Astronomy & Astrophysics manuscript no. ex0069 November 4, 2018
(DOI: will be inserted by hand later)
VLBI observations of nineteen GHz-Peaked-Spectrum radio
sources at 1.6 GHz
X. Liu1, L. Cui1,2, W. -F. Luo1,2, W. -Z. Shi1,2, and H. -G. Song1
1 National Astronomical Observatories/Urumqi Observatory, CAS, 40-5 South Beijing Road, Urumqi 830011, China
2 Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
Received / Accepted
Abstract. Aims and Methods: We present the results of VLBI observations of nineteen GHz-Peaked-Spectrum (GPS) radio
sources at 1.6 GHz. Of them, 15 sources are selected from the Parkes Half Jansky (PHJ) sample (Snellen 2002), 4 others
are from our previous observation list. We aimed at imaging the structure of GPS sources, searching for Compact Symmetric
Objects (CSOs) and studying the absorption for the convex radio spectra of GPS sources.
Results: We obtained total intensity 1.6 GHz VLBI images of 17 sources for the first time. Of them, 80% show mini-double-lobe
radio structure, indicating that they are CSOs or candidates, and their host AGNs could be edge-on to us. This result suggests
that there is a high incidence of mini double-lobe sources (or CSOs) in the PHJ sample. The sources J0323+0534, J1135−0021,
J1352+0232, J2058+0540, J2123−0112 and J2325−0344 with measured redshift, showing double-lobe structure with sizes of
< 1 kpc, are classified as CSOs. Three sources J1057+0012, J1600−0037 and J1753+2750 are considered as core-jet sources
according to their morphologies and flux variability.
Key words. galaxies: nuclei – quasars: general – radio continuum: galaxies
1. Introduction
GHz-Peaked-Spectrum (GPS) radio sources are powerful
(P1.4 GHz ≥ 10
25 W Hz−1), compact (≤ 1 kpc), and have con-
vex radio spectra, and they make up a significant fraction (≈
10%) of the bright radio source sample, see O’Dea (1998) for
a review. In general, the presence of large scale emission asso-
ciated with GPS galaxies is rare, about a few percent in a GPS
sample (Stanghellini et al. 2005). Most GPS sources appear to
be truly compact and isolated.
Their small size is most likely due to their youth (< 104
years) according to a spectral aging analysis (Murgia 2003). A
couple of GPS sources are certainly young radio sources whose
kinematic age from lobe proper motions has been measured
and these sources are also identified as Compact Symmetric
Objects (CSOs). There is compelling evidence in favour of the
youth scenario of GPS sources and CSOs, see e.g. Owsianik
& Conway (1998), Tschager et al. (2000), Polatidis & Conway
(2003), and Orienti et al. (2007). The GPS sources and CSOs
are the key objects to study the early evolution of power-
ful radio-loud AGN. A unification scenario assumes that GPS
sources evolve into Compact Steep Spectrum sources (1-15
kpc), which in turn, evolve into classical extended radio sources
(> 15 kpc), i.e. FR I/II radio sources (Fanti et al. 1995, Snellen
et al. 2000, de Vries et al. 2007).
Send offprint requests to: X. Liu: liux@ms.xjb.ac.cn
GPS galaxies are dominated by lobe/jet emission on both
sides of the central engine, and are thought to be relatively free
of beaming effects. The GPS galaxies show very low polariza-
tion (about less than 0.5% at 5 GHz, Dallacasa 2004, Xiang
et al. 2006). The low integrated polarization could be due to
large Faraday depths around the radio source, which would de-
polarize the radio emission, implying that their host-AGNs are
probably edge-on to us.
Since GPS sources live in the narrow line region of AGN,
it is likely that their low frequency radio emission will be ab-
sorbed due to either synchrotron self-absorption or free-free
absorption, giving rise to a peaked radio spectrum. Therefore,
GPS sources are also suitable for studying radio absorption and
scattering in AGNs.
We have carried out EVN (European VLBI Network) ob-
servations of 19 GPS sources, 15 of them are from the Parkes
Half Jansky (PHJ) sample (Snellen et al. 2002) with declina-
tion > −5◦ and not observed with VLBI before. Four sources
are from our previous observation list which we have observed
with the EVN at 2.3/8.4 GHz and/or 5 GHz (see Xiang et al.
2005, 2006). We aimed at imaging the GPS sources at 1.6 GHz,
in order to confirm whether the GPS sources are double-lobe
sources, and to find CSO candidates. For the sources with ob-
servations at 2.3, 5.0 and 8.4 GHz, the 1.6 GHz images will
further provide information on their source structure and inten-
http://arxiv.org/abs/0704.0310v1
2 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
sity at lower frequency, for further spectral study of the GPS
sources in the future.
2. Observations and data reduction
The observations were carried out on 3 March 2006 at
1.65 GHz using the MK5 recording system with a band-
width of 32 MHz and sample rate of 256 Mbps in dual
circular polarization. The EVN antennae in this experiment
were Effelsberg, Westerbork, Jodrell, Medicina, Noto, Onsala,
Torun, Hartebeesthoek, Urumqi and Shanghai. Snapshot obser-
vations of 19 sources (Table 1) in a total of 24 hours were made.
OQ208 and DA193 were observed as calibrators. The data cor-
relation was completed at JIVE.
The total flux densities of the sources were also measured
at 5 GHz with Urumqi 25m telescope in order to find any flux
variability. The values are listed in Table 2.
The Astronomical Image Processing System (AIPS) has
been used for editing, a-priori calibration, fringe-fitting, self-
calibration, imaging and model fitting of the data.
3. Results and comments on individual sources
We list the basic information of the sources in Table 1, and the
parameters derived from the VLBI images in Table 3. We com-
ment on the results of each source and give a short discussion.
We use S ∝ ν−α to define the spectral index. Optical informa-
tion and redshifts of the GPS sources in the PHJ sample are
given by de Vries et al. (2007), as listed in Table 1.
3.1. J0210+0419 (PKS B0208+040)
The 1.6 GHz VLBI image (Fig. 1) is the first VLBI image of
the source. It shows a double-lobe structure, and is most likely
a CSO. Optical observations did not result in an identification
with a lower limit of mR > 24.1, but it is identified with a mag-
nitude of Ks=18.3 (de Vries et al. 2007).
3.2. J0323+0534 (4C+05.14)
The 1.6 GHz VLBI image (Fig. 2) is the first VLBI image of the
source, and it exhibits a strong diffuse component and a weak
extended component in the south. Both are likely lobe emis-
sion. About 38% total flux density (estimated from Table 1) is
resolved out in the VLBI image, due to the diffuse components.
For its size of 490 pc, the source can be a CSO.
3.3. J0433−0229 (4C−02.17)
The 1.6 GHz VLBI image (Fig. 3) is the first VLBI image of the
source, and the main component is diffuse and extended in the
north-south direction, and a possible weak component in the
south. About 18% total flux density (estimated from Table 1)
is resolved out in the VLBI image. Either a core-jet or a CSO
classification is possible for the source.
3.4. J0913+1454 (PKS B0910+151)
The 1.6 GHz VLBI image (Fig. 4) is the first VLBI image of
the source. It shows double structure and both components are
further resolved. There is probably a hot-spot imbedded in the
bright one. We consider it as a CSO candidate.
3.5. J1057+0012 (PKS B1054+004)
The 1.6 GHz VLBI image (Fig. 5) is the first VLBI image of
the source. There is a bright compact component followed by
a secondary component and a series of possible weak compo-
nents in the east, indicating this is a core-jet source. A flux vari-
ability of (−11.4 ± 3.6)% over 15 years at 5 GHz, as reported
in Table 2, is consistent with the core-jet classification.
3.6. J1109+1043 (PKS B1107+109)
The 1.6 GHz VLBI image (Fig. 6) is the first VLBI image of
the source. It is a double structure, and can be a CSO candi-
date. The total flux density (1270 mJy estimated from Table 1)
is completely restored in the VLBI image (1370 mJy, increased
by 8%). There is also an indication of total flux increasing
(4.9 ± 12.4)% at 5 GHz in Table 2 but with a large error.
