To appear in ApJ
The Isophotal Structure of Early-Type Galaxies in the SDSS:
Dependence on AGN Activity and Environment.
Anna Pasquali, Frank C. van den Bosch and Hans-Walter Rix
Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
pasquali@mpia.de, vdbosch@mpia.de, rix@mpia.de
ABSTRACT
We study the dependence of the isophotal shape of early-type galaxies on their ab-
solute B-band magnitude, MB , their dynamical mass, Mdyn, and their nuclear activity
and environment, using an unprecedented large sample of 847 early-type galaxies identi-
fied in SDSS by Hao et al. (2006a). We find that the fraction of disky galaxies smoothly
decreases from fdisky ∼ 0.8 at MB − 5 log(h) = −18.7 (Mdyn = 6 × 1010h−1M⊙) to
∼ 0.5 at MB − 5 log(h) = −20.8 (Mdyn = 3 × 1011h−1M⊙). The large sample allows
us to describe these trends accurately with tight linear relations that are statistically
robust against the uncertainty in the isophotal shape measurements. There is also a
host of significant correlations between fdisky and indicators of nuclear activity (both
in the optical and in the radio) and environment (soft X-rays, group mass, group hier-
archy). Our analysis shows however that these correlations can be accurately matched
by assuming that fdisky only depends on galaxy luminosity or mass. We therefore con-
clude that neither the level of activity, nor group mass or group hierarchy help in better
predicting the isophotal shape of early-type galaxies.
Subject headings: galaxies: active — galaxies: elliptical and lenticular, cD — galaxies:
Seyfert — galaxies: structure
1. Introduction
Early-type galaxies form a remarkably homogeneous class of objects with a well-defined Funda-
mental Plane and with tight relations between colour and magnitude, between colour and velocity
dispersion, and between the amount of α-element enhancement and velocity dispersion (e.g., Faber
& Jackson 1976; Visvanathan & Sandage 1977; Dressler 1987; Djorgovski & Davis 1987; Bower et
al. 1992; Ellis et al. 1997). They have old stellar populations, though sometimes with a younger
component (Trager et al. 2000; Serra et al. 2006; Kuntschner et al. 2006), contain little ionized and
cold gas (Sarzi et al. 2006; Morganti et al. 2006), and are preferentially located in massive dark
matter halos (e.g., Dressler 1980; Weinmann et al. 2006).
http://arxiv.org/abs/0704.0931v1
– 2 –
Ever since the seminal work by Davies et al. (1983), however, it has become clear that early-
type galaxies encompasses two distinct families. Davies et al. showed that bright ellipticals typically
have little rotation, such that their flattening must originate from anisotropic pressure. This is
consistent with bright ellipticals being in general triaxial. Low luminosity ellipticals, on the other
hand, typically have rotation velocities that are consistent with them being oblate isotropic rotators.
With the advent of CCD detectors, it quickly became clear that these different kinematic classes also
have different morphologies. Although ellipticals have isophotes that are ellipses to high accuracy,
there are small deviations from perfect ellipses (e.g., Lauer 1985; Carter 1987; Bender & Möllenhoff
1987). In particular, bright, pressure-supported systems typically have boxy isophotes, while the
lower luminosity, rotation-supported systems often reveal disky isophotes (e.g., Bender 1988; Nieto
et al. 1988). With the high angular resolution of the Hubble Space Telescope it has become clear
that both types have different central surface brightness profiles as well. The bright, boxy ellipticals
typically have luminosity profiles that break from steep outer power-laws to shallow inner cusps
(often called ‘cores’). The fainter, disky ellipticals, on the other hand, have luminosity profiles that
lack a clear break and have a steep central cusp (e.g., Jaffe et al. 1994; Ferrarese et al. 1994; Lauer
et al. 1995; Gebhardt et al. 1996; Faber et al. 1997; Rest et al. 2001; Ravindranath et al. 2001;
Lauer et al. 2005).
The isophotal shapes of early-type galaxies have also been found to correlate with the radio
and X-ray properties of elliptical galaxies (Bender et al. 1989; Pellegrini 1999). Objects which
are radio-loud and/or bright in soft X-ray emission generally have boxy isophotes, while disky
ellipticals are mostly radio-quiet and faint in soft X-rays. As shown in Pellegrini (2005), the soft
X-ray emission of power-law (and hence disky) ellipticals is consistent with originating from X-ray
binaries. Ellipticals with a central core (which are mainly boxy), however, often have soft X-ray
emission in excess of what may be expected from X-ray binaries. This emission originates from
a corona of hot gas which is distributed beyond the optical radius of the galaxy (e.g., Trinchieri
& Fabbiano 1985, Canizares et al. 1987; Fabbiano 1989). In terms of the radio and hard X-ray
emission, thought to originate from active galactic nuclei (AGN), it is found that those ellipticals
with the highest luminosities in radio and/or hard X-rays are virtually always boxy (Bender et
al. 1989; Pellegrini 2005). This is consistent with the results of Ravindranath et al. (2001), Lauer
et al. (2005) and Pellegrini (2005), all of whom find a somewhat higher fraction of ellipticals with
optical AGN activity (i.e., nuclear line emission) among cored galaxies.
