the other
two quasars the data is too sparse and contamination adds uncer-
tainty to the derived η, but in principle QSO A should also show
a strong effect, because its Lyman limit flux penetrating the line
of sight is ∼ 8 times higher than the one of QSO C.
Heap et al. (2000) and Smette et al. (2002) presented simple
models of the decrease of η in front of a quasar taking into ac-
count the absorption of ionising photons by the IGM. In a highly
photoionised IGM with helium mass fraction Y ≃ 0.24 and tem-
perature T ≃ 2 × 104 K we have
4 (1 − Y)
αHe ii
ΓHe ii
≃ 0.42
ΓHe ii
, (4)
where Γi and αi are the photoionisation rate and the radiative
recombination coefficient for species i (Fardal et al. 1998). The
photoionisation rate is Γi = Γi,b + Γi,q with a contribution of the
background and the quasar. The contribution of the quasar to the
photoionisation rate of species i at the jth absorber in front of it
(z j > z j+1) is
Γi,q(z j) =
σi fν,i
h(1 + zq)
1 + zq
1 + z j
)−α+1 ( dL(zq, 0)
dL(zq, z j)
x−α−4exp
Ni,kσix
1 + zk
1 + z j
dx, (5)
with the photoionisation cross section at the Lyman limit σi, the
observed Lyman limit flux fν,i and x = ν/νi with the Lyman limit
frequency νi. Extrapolating the power law continuum flux to the
He ii Lyman limit yields fν,He ii = fν,H i4
−α. With the spectral in-
dex α from Table 3 we obtain ηmin ≃ 2.3 for QSO C.
We simulated η(z) for a set of 1000 Monte Carlo Lyα for-
est spectra generated with the procedure discussed in Sect. 4.2.
We assumed ΓH i,b = 1.75 × 10
−12 s−1 corresponding to the UV
background from Sect. 3 and ηb = 100, which agrees with the
median η towards HS 1700+6416 in the redshift range under
consideration (Fechner et al. 2006). The intervening absorbers
successively block the quasar flux. Especially, every absorber
with log NH i > 15.8 will truncate the quasar flux at hν > 4 ryd
due to a He ii Lyman limit system, leading to an abrupt softening
of the radiation field.
Figure 9 presents the simulated decrease of the median η
approaching QSO C assuming a constant quasar luminosity,
isotropic radiation and an infinite quasar lifetime together with
the upper and lower percentiles of the η distribution obtained
in bins of proper distance ∆d = 2 Mpc. The spread in the
simulated η is due to line-of-sight differences in the absorber
Fig. 9. Column density ratio η vs. proper distance d. The black
line shows the modelled decrease of the median η approaching
QSO C with respect to the ambient ηb = 100 (dashed line).
Green lines mark the upper and lower quartiles of the simu-
lated η distribution in bins of ∆d = 2 Mpc. QSO C is located at
7.75 Mpc. Filled circles show the median η from FR07 in con-
centric rings of ∆d = 2 Mpc around the quasar. Error bars are the
quartile distances to the median. The arrow marks the metal line
system at z = 2.7122 at d = 12.03 Mpc. [See the online edition
of the Journal for a colour version of this figure.]
properties. Since we consider the transverse proximity effect,
we are limited to a proper distance d >∼ 7.75 Mpc (Table 3).
The model agrees reasonably with the median η of the data ob-
tained in concentric rings around the quasar. As expected, in-
dividual η values strongly deviate from this simple model due
to the assumptions of the quasar properties (constant luminosity
and spectral index, isotropic radiation) and due to the unknown
real distribution of absorbers in transverse direction. Recently,
Hennawi & Prochaska (2007) found evidence for excess small-
scale clustering of high-column density systems in transverse
direction to quasar sightlines. In Sect. 4.3.3 we found indica-
tions that the η distribution around QSO C is not symmetric,
which could be due to such anisotropic shielding. However, this
does not imply an intrinsic anisotropy due to the unknown mat-
ter distribution around the quasar and the large uncertainties in
individual η values. Moreover, the line-of-sight variance at a
constant η = 100 is too small to explain the large observed
spread of the η values. Clearly, a self-consistent explanation
of the small-scale η fluctuations would require hydrodynami-
cal simulations of cosmological radiative transfer in order to in-
vestigate possible shielding effects and the statistical distribu-
tion of η values near quasars. While there is recent progress in
case of the UV background (Sokasian et al. 2003; Croft 2004;
Maselli & Ferrara 2005; Bolton et al. 2006), a proper treatment
of three-dimensional radiative transfer in the IGM around a
quasar is still in its infancy. However, our simplified approach
suggests that QSO C is capable of changing the spectral shape
of the UV radiation field by the right order of magnitude to ex-
plain the low η values in its vicinity. Also a variation in the sizes
and the centres of the bins chosen for Fig. 9 does not drastically
change the indicated excess of low η at d <∼ 14 Mpc. Figure 9
also shows very clearly that the sphere of influence for the trans-
verse proximity effect is not limited to the immediate vicinity of
the quasar.