3.7. J1135−0021 (4C−00.45)
The 1.6 GHz VLBI image (Fig. 7) is the first VLBI image of
the source. It shows a double-lobe structure, and with a size of
720 pc, we classify the source as a CSO.
3.8. J1203+0414 (PKS B1200+045)
The 1.6 GHz VLBI image (Fig. 8) is the first VLBI image of
the source. The triple structure may consist of a core and two
sided emission, or a one sided core-jet source. The quasar as
newly identified by de Vries et al (2007), is possibly a core-jet
one, but still we keep the source as a CSO candidate.
3.9. J1352+0232 (PKS B1349+027)
The 1.6 GHz VLBI image (Fig. 9) is the first VLBI image of
the source. It shows a double-lobe like structure, for its size of
918 pc we consider it as a CSO.
3.10. J1352+1107 (4C+11.46)
The 1.6 GHz VLBI image (Fig. 10) is the first VLBI image of
the source. It appears to have a compact double structure or a
core-jet alike, and seems diffuse emission around the source.
About 30% total flux density (estimated from Table 1) is re-
solved out in the VLBI image. Either a core-jet or a compact
double classification is possible.
3.11. J1600−0037 (PKS B1557−004)
The 1.6 GHz VLBI image (Fig. 11) is the first VLBI image of
the source, and it has an overall double structure, the eastern
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 3
Table 1. The GPS sources. Columns (1),(2) source names; (3) optical identification (G: galaxy, QSO: quasar, EF: empty field);
(4) optical magnitude; (5) redshift (de Vries et al. 2007, those with * are a photometric estimated by Tinti et al. 2005); (6) linear
scale factor pc/mas [H0 = 71kms
−1Mpc−1 and q0 = 0.5 have been assumed]; (7) maximum angular size from the observation; (8)
maximum linear size; (9) 1.4 GHz flux density from the NVSS; (10) 2.7 GHz flux density from Snellen sample and the NED; (11)
low frequency spectral index; (12) higher frequency spectral index ( computed from columns 9 and 10); (13) turnover frequency;
(14) peak flux density; (15) references for the spectral information, 1 Snellen et al. 2002, 2 de Vries et al. 1997, 3 Stanghellini et
al. 1998, where S ∝ ν−α.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
S ource id mR z pc/mas θ L S 1.4 S 2.7 αl αh νm S m re f
mas pc mJy Jy GHz Jy
J0210+0419 B0208+040 G 18.3Ks 1.5* 6.1 90 948 0.56 0.80 0.4 1.3 1
J0323+0534 4C+05.14 G 19.2 0.1785 2.7 180 490 2793 1.60 0.85 0.4 7.1 1
J0433−0229 4C−02.17 G 19.1 0.530 5.1 80 408 1462 1.04 0.52 0.4 3.0 1
J0913+1454 B0910+151 G 22.9 0.47* 4.9 80 881 0.54 0.75 0.6 1.1 1
J1057+0012 B1054+004 G 22.3 0.65* 5.5 80? 898 0.58 0.67 0.4 1.6 1
J1109+1043 B1107+109 G 22.6 0.55* 5.2 60 1481 0.80 0.94 0.5 2.4 1
J1135−0021 4C−00.45 G 21.9 0.975 6.0 120 720 1268 0.76 0.78 0.4 2.9 1
J1203+0414 B1200+045 QSO 18.8 1.221 6.1 75 458 1146 0.85 0.45 0.4 1.4 1
J1352+0232 B1349+027 G 20.0 0.607 5.4 170 918 1145 0.78 0.58 0.4 2.0 1
J1352+1107 4C+11.46 G 21.0 0.891 5.9 50 295 1538 0.78 1.03 0.4 3.6 1
J1600−0037 B1557−004 G 50 1168 0.54 1.17 1.0 1.2 1
J1648+0242 4C+02.43 G 22.1 0.824 5.8 0.61 0.4 3.4 1
J2058+0540 4C+05.78 G 23.4 1.381 6.1 160 970 1213 0.65 0.95 0.4 3.1 1
J2123−0112 B2121−014 G 23.3 1.158 6.1 80 488 1087 0.64 -0.56 0.75 0.5 1.8 2
J2325−0344 B2322−040 G 23.5 1.509 6.0 75 450 1224 0.91 -0.42 0.75 1.4 1.3 2
J0917+1113 B0914+114 EF 190 800 0.31 -0.1 1.6 0.3 2.3 3
J1753+2750 B1751+278 G 21.7 0.86* 5.9 50 625 0.46 -0.27 0.57 1.4 0.6 2
J1826+2708 B1824+271 G 22.9 45 332 0.23 -0.39 0.75 1.0 0.4 2
J2325+7917 B2323+790 G 19.5V 32 1136 -0.3 0.75 1.4 1.2 2
component has some extension in the west-east direction. A
flux variability of (13± 6.9)% at 5 GHz in Table 2 may suggest
this is a core-jet source.
3.12. J1648+0242 (4C+02.43)
The GPS source is not detected with VLBI. It is an NVSS
double-lobe source, and totally resolved out in the VLBI ob-
servation.
3.13. J2058+0540 (4C+05.78)
The 1.6 GHz VLBI image (Fig. 12) is the first VLBI image of
the source. It shows a double-lobe source, and for the size of
970 pc, we suggest this is a CSO.
3.14. PKS B2121−014
The 1.6 GHz VLBI image (Fig. 13) shows a double-lobe struc-
ture, it is similar to that at 2.3 and 5 GHz (Xiang et al. 2005,
2006), except that a weak jet-like emission ‘B’, which appears
at 2.3 and 5 GHz, is missing, probably due to absorption at the
lower frequency 1.6 GHz. The source is a CSO for the source
size of 488 pc.
3.15. PKS B2322−040
The 1.6 GHz VLBI image (Fig. 14) exposes a central emission
region between the two lobes ‘A’ and ‘B’, which is probably a
core embedded in the central region. The ‘core’ emission is not
detected at higher frequencies (Xiang et al. 2005, 2006), but it
emerges at 1.6 GHz near the peak frequency (1.4 GHz) of the
GPS source. There is a flux increase of (4.0 ± 1.9)% over 15
years at 5 GHz (Table 2), may suggest that the core is currently
active. The source can be a CSO for its size of 450 pc.
3.16. PKS B0914+114
The 1.6 GHz VLBI image (Fig. 15) exhibits a core ‘A’, jet fea-
ture ‘B’ and two lobes ‘C’, ‘E’. The western one ‘E’ emerges at
this frequency. Labiano et al. (2007) have identified an empty
field (> 25 mR) at the FIRST position of the source, and con-
cluded that the previously identified nearby disk galaxy (a red-
shift of 0.178) is not the host of this radio source 0914+114.
For the typical compact symmetric structure, we consider the
source is a CSO.
4 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
Table 2. Source flux and possible variability, columns 2-4 are flux densities at 5.0 GHz (PKS90), 4.85 GHz (Gregory &
Condon 1991, and Griffith et al. 1995) and 4.85 GHz flux measured with the Urumqi 25m telescope on 2007/1/24 (J1648+0242,
J2058+0540, 1824+271 and 2121−014 were not well measured due to source confusion or weak); column 5 is a flux variability
computed from columns 3,4.
S ource S 5.0 S 4.85 S 4.85Ur δS 4.85
mJy mJy mJy %
J0210+0419 300 298±19 302 ± 10 1.3 ±3.1
J0323+0534 830 819±44 868 ± 9 6.0±4.6
J0433−0229 640 640±35 637 ± 14 -0.5±3.3
J0913+1454 300 315±43 297 ± 8 -5.7±10.3
J1057+0012 370 396±23 351 ± 6 -11.4±3.6
J1109+1043 400 408±56 428 ± 8 4.9±12.4
J1135−0021 440 446±25 427 ± 8 -4.3±3.6
J1203+0414 520 640±35 611 ± 7 -4.5 ±4.1
J1352+0232 470 469 ± 7
J1352+1107 410 447±62 418 ± 5 -6.5±11.9
J1600−0037 180 187±14 212 ± 3 13.4±6.9
J1648+0242 260 337±20
J2058+0540 340 356±21
2121−014 320 345±21
2322−040 500 524±29 545 ± 20 4.0 ±1.9
0914+114 140 134±19 140 ± 1 4.5 ±14.1
1751+278 298±39 292 ± 6 -2.0±10.8
1824+271 122±17
2323+790 491 ± 7
OQ208 2421±217 2514 ± 12 3.8±8.8
3.17. 1751+278 (MG2 J175301+2750)
The 1.6 GHz structure (Fig. 16) is similar to what we got be-
fore at 1.6 GHz (Xiang et al. 2002), and confirms that there
is jet-like emission ‘C’ and ‘D’ associated with the southern
component ‘B’, indicating this is a core-jet source.