The above mentioned trends between isophotal shape and galaxy properties have mainly been
based on relatively small, somewhat heterogenious samples of relatively few objects ( <∼ 100). Re-
cently, however, Hao et al. (2006a, hereafter H06) compiled a sample of 847 nearby, early-type
galaxies from the Sloan Digital Sky Survey (SDSS) for which they measured the isophotal shapes.
Largely in agreement with the aforementioned studies they find that (i) more luminous galaxies
are on average rounder and are more likely to have boxy isophotes (ii) disky ellipticals favor field
environments, while boxy ellipticals prefer denser environments, and (iii) disky ellipticals tend to
lack powerful radio emission, although this latter trend is weak.
– 3 –
The prevailing idea as to the origin of this disky-boxy dichotomy is that it reflects the galaxy’s
assembly history. Within the standard, hierarchical formation picture, in which ellipticals are
formed via mergers, the two main parameters that determine whether an elliptical will be boxy
and cored or disky and cuspy are the progenitor mass ratio and the progenitor gas mass fractions.
Pure N -body simulations without gas show that the isophotal shapes of merger remnants depend
sensitively on the progenitor mass ratio: major mergers create ellipticals with boxy isophotes,
while minor mergers mainly result in systems with disky isophotes (Khochfar & Burkert 2005,
Jesseit et al. 2005). As shown by Naab et al. (2006), including even modest amounts of gas
has a dramatic impact on the isophotal shape of equal-mass merger remnants. The gas causes a
significant reduction of the fraction of box and boxlet orbits with respect to collisionless mergers,
and the remnant appears disky rather than boxy. Therefore, it seems that the massive, boxy
ellipticals can only be produced via dry, major mergers. The cores in these boxy ellipticals are
thought to arise from the binding energy liberated by the coalescence of supermassive binary black
holes during the major merger event (e.g., Faber et al. 1997; Graham et al. 2001; Milosavljević et
al. 2002). When sufficient gas is present, however, dissipation and sub-sequent star formation may
regenerate a central cusp. Alternatively, the gas may serve as an energy sink for the binding energy
of the black hole binary, leaving the original stellar cusp largely intact. Thus, following Lauer
et al. (2005), we may summarize this picture as implying that power-laws reflect the outcome of
dissipation and concentration, while cores owe to mixing and scattering.
But what about the correlation between isophotal shape and AGN activity? It is tempting
to believe that this correlation simply derives from the fact that both isophotal shape and AGN
activity may be related to mergers. After all, it is well known that mergers can drive nuclear inflows
of gas, which produce starbursts and feed the central supermassive black hole(s) (Toomre & Toomre
1972, Barnes & Hernquist 1991,1996, Mihos & Hernquist 1994,1996, Springel 2000, Cattaneo et
al. 2005). However, since the onset of such AGN activity requires wet mergers, this would predict
a higher frequency of AGN among disky ellipticals, contrary to the observed trend. Another
argument against mergers being responsible for the AGN-boxiness correlation is that the time scale
for merger-induced AGN activity is relatively short ( <∼ 10
8 yrs) compared to the dynamical time
in the outer parts of the merger remnant. This implies that active ellipticals should reveal strongly
distorted isophotes, which is not the case.
An important hint may come from the strong correlation between the presence of dust (either
clumpy, filamentary, or in well defined rings and disks) and the presence of optical emission line
activity (Tran et al. 2001; Ravindranath et al. 2001; Lauer et al. 2005). Although this suggests
that this dust is (related to) the actual fuel for the AGN activity, many questions remain. For
instance, it is unclear whether the origin of the dust is internal (shed by stellar winds) or external
(see Lauer et al. 2005 for a detailed discussion). In addition, it is not clear why the presence of dust,
and hence the AGN activity, would be more prevalent in boxy ellipticals. One option is that boxy
ellipticals are preferentially central galaxies (as opposed to satellites), so that they are more efficient
at accreting external gas (and dust). This is consistent with the fact that boxy ellipticals (i) are, on
– 4 –
average, brighter, (ii) reside in dense environments (Shioya & Taniguchi 1993; H06), and (iii) more
often contain hot, soft X-ray emitting halos. Another, more benign possibility, is that the relation
between morphology and AGN activity is merely a reflection of the fact that both morphology and
AGN activity depend on the magnitude of the galaxy (or on its stellar or dynamical mass). In this
case, AGN activity is only indirectly related to the morphology of its host galaxy.