Figure 10 shows a two-dimensional cut in comoving space
near QSO C in the plane spanned by both lines of sight. The
minimum separation of both lines of sight corresponds to a light
travel time of ≃ 25.2 Myr, but the lifetime of QSO C could be
>∼ 40 Myr due to the low η values at larger distances. The fluctua-
12 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Fig. 10. Transverse comoving separation ∆r⊥ vs. line-of-sight
comoving separation ∆r‖ with respect to QSO C. Black (green)
points denote absorbers with η < 100 (η ≥ 100) on the line of
sight towards HE 2347−4342 (curved line) with indicated red-
shifts. The blue arrow points to the metal line system at z =
2.712. The half circles show the distance travelled by light emit-
ted at the indicated times prior to our observation. The minimum
light travel time between the two lines of sight is 25.19 Myr. [See
the online edition of the Journal for a colour version of this fig-
ure.]
tions of the UV spectral shape could be explained by shadowing
of the hard QSO radiation by unknown intervening structures
between both lines of sight.
6.3. Other regions with an inferred hard UV radiation field
In Fig. 4 we note two additional regions at z ∼ 2.48 and z ∼ 2.53
where R is prominently small and where there is no nearby
quasar. Also the fitted η(z) shows very low values apparently
unrelated to a known foreground quasar. Figure 11 displays the
redshift distribution of the η ≤ 10 subsample. The low η values
are clustered with two peaks near the foreground quasars, but
also at z ∼ 2.40, z ∼ 2.48 and z ∼ 2.53. At the first glance the ex-
istence of such regions seems to undermine the relation between
the foreground quasars and a low η in their vicinity. However,
there are several plausible explanations for the remaining low η
values:
1. Unknown quasars: We can conclude from Paper I that the
quasars responsible for hardness fluctuations may be very
faint (like Q 0302-D113 in Paper I) or may reside at large
distances (Q 0301−005 in Paper I). QSO C is located near
the edge of our survey area centred on HE 2347−4342, so
other quasars capable of influencing the UV spectral shape
might be located outside the field of view. Moreover, in or-
der to sample the full quasar luminosity domain (MB ≤ −23)
at z ∼ 2.5 our survey is still too shallow by ∼ 1 mag-
nitude. Therefore, a larger and/or deeper survey around
HE 2347−4342 is desirable.
2. Quasar lifetime: Assuming that quasars are long-lived and
radiate isotropically, every statistically significant low η fluc-
tuation should be due to a nearby quasar. On the other hand,
short-lived quasars will not be correlated with a hard radia-
tion field due to the light travel time from the quasar to the
background line of sight. Quasar lifetimes are poorly con-
strained by observations to 1 <∼ tq <∼ 100 Myr (Martini 2004).
This could be short enough to create relic light echoes from
extinct quasars. The comoving space density of quasars with
Fig. 11. Redshift distribution of low-η absorbers. The open
(hashed) histogram shows all (NH i ≤ 10
14 cm−2) absorbers with
η ≤ 10. Letters and dotted lines mark foreground quasars. The
horizontal dashed line denotes the estimated average number of
absorbers scattered from η = 80 to η ≤ 10 (≃ 0.64 per bin).
MB < −23 at z ≃ 2.5 is ≃ 3.7×10
−6 Mpc−3 (Wolf et al. 2003)
resulting in an average proper separation of ∼ 18.5 Mpc be-
tween two lines of sight. This translates into a light travel
time of ∼ 60 Myr which is of the same order as the quasar
lifetime. So it is quite possible that some quasars have al-
ready turned off, but their hard radiation is still present.