3.18. B2 1824+271
The 1.6 GHz VLBI image (Fig. 17) exposes a symmetric dou-
ble structure and jet-like emission associated with the two
lobes, confirming this is a CSO as we have suggested (Xiang et
al. 2006).
3.19. [WB92] 2323+790
The 1.6 GHz image (Fig. 18) shows a central component ‘A’
and a weak one ‘B+C’ in the north-west, and the components
‘A’ and ‘B+C’ show steep spectra between 1.6 GHz and 5 GHz
(Xiang et al. 2006). The source can be a CSO candidate.
4. Discussion
In the sample (Table 1), J1648+0242 is an NVSS double source
and is not detected in this VLBI observation; all others are
point-like in the NVSS images, indicating that GPS sources
are compact. Except four sources (J1057+0012, J1352+1107,
J1600−0037 and 1751+278), 14 out of 18 sources exhibit dou-
ble or triple VLBI structure and can be CSOs or CSO can-
didates though some of them have no measured redshift. The
sources with redshift show double or triple structure with sizes
< 1 kpc, suggesting these GPS sources are certainly compact
and likely CSOs.
The mini double-lobe sources or CSOs could be more sta-
ble in flux density than other type of compact sources. We have
measured the flux densities for the sources (Table 2) at 4.85
GHz and compared with the values observed 15 years ago, we
found that 12 among 14 GPS sources are likely stable in flux
(1σ level), two sources (J1057+0012 and J1600−0037) show
about 10% variability in 3σ and 2σ level respectively. The flux
variability on J1057+0012 and J1600−0037 is consistent with
their core-jet classification. ‘Core-jet’ sources are defined to
show a one-sided jet, and the jet is often closely pointing to us
(from a pole-on AGN). It is hard to estimate the real source size
due to Doppler boosting, hence the ‘core-jet’ sources might not
be young radio sources even if they appear to be compact in
some cases.
In addition, some sources are resolved out in our VLBI
image by more than 10% of total flux estimated from
Table 1, probably due to diffuse emission associated with
lobes and tail/jet emission. They are J0210+0419 (-14%),
J0323+0534 (-38%), J0433−0229 (-18%), J1352+0232 (-
15%), J1135+1107 (-31%), J2058+0540 (-12%), 2322−040 (-
15%), and J1648+0242 is completely resolved out. The VLBI
flux densities of the other nine sources at 1.6 GHz are consis-
tent with the estimated total flux densities within an error of
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 5
Table 3. The component parameters of the VLBI images at 1.6 GHz. The columns give: (1) source name and possible classifica-
tion (CSOc: CSO candidate, cj: core-jet); (2) total cleaned flux density of image at 1.6 GHz; (3) component identification labled
to Xiang et al. 2002, 2005, 2006; (4),(5) peak and integral intensity of a fitted Gaussian component at 1.6 GHz in the AIPS task
JMFIT; (6),(7) major/minor axes and position angle of component at 1.6 GHz; (8),(9) distance and position angle relative to the
first component; (10) brightness temperature of component.
1 2 3 4 5 6 7 8 9 10
Name S vlbi Comp S p S int θ1 × θ2 PA d PA T b
class mJy mJy mJy mas×mas ◦ mas ◦ 108K◦
J0210+0419 715 A 290 375 5.7 × 2.9 169 0 11.9
CSOc B 118 205 10 × 3.6 175 68.2 ± 0.1 -153.2 ± 0.1 2.2
J0323+0534 1497 A 536 1270 32.6 × 11.1 70 0 0.5
CSO B 117 548 57.5 × 21.7 18 122.7 ± 2.9 -167.2 ± 0.4 0.1
J0433−0229 1095 A 407 1045 14.5 × 4.7 171 0 2.4
CSOc/cj B 49 111 9.9 × 5.5 88 53.0 ± 0.3 164.4 ± 0.2 0.3
J0913+1454 796 A 211 501 8.2 × 4.9 65 0 2.1
CSOc B 55 157 8.9 × 6.2 80 56.8 ± 0.1 73.3 ± 0.1 0.4
J1057+0012 810 A 381 550 3.6 × 2.7 6.6 0 17.5
cj B 37 58 6.2 × 2.1 9.7 9.7 ± 0.2 114.8 ± 0.9 1.3
J1109+1043 1370 A 420 984 5.7 × 4.7 110 0 6.6
CSOc B 143 311 5.4 × 4.2 91 46.3 ± 0.1 104.4 ± 0.1 2.6
J1135−0021 1025 A 279 454 6.6 × 2.7 142 0 4.9
CSO B 143 271 8.6 × 3.0 153 85.8 ± 0.1 164.4 ± 0.1 1.7
J1203+0414 1029 A 571 850 4.7 × 2.9 107 0 25.1
CSOc B 51 81 5.0 × 3.8 78 18.1 ± 0.2 103.0 ± 0.4 1.6
C 31 35 6 × 6 0 58.2 ± 0.2 104.2 ± 0.2 1.9
J1352+0232 885 A 173 480 7.4 × 4.5 53 0 2.1
CSO B 30 120 8.9 × 6.3 110 165.7 ± 0.3 -111.9 ± 0.1 0.2
J1352+1107 896 A 198 395 8.1 × 5.4 11.7 0 2.0
CSOc/cj B 106 202 6.8 × 6.3 10 3.5 ± 0.1 53.1 ± 0.1 1.1
J1600−0037 936 A 394 607 5.4 × 4.1 157 0
cj B 125 255 7.9 × 5.0 86 24.7 ± 0.1 83.5 ± 0.1
J2058+0540 914 A 356 513 7.6 × 3.7 163 0 8.2
CSO B 182 403 12.6 × 4.5 128 127.2 ± 0.1 172.1 ± 0.1 2.3
2121−014 976 A 363 594 5.1 × 3.9 123 0 10.4
CSO C 191 415 7.4 × 5.0 126 59.4 ± 0.1 85.4 ± 0.1 2.9
2322−040 965 A 229 509 14.6 × 3.3 161 0 3.6
CSO B 74 160 11.7 × 5.3 162 40.6 ± 0.2 171.3 ± 0.1 0.8
C 65 196 19.3 × 4.7 1 22.0 ± 0.4 171.1 ± 0.2 0.5
0914+114 578 A 37 50 4.9 × 2.7 16 0
CSO B 29 65 9.1 × 4.3 66 45.6 ± 0.1 80.1 ± 0.1
C 242 360 4.2 × 3.9 90 84.2 ± 0.1 81.6 ± 0.1
E 28 40 4.4 × 3.4 63 86.1 ± 0.1 -95.9 ± 0.1
1751+278 596 A 400 522 4.8 × 2.9 71 0 14.5
cj B 26 37 8 × 3 15 20.5 ± 0.1 -128.6 ± 0.2 0.5
C 12.4 21 7 × 5 59 26.6 ± 0.3 -119.8 ± 0.3 0.2
D 8 20 19.5 × 3.6 176 41.4 ± 0.4 -104.4 ± 0.8 0.1
1824+271 296 A 145 174 3.9 × 2.1 164 0
CSO B 42 69 6.1 × 4.3 137 21.8 ± 0.1 -83.4 ± 0.1
2323+790 900 A 439 621 5.8 × 2.1 159 0
CSOc B+C 107 155 7.9 × 3.1 118 19.2 ± 0.1 -71.1 ± 0.1
10% the estimated amplitude uncertainty of the EVN observa-
tions.