3. Obscured quasars: Anisotropic emission of type I quasars
may lead to redshift offsets between regions with an inferred
hard radiation field and quasars close to the line of sight. In
the extreme case the putative quasar radiates in transverse di-
rection, but is obscured on our line of sight (type II quasar).
The space density of type II AGN at z > 2 is very uncer-
tain due to the challenging optical follow-up that limits the
survey completeness (e.g. Barger et al. 2003; Szokoly et al.
2004; Krumpe et al. 2007). Thus, the fraction of obscured
AGN at high redshift is highly debated (Akylas et al. 2006;
Treister & Urry 2006), but may well equal that of type I
AGN in the luminosity range of interest (Ueda et al. 2003).
We believe that a combination of the above effects is respon-
sible for the loose correlation between low η values and active
quasars. In particular, at z ∼ 2.4 we infer a hard radiation field in
a H i void (FR07), which may have been created by a luminous
quasar that is unlikely to be missed by our survey (V <∼ 22).
In Fig. 11 we also indicate the error level due to inaccurate
line fitting and noise in the He ii data (dashed line) obtained from
simulated data (see below). The low number of η values scattered
from a simulated η = 80 to η ≤ 10 implies that the overdensities
of such small η values are statistically significant. Constraining
the sample to lines with log(NH i) < 14 due to a possible bias
caused by thermal broadening does not remove the significant
clusters of lines with small η.
6.4. Uncertainties in the spectral hardness
Our findings are likely to be affected by random errors and possi-
bly also by systematic errors mostly related to the He ii data. The
poor quality of the FUSE spectrum of HE 2347−4342 (S/N <∼ 5)
contributes to the fluctuations in η even if the η value was con-
stant (Fechner et al. 2006, see also below). The optical depth
ratio R should be less affected by noise, since it is an average
over a broader redshift range ∆z = 0.005. The low S/N and the
generally high absorption at η ≫ 1 provide uncertainty for the
continuum determination in the He ii spectrum. The extrapolated
reddened power law is certainly an approximation.
Although the η fitting results from FR07 are broadly consis-
tent with those of Z04 and agree well in the regions near the fore-
G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342 13
ground quasars, there are substantial differences in some redshift
ranges. This is probably due to the combined effects of low He ii
data quality, different data analysis software and ambiguities in
the deblending of lines. At present, η cannot be reliably deter-
mined at individual absorbers unless metal transitions provide
further constraints.
In order to assess the random scatter in η due to the low S/N
He ii data and ambiguities in the line deblending of both species,
we again used Monte Carlo simulations. Ten H i Lyα forest spec-
tra were generated in the range 2 < z < 3 via the Monte Carlo
procedure outlined in Sect. 4.2. The resolution R ∼ 42000 and
S/N = 100 closely resembles the optical data of HE 2347−4342.
We also generated the corresponding He ii forests at FUSE res-
olution and S/N = 4. We assumed pure non-thermal line broad-
ening and η = 80. Voigt profiles were automatically fitted to
the H i spectra using AUTOVP 1 (Davé et al. 1997). The He ii
spectra were then automatically fitted with redshifts z and non-
thermal Doppler parameters bH i fixed from the fitted H i line
lists, yielding 7565 simulated η values. On average the recov-
ered η is slightly higher than the simulated one (median η ≃ 89)
with a large spread (0 < η <∼ 8000), but only 285 lines have
η ≤ 10. Thus, we estimate a probability P ≃ 3.8% that η is
scattered randomly from η = 80 to η ≤ 10 if the assumption of
non-thermal broadening is correct. Note that this probability is
likely an upper limit due to the fact that only H i Lyα was used
to obtain the line parameters, which results in large error bars
for saturated lines on the flat part of the curve of growth. In the
real data, these errors were avoided by fitting unsaturated higher
orders of the Lyman series wherever possible.
In the line sample by FR07, 94 out of the 526 absorbers have
η ≤ 10, whereas our simulation implies that only ∼ 20 are ex-
pected to be randomly scattered to η ≤ 10 if η was constant.
Thus, the major part of the scatter of η in the data is due to real
fluctuations in the UV spectral shape. The majority of the low
η ≤ 10 values is inconsistent with η ≥ 80, so they indicate a
hard radiation field in spite of the low S/N in the He ii data. Yet,
due to the large intrinsic scatter obtained from the simulations,
individual η values hardly trace the variations of the UV spectral
shape. Local spatial averages should be more reliable (FR07).