5. Summary and conclusion
1. We obtained total intensity 1.6 GHz VLBI images of 17
GPS sources for the first time. The majority (80%) show
mini-double-lobe radio structure, indicating that they are
CSOs or candidates, and their host AGNs could be edge-
on to us. This result suggests that there is a high incidence
of mini double-lobe sources and CSOs in the GPS source
sample.
6 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
2. The sources J0323+0534, J1135−0021, J1352+0232,
J2058+0540, 2121−014 and 2322−040 with measured red-
shift, are double-lobed with sizes of < 1 kpc, and are clas-
sified as CSOs.
3. Three sources (J1057+0012, J1600−0037 and 1751+278)
are classified as core-jet sources according to their mor-
phologies and flux variability.
4. The 1.6 GHz images of the sources 0914+114, 1824+271,
2121−014 and 2322−040, for which we had observations
at 2.3, 5.0 and 8.4 GHz, have provided information on their
source structure and spectra at the lower frequency, permit-
ting further spectral study in the future.
Acknowledgements. We thank the referee Alvaro Labiano, and
Nathan de Vries for comments. The European VLBI Network is a
joint facility of European, Chinese, South African and other radio as-
tronomy institutes funded by their national research councils. This re-
search has made use of the NASA/IPAC Extragalatic Database (NED)
which is operated by the Jet Propulsion Laboratory, Caltech, under
contract with NASA. This work was partly supported by the Natural
Science Foundation of China (NSFC).
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Orienti M., Dallacasa D., Stanghellini C., 2007, A&A 461, 923
Polatidis A. G., Conway J. E., 2003, PASA 20, 69
Owsianik I., Conway J. E., 1998, A&A 337, 69
Snellen I. A. G., Lehnert M. D., Bremer M. N., Schilizzi R. T., 2002,
MNRAS 337, 981
Snellen I. A. G., Schilizzi R. T., Miley G. K., de Bruyn A. G., Bremer
M. N., Röttgering H. J. A., 2000, MNRAS 319, 445
Tschager W., Schilizzi R. T., Röttgering H. J. A., Snellen I. A. G.,
Miley G. K., 2000, A&A 360, 887
Stanghellini C., O’Dea C. P., Dallacasa D., Cassaro P., Baum S. A.,
Fanti R., Fanti C., 2005, A&A 443, 891
Stanghellini C., O’Dea C. P., Dallacasa D., Baum S. A., Fanti R., Fanti
C., 1998, A&AS 131, 303
Tinti S.,Dallacasa D., de Zotti G., Celotti A., Stanghellini C., 2005,
A&A 432, 31
Xiang L., Stanghellini C., Dallacasa D., Haiyan Z., 2002, A&A 385,
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434, 123
Xiang L., Reynolds C., Strom R. G., Dallacasa D., 2006, A&A 454,
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 7
J0210+0419
0 100 200
MilliARC SEC
60 40 20 0 -20 -40 -60 -80 -100
Fig. 1. J0210+0419 at 1.65 GHz, the restoring beam is 10.2 ×
7.5 mas with PA −75.5◦, the peak is 293 mJy/beam, the con-
tours are 12 mJy/beam times levels -1, 1, 2, 4, 8, 16, 32, 64,
100, 200, 400, 800, and the same levels are used in the follow-
ing images, the grey scale unit is mJy.
J0323+0534
MilliARC SEC
150 100 50 0 -50 -100 -150
Fig. 2. J0323+0534 at 1.65 GHz, the restoring beam is 21.3 ×
17.7 mas with PA −20.2◦, the peak is 586 mJy/beam, the first
contour is 50 mJy/beam.
J0433-0229
0 100 200 300 400
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60 -80
Fig. 3. J0433−0229 at 1.65 GHz, the restoring beam is 8.0×6.3
mas with PA −1.7◦, the peak is 435 mJy/beam, the first contour
is 15 mJy/beam.
J0913+1454
0 50 100 150 200
MilliARC SEC
100 80 60 40 20 0 -20 -40
Fig. 4. J0913+1454 at 1.65 GHz, the restoring beam is 6.9×4.6
mas with PA 24.4◦, the peak is 217 mJy/beam, the first contour
is 6 mJy/beam.
8 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
J1057+0012
0 100 200 300
MilliARC SEC
100 80 60 40 20 0 -20 -40
Fig. 5. J1057+0012 at 1.65 GHz, the restoring beam is 7.3×3.3
mas with PA 13.4◦, the peak is 390 mJy/beam, the first contour
is 6 mJy/beam.
J1109+1043
0 100 200 300 400
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60
Fig. 6. J1109+1043 at 1.65 GHz, the restoring beam is 7.9×3.3
mas with PA 16.5◦, the peak is 434 mJy/beam, the first contour
is 10 mJy/beam.
J1135-0021
0 100 200
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60 -80
Fig. 7. J1135−0021 at 1.65 GHz, the restoring beam is 7.8×5.3
mas with PA 20.4◦, the peak is 281 mJy/beam, the first contour
is 8 mJy/beam.
J1203+0414
0 200 400
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60
Fig. 8. J1203+0414 at 1.65 GHz, the restoring beam is 7.0×4.9
mas with PA 15.2◦, the peak is 575 mJy/beam, the first contour
is 10 mJy/beam.
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 9
J1352+0232
MilliARC SEC
50 0 -50 -100 -150 -200
Fig. 9. J1352+0232 at 1.65 GHz, the restoring beam is 6.7×3.2
mas with PA 9.9◦, the peak is 183 mJy/beam, the first contour
is 8 mJy/beam.
J1352+1107
MilliARC SEC
100 50 0 -50 -100
Fig. 10. J1352+1107 at 1.65 GHz, the restoring beam is 14.0×
12.5 mas with PA −75.3◦, the peak is 379 mJy/beam, the first
contour is 20 mJy/beam.
J1600-0037
0 100 200 300 400
MilliARC SEC
80 60 40 20 0 -20 -40 -60
Fig. 11. J1600−0037 at 1.65 GHz, the restoring beam is 7.4 ×
5.5 mas with PA −39.8◦, the peak is 400 mJy/beam, the first
contour is 8 mJy/beam.
J2058+0540
MilliARC SEC
150 100 50 0 -50 -100 -150
Fig. 12. J2058+0540 at 1.65 GHz, the restoring beam is 10.6×
6.5 mas with PA 0.7◦, the peak is 362 mJy/beam, the first con-
tour is 10 mJy/beam.
10 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
B2121-014
0 100 200 300
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60
Fig. 13. 2121−014 at 1.65 GHz, the restoring beam is 5.9× 5.5
mas with PA −1.4◦, the peak is 370 mJy/beam, the first contour
is 15 mJy/beam.
2322-040
MilliARC SEC
80 60 40 20 0 -20 -40 -60 -80
Fig. 14. 2322−040 at 1.65 GHz, the restoring beam is 8.6× 6.8
mas with PA −2.1◦, the peak is 233 mJy/beam, the first contour
is 10 mJy/beam.
0914+114
MilliARC SEC
50 0 -50 -100 -150 -200
Fig. 15. 0914+114 at 1.65 GHz, the restoring beam is 8.3× 4.7
mas with PA 16.8◦, the peak is 246 mJy/beam, the first contour
is 1 mJy/beam.
MilliARC SEC
80 60 40 20 0 -20 -40 -60 -80
1751+278
Fig. 16. 1751+278 at 1.65 GHz, the restoring beam is 10.7×6.0
mas with PA 7.0◦, the peak is 406 mJy/beam, the first contour
is 3 mJy/beam.
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 11
1824+271
MilliARC SEC
60 40 20 0 -20 -40 -60 -80
Fig. 17. 1824+271 at 1.65 GHz, the restoring beam is 9.0× 5.1
mas with PA 8.1◦, the peak is 146 mJy/beam, the first contour
is 1 mJy/beam.
2323+790
MilliARC SEC
60 40 20 0 -20 -40 -60 -80
Fig. 18. 2323+790 at 1.65 GHz, the restoring beam is 11.6×5.4
mas with PA −82◦, the peak is 438 mJy/beam, the first contour
is 10 mJy/beam.
	Introduction
	Observations and data reduction
	Results and comments on individual sources
	J0210+0419 (PKS B0208+040)
	J0323+0534 (4C+05.14)
	J0433-0229 (4C-02.17)
	J0913+1454 (PKS B0910+151)
	J1057+0012 (PKS B1054+004)
	J1109+1043 (PKS B1107+109)
	J1135-0021 (4C-00.45)
	J1203+0414 (PKS B1200+045)
	J1352+0232 (PKS B1349+027)
	J1352+1107 (4C+11.46)
	J1600-0037 (PKS B1557-004)
	J1648+0242 (4C+02.43)
	J2058+0540 (4C+05.78)
	PKS B2121-014
	PKS B2322-040
	PKS B0914+114
	1751+278 (MG2 J175301+2750)
	B2 1824+271 
	[WB92] 2323+790
	Discussion
	Summary and conclusion

  Aims and Methods: We present the results of VLBI observations of nineteen
GHz-Peaked-Spectrum (GPS) radio sources at 1.6 GHz. Of them, 15 sources are
selected from the Parkes Half Jansky (PHJ) sample (Snellen 2002), 4 others are
from our previous observation list. We aimed at imaging the structure of GPS
sources, searching for Compact Symmetric Objects (CSOs) and studying the
absorption for the convex radio spectra of GPS sources.
  Results: We obtained total intensity 1.6 GHz VLBI images of 17 sources for
the first time. Of them, 80% show mini-double-lobe radio structure, indicating
that they are CSOs or candidates, and their host AGNs could be edge-on to us.
This result suggests that there is a high incidence of mini double-lobe sources
(or CSOs) in the PHJ sample. The sources J0323+0534, J1135-0021, J1352+0232,
J2058+0540, J2123-0112 and J2325-0344 with measured redshift, showing
double-lobe structure with sizes of <1 kpc, are classified as CSOs. Three
sources J1057+0012, J1600-0037 and J1753+2750 are considered as core-jet
sources according to their morphologies and flux variability.

Introduction
GHz-Peaked-Spectrum (GPS) radio sources are powerful
(P1.4 GHz ≥ 10
25 W Hz−1), compact (≤ 1 kpc), and have con-
vex radio spectra, and they make up a significant fraction (≈
10%) of the bright radio source sample, see O’Dea (1998) for
a review. In general, the presence of large scale emission asso-
ciated with GPS galaxies is rare, about a few percent in a GPS
sample (Stanghellini et al. 2005). Most GPS sources appear to
be truly compact and isolated.
Their small size is most likely due to their youth (< 104
years) according to a spectral aging analysis (Murgia 2003). A
couple of GPS sources are certainly young radio sources whose
kinematic age from lobe proper motions has been measured
and these sources are also identified as Compact Symmetric
Objects (CSOs). There is compelling evidence in favour of the
youth scenario of GPS sources and CSOs, see e.g. Owsianik
& Conway (1998), Tschager et al. (2000), Polatidis & Conway
(2003), and Orienti et al. (2007). The GPS sources and CSOs
are the key objects to study the early evolution of power-
ful radio-loud AGN. A unification scenario assumes that GPS
sources evolve into Compact Steep Spectrum sources (1-15
kpc), which in turn, evolve into classical extended radio sources
(> 15 kpc), i.e. FR I/II radio sources (Fanti et al. 1995, Snellen
et al. 2000, de Vries et al. 2007).
Send offprint requests to: X. Liu: liux@ms.xjb.ac.cn
GPS galaxies are dominated by lobe/jet emission on both
sides of the central engine, and are thought to be relatively free
of beaming effects. The GPS galaxies show very low polariza-
tion (about less than 0.5% at 5 GHz, Dallacasa 2004, Xiang
et al. 2006). The low integrated polarization could be due to
large Faraday depths around the radio source, which would de-
polarize the radio emission, implying that their host-AGNs are
probably edge-on to us.
Since GPS sources live in the narrow line region of AGN,
it is likely that their low frequency radio emission will be ab-
sorbed due to either synchrotron self-absorption or free-free
absorption, giving rise to a peaked radio spectrum. Therefore,
GPS sources are also suitable for studying radio absorption and
scattering in AGNs.
We have carried out EVN (European VLBI Network) ob-
servations of 19 GPS sources, 15 of them are from the Parkes
Half Jansky (PHJ) sample (Snellen et al. 2002) with declina-
tion > −5◦ and not observed with VLBI before. Four sources
are from our previous observation list which we have observed
with the EVN at 2.3/8.4 GHz and/or 5 GHz (see Xiang et al.
2005, 2006). We aimed at imaging the GPS sources at 1.6 GHz,
in order to confirm whether the GPS sources are double-lobe
sources, and to find CSO candidates. For the sources with ob-
servations at 2.3, 5.0 and 8.4 GHz, the 1.6 GHz images will
further provide information on their source structure and inten-
http://arxiv.org/abs/0704.0310v1
2 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
sity at lower frequency, for further spectral study of the GPS
sources in the future.
2. Observations and data reduction
The observations were carried out on 3 March 2006 at
1.65 GHz using the MK5 recording system with a band-
width of 32 MHz and sample rate of 256 Mbps in dual
circular polarization. The EVN antennae in this experiment
were Effelsberg, Westerbork, Jodrell, Medicina, Noto, Onsala,
Torun, Hartebeesthoek, Urumqi and Shanghai. Snapshot obser-
vations of 19 sources (Table 1) in a total of 24 hours were made.
OQ208 and DA193 were observed as calibrators. The data cor-
relation was completed at JIVE.
The total flux densities of the sources were also measured
at 5 GHz with Urumqi 25m telescope in order to find any flux
variability. The values are listed in Table 2.
The Astronomical Image Processing System (AIPS) has
been used for editing, a-priori calibration, fringe-fitting, self-
calibration, imaging and model fitting of the data.
3. Results and comments on individual sources
We list the basic information of the sources in Table 1, and the
parameters derived from the VLBI images in Table 3. We com-
ment on the results of each source and give a short discussion.
We use S ∝ ν−α to define the spectral index. Optical informa-
tion and redshifts of the GPS sources in the PHJ sample are
given by de Vries et al. (2007), as listed in Table 1.
3.1. J0210+0419 (PKS B0208+040)
The 1.6 GHz VLBI image (Fig. 1) is the first VLBI image of
the source. It shows a double-lobe structure, and is most likely
a CSO. Optical observations did not result in an identification
with a lower limit of mR > 24.1, but it is identified with a mag-
nitude of Ks=18.3 (de Vries et al. 2007).
3.2. J0323+0534 (4C+05.14)
The 1.6 GHz VLBI image (Fig. 2) is the first VLBI image of the
source, and it exhibits a strong diffuse component and a weak
extended component in the south. Both are likely lobe emis-
sion. About 38% total flux density (estimated from Table 1) is
resolved out in the VLBI image, due to the diffuse components.
For its size of 490 pc, the source can be a CSO.
3.3. J0433−0229 (4C−02.17)
The 1.6 GHz VLBI image (Fig. 3) is the first VLBI image of the
source, and the main component is diffuse and extended in the
north-south direction, and a possible weak component in the
south. About 18% total flux density (estimated from Table 1)
is resolved out in the VLBI image. Either a core-jet or a CSO
classification is possible for the source.
3.4. J0913+1454 (PKS B0910+151)
The 1.6 GHz VLBI image (Fig. 4) is the first VLBI image of
the source. It shows double structure and both components are
further resolved. There is probably a hot-spot imbedded in the
bright one. We consider it as a CSO candidate.
3.5. J1057+0012 (PKS B1054+004)
The 1.6 GHz VLBI image (Fig. 5) is the first VLBI image of
the source. There is a bright compact component followed by
a secondary component and a series of possible weak compo-
nents in the east, indicating this is a core-jet source. A flux vari-
ability of (−11.4 ± 3.6)% over 15 years at 5 GHz, as reported
in Table 2, is consistent with the core-jet classification.
3.6. J1109+1043 (PKS B1107+109)
The 1.6 GHz VLBI image (Fig. 6) is the first VLBI image of
the source. It is a double structure, and can be a CSO candi-
date. The total flux density (1270 mJy estimated from Table 1)
is completely restored in the VLBI image (1370 mJy, increased
by 8%). There is also an indication of total flux increasing
(4.9 ± 12.4)% at 5 GHz in Table 2 but with a large error.