Since the transverse proximity effect zones always extend over
some redshift range, this requirement is fulfilled and on average
we reveal a harder radiation field than expected.
Concerning the high tail of the simulated distribution at
η = 80, ∼ 15% of the lines are returned with η >∼ 200. This
may indicate that a fraction of the observed high η values is still
consistent with a substantially harder radiation field, underlining
that single η values poorly constrain the spectral shape.
Possibly, some η values are systematically too low due to
the assumption of non-thermal broadening (bHe ii = bH i) when
fitting the He ii forest. FR07 found that this leads to underes-
timated η values at NH i >∼ 10
13 cm−2 if the lines are in fact
thermally broadened (bHe ii = 0.5bH i). Non-thermal broaden-
ing is caused by turbulent gas motions or the differential Hubble
flow, with the latter affecting in particular the low-column den-
sity forest. Thermal broadening becomes important in collapsed
structures at high column densities. In simulations of the Lyα
forest, non-thermal broadening has been found to dominate
(Zhang et al. 1995, 1998; Hernquist et al. 1996; Weinberg et al.
1997; Bolton et al. 2006; Liu et al. 2006). This has been con-
firmed observationally for the low-column density forest (Z04;
Rauch et al. 2005). On the other hand, eight out of eleven ab-
sorbers with NH i > 10
14 cm−2 in the vicinity of QSO C have
1 http://ursa.as.arizona.edu/˜rad/autovp.tar
η ≤ 10 (Fig. 7). Although the column density ratio of these
absorbers could be underestimated due to an unknown contri-
bution of thermal broadening, the statistical evidence for a hard
radiation field is based on the vast majority of low-column den-
sity lines. The median η obtained in this region does not in-
crease significantly after excluding the suspected lines (∼ 53
vs. ∼ 40). This is still much lower than the median η ∼ 100
towards HS 1700+6416 in this redshift range (Fechner et al.
2006). Therefore, it is unlikely that our results are biased due
to the assumed line broadening.
7. Conclusions
Traditionally, the transverse proximity effect of a quasar has
been claimed to be detectable as a radiation-induced void in the
H i Lyα forest. But due to several systematic effects like quasar
variability, finite quasar lifetime, intrinsic overdensities around
quasars, or anisotropic radiation, most searches yielded negative
results (e.g. Schirber et al. 2004; Croft 2004).
In this paper, we have analysed the fluctuating spectral shape
of the UV background in the projected vicinity of the three
foreground quasars QSO J23503−4328, QSO J23500−4319 and
QSO J23495−4338 (dubbed QSO A, B and C) on the line of
sight towards HE 2347−4342 (z = 2.885). By comparing the
H i absorption and the corresponding He ii absorption, we have
presented evidence for a statistical excess of hard UV radiation
near the foreground quasars. However, due to contamination of
the forests near QSO A (z = 2.282) and QSO B (z = 2.302),
the evidence is strongest for QSO C (z = 2.690). We interpret
these indicators for an excess of hard radiation near the fore-
ground quasars as a manifestation of the transverse proximity
effect. A simple model indicates that the foreground quasars are
capable of generating the observed hard radiation over the ob-
served distances of several Mpc. Furthermore, we tried to model
the ionising radiation field of three metal line systems close to
the foreground quasars. Two of those are strongly affected by
line blending and do not allow for reliable photoionisation mod-
els. The remaining system can be modelled reasonably with or
without a contribution by a local quasar. Future larger samples of
highly ionised unblended metal systems near foreground quasars
may provide evidence for local hardness fluctuations.
In Worseck & Wisotzki (2006) we revealed the transverse
proximity effect as a systematic local hardness fluctuation
around four foreground quasars near Q 0302−003 and pointed
out that the relative UV spectral hardness is a sensitive phys-
ical indicator of the proximity effect over distances of several
Mpc. In this study we are able to confirm this on a second line
of sight. Evidently, small-scale transmission fluctuations in the
H i forest can dilute the small predicted signature of the effect.
However, the hard spectral shape of the UV radiation field still
indicates the transverse proximity effect despite the H i density
fluctuations. Thus, we confirm our previous result that the spec-
tral hardness breaks the density degeneracy that affects the tradi-
tional searches for the proximity effect. Moreover, the predicted
transverse proximity effect of the quasars in the H i forest is weak
due to the high UV background at 1 ryd. Still the UV spectral
shape is able to discriminate local UV sources independent of
the amplitude of the UV background.
Bolton et al. (2006) find that the large UV spectral shape
fluctuations in the IGM are likely due to the small number of
quasars contributing to the He ii ionisation rate at any given
point, whereas the H i ionisation rate is rather homogeneous
due to the probable contribution of star-forming galaxies (e.g.
Bianchi et al. 2001; Sokasian et al. 2003; Shapley et al. 2006).
14 G. Worseck et al.: The transverse proximity effect in spectral hardness towards HE 2347−4342
Our findings confirm the picture that AGN create the hard part of
the intergalactic UV radiation field. If the quasar is active long
enough, its hard radiation field can be observed penetrating a
background line of sight. It is also likely that light echoes from
already extinguished quasars are responsible for some locations
of hard radiation without an associated quasar. The transverse
proximity effect of QSO C implies a minimum quasar lifetime
of ∼ 25 Myr (probably even ∼ 40 Myr), providing additional
constraints to more indirect estimates (e.g. Martini 2004, and
references therein).
However, the UV radiation field near the foreground quasars
is not homogeneously hard as naively expected, but still shows
fluctuations. Apart from substantial measurement uncertainties,
the unknown density structure around the quasar could shield the
ionising radiation in some directions, maybe even preferentially
in transverse direction to the line of sight (Hennawi & Prochaska
2007). Thus, radiative transfer effects may become important
to explain a fluctuating UV spectral shape in the presence of
a nearby quasar. Large-scale simulations of cosmological radia-
tive transfer with discrete ionising sources are required to adress
these issues in detail.
Moreover, the He ii forest has been resolved so far only to-
wards two quasars at a very low S/N <∼ 5. While the low data
quality primarily creates uncertainties in the spectral shape on
small spatial scales, large scales could be affected by cosmic
variance. Thus, the general redshift evolution of the UV spec-
tral shape is not well known and estimates obtained from single
lines of sight may well be biased by local sources.
Acknowledgements. We thank the staff of the ESO observatories La Silla and
Paranal for their professional assistance in obtaining the optical data discussed in
this paper. We are grateful to Peter Jakobsen for agreeing to publish the quasars
from his survey. We thank Gerard Kriss and Wei Zheng for providing the re-
duced FUSE spectrum of HE 2347−4342. Tae-Sun Kim kindly supplied an addi-
tional line list of HE 2347−4342. GW and ADA acknowledge support by a HWP
grant from the state of Brandenburg, Germany. CF is supported by the Deutsche
Forschungsgemeinschaft under RE 353/49-1. We thank the anonymous referee
for helpful comments.
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List of Objects
‘HE 2347−4342’ on page 1
‘HS 1700+6416’ on page 1
‘Q 0302−003’ on page 2
‘QSO J23510−4336’ on page 4
‘QSO J23507−4319’ on page 4
‘QSO J23507−4326’ on page 4
‘QSO J23509−4330’ on page 4
‘QSO J23502−4334’ on page 4
‘QSO J23503−4328’ on page 4
‘QSO J23495−4338’ on page 4
‘QSO J23511−4319’ on page 4
‘QSO J23514−4339’ on page 4
‘QSO J23503−4317’ on page 4
‘QSO J23515−4324’ on page 4
‘QSO J23512−4332’ on page 4
‘QSO J23508−4335’ on page 4
‘QSO J23500−4319’ on page 4
‘Q 0302-D113’ on page 12
‘Q 0301−005’ on page 12
	Introduction
	Observations and data reduction
	Search for QSO candidates near HE 2347-4342
	Spectroscopic follow-up
	Additional quasars
	Redshifts and magnitudes
	Optical spectra of HE 2347-4342
	Far-UV spectra of HE 2347-4342
	The Ly forest near the foreground quasars
	The fluctuating shape of the UV radiation field towards HE 2347-4342
	Diagnostics
	Fluctuations in R and  along the line of sight
	The UV radiation field near the quasars
	HE 2347-4342
	QSOs A and B
	QSO C
	Constraints from metal line systems
	The system at z=2.275 near QSO A
	The systems near QSO B and QSO C
	Discussion
	The transverse proximity effect in spectral hardness
	The decrease of  near QSO C
	Other regions with an inferred hard UV radiation field
	Uncertainties in the spectral hardness
	Conclusions