3.7. J1135−0021 (4C−00.45)
The 1.6 GHz VLBI image (Fig. 7) is the first VLBI image of
the source. It shows a double-lobe structure, and with a size of
720 pc, we classify the source as a CSO.
3.8. J1203+0414 (PKS B1200+045)
The 1.6 GHz VLBI image (Fig. 8) is the first VLBI image of
the source. The triple structure may consist of a core and two
sided emission, or a one sided core-jet source. The quasar as
newly identified by de Vries et al (2007), is possibly a core-jet
one, but still we keep the source as a CSO candidate.
3.9. J1352+0232 (PKS B1349+027)
The 1.6 GHz VLBI image (Fig. 9) is the first VLBI image of
the source. It shows a double-lobe like structure, for its size of
918 pc we consider it as a CSO.
3.10. J1352+1107 (4C+11.46)
The 1.6 GHz VLBI image (Fig. 10) is the first VLBI image of
the source. It appears to have a compact double structure or a
core-jet alike, and seems diffuse emission around the source.
About 30% total flux density (estimated from Table 1) is re-
solved out in the VLBI image. Either a core-jet or a compact
double classification is possible.
3.11. J1600−0037 (PKS B1557−004)
The 1.6 GHz VLBI image (Fig. 11) is the first VLBI image of
the source, and it has an overall double structure, the eastern
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 3
Table 1. The GPS sources. Columns (1),(2) source names; (3) optical identification (G: galaxy, QSO: quasar, EF: empty field);
(4) optical magnitude; (5) redshift (de Vries et al. 2007, those with * are a photometric estimated by Tinti et al. 2005); (6) linear
scale factor pc/mas [H0 = 71kms
−1Mpc−1 and q0 = 0.5 have been assumed]; (7) maximum angular size from the observation; (8)
maximum linear size; (9) 1.4 GHz flux density from the NVSS; (10) 2.7 GHz flux density from Snellen sample and the NED; (11)
low frequency spectral index; (12) higher frequency spectral index ( computed from columns 9 and 10); (13) turnover frequency;
(14) peak flux density; (15) references for the spectral information, 1 Snellen et al. 2002, 2 de Vries et al. 1997, 3 Stanghellini et
al. 1998, where S ∝ ν−α.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
S ource id mR z pc/mas θ L S 1.4 S 2.7 αl αh νm S m re f
mas pc mJy Jy GHz Jy
J0210+0419 B0208+040 G 18.3Ks 1.5* 6.1 90 948 0.56 0.80 0.4 1.3 1
J0323+0534 4C+05.14 G 19.2 0.1785 2.7 180 490 2793 1.60 0.85 0.4 7.1 1
J0433−0229 4C−02.17 G 19.1 0.530 5.1 80 408 1462 1.04 0.52 0.4 3.0 1
J0913+1454 B0910+151 G 22.9 0.47* 4.9 80 881 0.54 0.75 0.6 1.1 1
J1057+0012 B1054+004 G 22.3 0.65* 5.5 80? 898 0.58 0.67 0.4 1.6 1
J1109+1043 B1107+109 G 22.6 0.55* 5.2 60 1481 0.80 0.94 0.5 2.4 1
J1135−0021 4C−00.45 G 21.9 0.975 6.0 120 720 1268 0.76 0.78 0.4 2.9 1
J1203+0414 B1200+045 QSO 18.8 1.221 6.1 75 458 1146 0.85 0.45 0.4 1.4 1
J1352+0232 B1349+027 G 20.0 0.607 5.4 170 918 1145 0.78 0.58 0.4 2.0 1
J1352+1107 4C+11.46 G 21.0 0.891 5.9 50 295 1538 0.78 1.03 0.4 3.6 1
J1600−0037 B1557−004 G 50 1168 0.54 1.17 1.0 1.2 1
J1648+0242 4C+02.43 G 22.1 0.824 5.8 0.61 0.4 3.4 1
J2058+0540 4C+05.78 G 23.4 1.381 6.1 160 970 1213 0.65 0.95 0.4 3.1 1
J2123−0112 B2121−014 G 23.3 1.158 6.1 80 488 1087 0.64 -0.56 0.75 0.5 1.8 2
J2325−0344 B2322−040 G 23.5 1.509 6.0 75 450 1224 0.91 -0.42 0.75 1.4 1.3 2
J0917+1113 B0914+114 EF 190 800 0.31 -0.1 1.6 0.3 2.3 3
J1753+2750 B1751+278 G 21.7 0.86* 5.9 50 625 0.46 -0.27 0.57 1.4 0.6 2
J1826+2708 B1824+271 G 22.9 45 332 0.23 -0.39 0.75 1.0 0.4 2
J2325+7917 B2323+790 G 19.5V 32 1136 -0.3 0.75 1.4 1.2 2
component has some extension in the west-east direction. A
flux variability of (13± 6.9)% at 5 GHz in Table 2 may suggest
this is a core-jet source.
3.12. J1648+0242 (4C+02.43)
The GPS source is not detected with VLBI. It is an NVSS
double-lobe source, and totally resolved out in the VLBI ob-
servation.
3.13. J2058+0540 (4C+05.78)
The 1.6 GHz VLBI image (Fig. 12) is the first VLBI image of
the source. It shows a double-lobe source, and for the size of
970 pc, we suggest this is a CSO.
3.14. PKS B2121−014
The 1.6 GHz VLBI image (Fig. 13) shows a double-lobe struc-
ture, it is similar to that at 2.3 and 5 GHz (Xiang et al. 2005,
2006), except that a weak jet-like emission ‘B’, which appears
at 2.3 and 5 GHz, is missing, probably due to absorption at the
lower frequency 1.6 GHz. The source is a CSO for the source
size of 488 pc.
3.15. PKS B2322−040
The 1.6 GHz VLBI image (Fig. 14) exposes a central emission
region between the two lobes ‘A’ and ‘B’, which is probably a
core embedded in the central region. The ‘core’ emission is not
detected at higher frequencies (Xiang et al. 2005, 2006), but it
emerges at 1.6 GHz near the peak frequency (1.4 GHz) of the
GPS source. There is a flux increase of (4.0 ± 1.9)% over 15
years at 5 GHz (Table 2), may suggest that the core is currently
active. The source can be a CSO for its size of 450 pc.
3.16. PKS B0914+114
The 1.6 GHz VLBI image (Fig. 15) exhibits a core ‘A’, jet fea-
ture ‘B’ and two lobes ‘C’, ‘E’. The western one ‘E’ emerges at
this frequency. Labiano et al. (2007) have identified an empty
field (> 25 mR) at the FIRST position of the source, and con-
cluded that the previously identified nearby disk galaxy (a red-
shift of 0.178) is not the host of this radio source 0914+114.
For the typical compact symmetric structure, we consider the
source is a CSO.
4 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
Table 2. Source flux and possible variability, columns 2-4 are flux densities at 5.0 GHz (PKS90), 4.85 GHz (Gregory &
Condon 1991, and Griffith et al. 1995) and 4.85 GHz flux measured with the Urumqi 25m telescope on 2007/1/24 (J1648+0242,
J2058+0540, 1824+271 and 2121−014 were not well measured due to source confusion or weak); column 5 is a flux variability
computed from columns 3,4.
S ource S 5.0 S 4.85 S 4.85Ur δS 4.85
mJy mJy mJy %
J0210+0419 300 298±19 302 ± 10 1.3 ±3.1
J0323+0534 830 819±44 868 ± 9 6.0±4.6
J0433−0229 640 640±35 637 ± 14 -0.5±3.3
J0913+1454 300 315±43 297 ± 8 -5.7±10.3
J1057+0012 370 396±23 351 ± 6 -11.4±3.6
J1109+1043 400 408±56 428 ± 8 4.9±12.4
J1135−0021 440 446±25 427 ± 8 -4.3±3.6
J1203+0414 520 640±35 611 ± 7 -4.5 ±4.1
J1352+0232 470 469 ± 7
J1352+1107 410 447±62 418 ± 5 -6.5±11.9
J1600−0037 180 187±14 212 ± 3 13.4±6.9
J1648+0242 260 337±20
J2058+0540 340 356±21
2121−014 320 345±21
2322−040 500 524±29 545 ± 20 4.0 ±1.9
0914+114 140 134±19 140 ± 1 4.5 ±14.1
1751+278 298±39 292 ± 6 -2.0±10.8
1824+271 122±17
2323+790 491 ± 7
OQ208 2421±217 2514 ± 12 3.8±8.8
3.17. 1751+278 (MG2 J175301+2750)
The 1.6 GHz structure (Fig. 16) is similar to what we got be-
fore at 1.6 GHz (Xiang et al. 2002), and confirms that there
is jet-like emission ‘C’ and ‘D’ associated with the southern
component ‘B’, indicating this is a core-jet source.
3.18. B2 1824+271
The 1.6 GHz VLBI image (Fig. 17) exposes a symmetric dou-
ble structure and jet-like emission associated with the two
lobes, confirming this is a CSO as we have suggested (Xiang et
al. 2006).
3.19. [WB92] 2323+790
The 1.6 GHz image (Fig. 18) shows a central component ‘A’
and a weak one ‘B+C’ in the north-west, and the components
‘A’ and ‘B+C’ show steep spectra between 1.6 GHz and 5 GHz
(Xiang et al. 2006). The source can be a CSO candidate.
4. Discussion
In the sample (Table 1), J1648+0242 is an NVSS double source
and is not detected in this VLBI observation; all others are
point-like in the NVSS images, indicating that GPS sources
are compact. Except four sources (J1057+0012, J1352+1107,
J1600−0037 and 1751+278), 14 out of 18 sources exhibit dou-
ble or triple VLBI structure and can be CSOs or CSO can-
didates though some of them have no measured redshift. The
sources with redshift show double or triple structure with sizes
< 1 kpc, suggesting these GPS sources are certainly compact
and likely CSOs.
The mini double-lobe sources or CSOs could be more sta-
ble in flux density than other type of compact sources. We have
measured the flux densities for the sources (Table 2) at 4.85
GHz and compared with the values observed 15 years ago, we
found that 12 among 14 GPS sources are likely stable in flux
(1σ level), two sources (J1057+0012 and J1600−0037) show
about 10% variability in 3σ and 2σ level respectively. The flux
variability on J1057+0012 and J1600−0037 is consistent with
their core-jet classification. ‘Core-jet’ sources are defined to
show a one-sided jet, and the jet is often closely pointing to us
(from a pole-on AGN). It is hard to estimate the real source size
due to Doppler boosting, hence the ‘core-jet’ sources might not
be young radio sources even if they appear to be compact in
some cases.
In addition, some sources are resolved out in our VLBI
image by more than 10% of total flux estimated from
Table 1, probably due to diffuse emission associated with
lobes and tail/jet emission. They are J0210+0419 (-14%),
J0323+0534 (-38%), J0433−0229 (-18%), J1352+0232 (-
15%), J1135+1107 (-31%), J2058+0540 (-12%), 2322−040 (-
15%), and J1648+0242 is completely resolved out. The VLBI
flux densities of the other nine sources at 1.6 GHz are consis-
tent with the estimated total flux densities within an error of
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 5
Table 3. The component parameters of the VLBI images at 1.6 GHz. The columns give: (1) source name and possible classifica-
tion (CSOc: CSO candidate, cj: core-jet); (2) total cleaned flux density of image at 1.6 GHz; (3) component identification labled
to Xiang et al. 2002, 2005, 2006; (4),(5) peak and integral intensity of a fitted Gaussian component at 1.6 GHz in the AIPS task
JMFIT; (6),(7) major/minor axes and position angle of component at 1.6 GHz; (8),(9) distance and position angle relative to the
first component; (10) brightness temperature of component.
1 2 3 4 5 6 7 8 9 10
Name S vlbi Comp S p S int θ1 × θ2 PA d PA T b
class mJy mJy mJy mas×mas ◦ mas ◦ 108K◦
J0210+0419 715 A 290 375 5.7 × 2.9 169 0 11.9
CSOc B 118 205 10 × 3.6 175 68.2 ± 0.1 -153.2 ± 0.1 2.2
J0323+0534 1497 A 536 1270 32.6 × 11.1 70 0 0.5
CSO B 117 548 57.5 × 21.7 18 122.7 ± 2.9 -167.2 ± 0.4 0.1
J0433−0229 1095 A 407 1045 14.5 × 4.7 171 0 2.4
CSOc/cj B 49 111 9.9 × 5.5 88 53.0 ± 0.3 164.4 ± 0.2 0.3
J0913+1454 796 A 211 501 8.2 × 4.9 65 0 2.1
CSOc B 55 157 8.9 × 6.2 80 56.8 ± 0.1 73.3 ± 0.1 0.4
J1057+0012 810 A 381 550 3.6 × 2.7 6.6 0 17.5
cj B 37 58 6.2 × 2.1 9.7 9.7 ± 0.2 114.8 ± 0.9 1.3
J1109+1043 1370 A 420 984 5.7 × 4.7 110 0 6.6
CSOc B 143 311 5.4 × 4.2 91 46.3 ± 0.1 104.4 ± 0.1 2.6
J1135−0021 1025 A 279 454 6.6 × 2.7 142 0 4.9
CSO B 143 271 8.6 × 3.0 153 85.8 ± 0.1 164.4 ± 0.1 1.7
J1203+0414 1029 A 571 850 4.7 × 2.9 107 0 25.1
CSOc B 51 81 5.0 × 3.8 78 18.1 ± 0.2 103.0 ± 0.4 1.6
C 31 35 6 × 6 0 58.2 ± 0.2 104.2 ± 0.2 1.9
J1352+0232 885 A 173 480 7.4 × 4.5 53 0 2.1
CSO B 30 120 8.9 × 6.3 110 165.7 ± 0.3 -111.9 ± 0.1 0.2
J1352+1107 896 A 198 395 8.1 × 5.4 11.7 0 2.0
CSOc/cj B 106 202 6.8 × 6.3 10 3.5 ± 0.1 53.1 ± 0.1 1.1
J1600−0037 936 A 394 607 5.4 × 4.1 157 0
cj B 125 255 7.9 × 5.0 86 24.7 ± 0.1 83.5 ± 0.1
J2058+0540 914 A 356 513 7.6 × 3.7 163 0 8.2
CSO B 182 403 12.6 × 4.5 128 127.2 ± 0.1 172.1 ± 0.1 2.3
2121−014 976 A 363 594 5.1 × 3.9 123 0 10.4
CSO C 191 415 7.4 × 5.0 126 59.4 ± 0.1 85.4 ± 0.1 2.9
2322−040 965 A 229 509 14.6 × 3.3 161 0 3.6
CSO B 74 160 11.7 × 5.3 162 40.6 ± 0.2 171.3 ± 0.1 0.8
C 65 196 19.3 × 4.7 1 22.0 ± 0.4 171.1 ± 0.2 0.5
0914+114 578 A 37 50 4.9 × 2.7 16 0
CSO B 29 65 9.1 × 4.3 66 45.6 ± 0.1 80.1 ± 0.1
C 242 360 4.2 × 3.9 90 84.2 ± 0.1 81.6 ± 0.1
E 28 40 4.4 × 3.4 63 86.1 ± 0.1 -95.9 ± 0.1
1751+278 596 A 400 522 4.8 × 2.9 71 0 14.5
cj B 26 37 8 × 3 15 20.5 ± 0.1 -128.6 ± 0.2 0.5
C 12.4 21 7 × 5 59 26.6 ± 0.3 -119.8 ± 0.3 0.2
D 8 20 19.5 × 3.6 176 41.4 ± 0.4 -104.4 ± 0.8 0.1
1824+271 296 A 145 174 3.9 × 2.1 164 0
CSO B 42 69 6.1 × 4.3 137 21.8 ± 0.1 -83.4 ± 0.1
2323+790 900 A 439 621 5.8 × 2.1 159 0
CSOc B+C 107 155 7.9 × 3.1 118 19.2 ± 0.1 -71.1 ± 0.1
10% the estimated amplitude uncertainty of the EVN observa-
tions.
5. Summary and conclusion
1. We obtained total intensity 1.6 GHz VLBI images of 17
GPS sources for the first time. The majority (80%) show
mini-double-lobe radio structure, indicating that they are
CSOs or candidates, and their host AGNs could be edge-
on to us. This result suggests that there is a high incidence
of mini double-lobe sources and CSOs in the GPS source
sample.
6 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
2. The sources J0323+0534, J1135−0021, J1352+0232,
J2058+0540, 2121−014 and 2322−040 with measured red-
shift, are double-lobed with sizes of < 1 kpc, and are clas-
sified as CSOs.
3. Three sources (J1057+0012, J1600−0037 and 1751+278)
are classified as core-jet sources according to their mor-
phologies and flux variability.
4. The 1.6 GHz images of the sources 0914+114, 1824+271,
2121−014 and 2322−040, for which we had observations
at 2.3, 5.0 and 8.4 GHz, have provided information on their
source structure and spectra at the lower frequency, permit-
ting further spectral study in the future.
Acknowledgements. We thank the referee Alvaro Labiano, and
Nathan de Vries for comments. The European VLBI Network is a
joint facility of European, Chinese, South African and other radio as-
tronomy institutes funded by their national research councils. This re-
search has made use of the NASA/IPAC Extragalatic Database (NED)
which is operated by the Jet Propulsion Laboratory, Caltech, under
contract with NASA. This work was partly supported by the Natural
Science Foundation of China (NSFC).
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Tinti S.,Dallacasa D., de Zotti G., Celotti A., Stanghellini C., 2005,
A&A 432, 31
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X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 7
J0210+0419
0 100 200
MilliARC SEC
60 40 20 0 -20 -40 -60 -80 -100
Fig. 1. J0210+0419 at 1.65 GHz, the restoring beam is 10.2 ×
7.5 mas with PA −75.5◦, the peak is 293 mJy/beam, the con-
tours are 12 mJy/beam times levels -1, 1, 2, 4, 8, 16, 32, 64,
100, 200, 400, 800, and the same levels are used in the follow-
ing images, the grey scale unit is mJy.
J0323+0534
MilliARC SEC
150 100 50 0 -50 -100 -150
Fig. 2. J0323+0534 at 1.65 GHz, the restoring beam is 21.3 ×
17.7 mas with PA −20.2◦, the peak is 586 mJy/beam, the first
contour is 50 mJy/beam.
J0433-0229
0 100 200 300 400
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60 -80
Fig. 3. J0433−0229 at 1.65 GHz, the restoring beam is 8.0×6.3
mas with PA −1.7◦, the peak is 435 mJy/beam, the first contour
is 15 mJy/beam.
J0913+1454
0 50 100 150 200
MilliARC SEC
100 80 60 40 20 0 -20 -40
Fig. 4. J0913+1454 at 1.65 GHz, the restoring beam is 6.9×4.6
mas with PA 24.4◦, the peak is 217 mJy/beam, the first contour
is 6 mJy/beam.
8 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
J1057+0012
0 100 200 300
MilliARC SEC
100 80 60 40 20 0 -20 -40
Fig. 5. J1057+0012 at 1.65 GHz, the restoring beam is 7.3×3.3
mas with PA 13.4◦, the peak is 390 mJy/beam, the first contour
is 6 mJy/beam.
J1109+1043
0 100 200 300 400
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60
Fig. 6. J1109+1043 at 1.65 GHz, the restoring beam is 7.9×3.3
mas with PA 16.5◦, the peak is 434 mJy/beam, the first contour
is 10 mJy/beam.
J1135-0021
0 100 200
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60 -80
Fig. 7. J1135−0021 at 1.65 GHz, the restoring beam is 7.8×5.3
mas with PA 20.4◦, the peak is 281 mJy/beam, the first contour
is 8 mJy/beam.
J1203+0414
0 200 400
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60
Fig. 8. J1203+0414 at 1.65 GHz, the restoring beam is 7.0×4.9
mas with PA 15.2◦, the peak is 575 mJy/beam, the first contour
is 10 mJy/beam.
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 9
J1352+0232
MilliARC SEC
50 0 -50 -100 -150 -200
Fig. 9. J1352+0232 at 1.65 GHz, the restoring beam is 6.7×3.2
mas with PA 9.9◦, the peak is 183 mJy/beam, the first contour
is 8 mJy/beam.
J1352+1107
MilliARC SEC
100 50 0 -50 -100
Fig. 10. J1352+1107 at 1.65 GHz, the restoring beam is 14.0×
12.5 mas with PA −75.3◦, the peak is 379 mJy/beam, the first
contour is 20 mJy/beam.
J1600-0037
0 100 200 300 400
MilliARC SEC
80 60 40 20 0 -20 -40 -60
Fig. 11. J1600−0037 at 1.65 GHz, the restoring beam is 7.4 ×
5.5 mas with PA −39.8◦, the peak is 400 mJy/beam, the first
contour is 8 mJy/beam.
J2058+0540
MilliARC SEC
150 100 50 0 -50 -100 -150
Fig. 12. J2058+0540 at 1.65 GHz, the restoring beam is 10.6×
6.5 mas with PA 0.7◦, the peak is 362 mJy/beam, the first con-
tour is 10 mJy/beam.
10 X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz
B2121-014
0 100 200 300
MilliARC SEC
100 80 60 40 20 0 -20 -40 -60
Fig. 13. 2121−014 at 1.65 GHz, the restoring beam is 5.9× 5.5
mas with PA −1.4◦, the peak is 370 mJy/beam, the first contour
is 15 mJy/beam.
2322-040
MilliARC SEC
80 60 40 20 0 -20 -40 -60 -80
Fig. 14. 2322−040 at 1.65 GHz, the restoring beam is 8.6× 6.8
mas with PA −2.1◦, the peak is 233 mJy/beam, the first contour
is 10 mJy/beam.
0914+114
MilliARC SEC
50 0 -50 -100 -150 -200
Fig. 15. 0914+114 at 1.65 GHz, the restoring beam is 8.3× 4.7
mas with PA 16.8◦, the peak is 246 mJy/beam, the first contour
is 1 mJy/beam.
MilliARC SEC
80 60 40 20 0 -20 -40 -60 -80
1751+278
Fig. 16. 1751+278 at 1.65 GHz, the restoring beam is 10.7×6.0
mas with PA 7.0◦, the peak is 406 mJy/beam, the first contour
is 3 mJy/beam.
X. Liu et al.: VLBI observations of nineteen GHz-Peaked-Spectrum radio sources at 1.6 GHz 11
1824+271
MilliARC SEC
60 40 20 0 -20 -40 -60 -80
Fig. 17. 1824+271 at 1.65 GHz, the restoring beam is 9.0× 5.1
mas with PA 8.1◦, the peak is 146 mJy/beam, the first contour
is 1 mJy/beam.
2323+790
MilliARC SEC
60 40 20 0 -20 -40 -60 -80
Fig. 18. 2323+790 at 1.65 GHz, the restoring beam is 11.6×5.4
mas with PA −82◦, the peak is 438 mJy/beam, the first contour
is 10 mJy/beam.
	Introduction
	Observations and data reduction
	Results and comments on individual sources
	J0210+0419 (PKS B0208+040)
	J0323+0534 (4C+05.14)
	J0433-0229 (4C-02.17)
	J0913+1454 (PKS B0910+151)
	J1057+0012 (PKS B1054+004)
	J1109+1043 (PKS B1107+109)
	J1135-0021 (4C-00.45)
	J1203+0414 (PKS B1200+045)
	J1352+0232 (PKS B1349+027)
	J1352+1107 (4C+11.46)
	J1600-0037 (PKS B1557-004)
	J1648+0242 (4C+02.43)
	J2058+0540 (4C+05.78)
	PKS B2121-014
	PKS B2322-040
	PKS B0914+114
	1751+278 (MG2 J175301+2750)
	B2 1824+271 
	[WB92] 2323+790
	Discussion
	Summary and conclusion