In this paper we use the large data set of H06 to re-investigate the correlations between
morphology and (i) luminosity, (ii) dynamical mass, and (iii) emission line activity in the optical,
where we discriminate between AGN activity and star formation. In addition, we also examine to
what extent morphology correlates with X-ray emission (using data from ROSAT), with 1.4GHz
radio emission (using data from FIRST), and with the mass of the dark matter halo in which the
galaxy resides (using a SDSS galaxy group catalog). The outline of this paper is as follows. In § 2
we describe the data of H06; in § 3 we present the fraction of disky galaxies across the full sample
as a function of galaxy luminosity, dynamical mass and environment. In § 4 we split the sample
galaxies according to their activity in the optical, radio and X-rays, and investigate how the disky-
boxy morphology correlates with these various levels of ‘activity’. Finally, in § 5 we summarize
and discuss our findings. Throughout this paper we adopt a ΛCDM cosmology with Ωm = 0.3,
ΩΛ = 0.7, and H0 = 100h kms
−1 Mpc−1. Magnitudes are given in the AB system.
2. Data
2.1. Sample Selection
In order to investigate the interplay among AGN activity, morphology and environment for
early-type galaxies, we have analyzed the sample of H06, which consists of 847 galaxies in the SDSS
DR4 (Adelman-McCarthy et al. 2006) classified as ellipticals (E) or lenticulars (S0). As described
in H06, these objects are selected to be at z < 0.05, in order to ensure sufficient spatial resolution
to allow for a meaningful measurement of the isophotal parameters. In addition, the galaxies are
selected to have an observed velocity dispersion between 200 km s−1 and 420 km s−1 (where the
upper limit corresponds to the largest velocity dispersion that can be reliably measured from the
SDSS spectra), and are not allowed to be saturated. Note that, for the median sample distance,
the fiber radius of 1.5 arcsec corresponds to about 30% of the sample mean effective radius. From
all galaxies that obey these criteria, early-types have been selected by H06 using visual inspection.
Galaxies with prominent dust lanes have been excluded from the final sample in order to reduce
the effects of dust on the isophotal analysis.
– 5 –
2.2. Isophotal Analysis
Isophotes are typically parameterized by their corresponding surface brightness, I0, their semi-
major axis length, a, their ellipticity, ǫ, and their major axis position angle, θ0. In addition, since
isophotes are not perfectly elliptical, it is common practice to expand the angular intensity variation
along the best fit ellipse, δI(θ), in a Fourier series:
δI(θ) =
A′n cosn(θ − θ0) +B
n sinn(θ − θ0)
(e.g., Carter 1987; Jedrzejewski 1987; Bender & Möllenhoff 1987). Only the terms with n = 3 and
n = 4 are usually computed, as the data is often too noisy to reliably measure higher-order terms
(but see Scorza & Bender 1995 and Scorza & van den Bosch 1999). Note that, by definition, the
terms with n = 0, 1, and 2 are equal to zero within the errors. If the isophote is perfectly elliptical,
then A′n and B
n are also equal to zero for n ≥ 3. Non-zero A′3 and B′3 express deviations from
a pure ellipse that occur along the observed isophote every 120o. Typically, such deviations arise
from the presence of dust features or clumps. The most important Fourier coefficient, however, is
A′4, which quantifies the deviations taking place along the major and minor axes. Isophotes with
A′4 < 0 have a ‘boxy’ shape, while those with a positive A
4 parameter are ‘disk’-shaped.
For each of the 847 E/S0 galaxies in their sample H06 measured the isophotal parameters
using the IRAF1 task ELLIPSE. In particular, for each galaxy they provide the ellipticity, ǫ, the
position angle of the major axis, θ0, and the third and fourth order Fourier coefficients A3 and A4,
which are equal to A′3 and A
4, respectively, divided by the semi-major axis length and the local
intensity gradient. All the available parameters are intensity-weighted averages over the radial
interval 2Rs < R < 1.5R50. Here Rs is the seeing radius (typically lower than 1.5 arcsec, Stoughton
et al. 2002) and R50 is the Petrosian half-light radius
2. The Petrosian radius is defined as the
radius at which the ratio of the local surface brightness to the mean interior surface brightness is
0.2 (cf. Strauss et al. 2002). Therefore, R50 is the radius enclosing half of the flux measured within
a Petrosian radius and can be used as a proxy for the galaxy effective radius Re. In what follows,
we refer to galaxies with A4 ≤ 0 and A4 > 0 as ‘boxy’ and ‘disky’, respectively.
In their seminal papers on the isophotal shapes of elliptical galaxies, Bender & Möllenhoff
(1987), Bender et al. (1988) and Bender et al. (1989) define alternative structural parameters,
an/a and bn/a, which are related to the An and Bn parameters defined here as
1− ǫAn
1− ǫBn (2)
(Bender et al. 1988; Hao et al. 2006b).
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Asssociation of
Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation
2These data are publicly available at http://www.jb.man.ac.uk/∼smao/isophote.html
http://www.jb.man.ac.uk/~smao/isophote.html
– 6 –
2.3. Additional data
For all galaxies in the H06 sample we determined the absolute magnitudes in the SDSS g, r and
i bands, corrected for Galactic extinction, and K-corrected to z = 0, using the luminosity distances
corrected for Virgo-centric infall of the Local Group (LG) following Blanton et al. (2005). In order
to allow for a comparison with the samples of Bender et al. (1989) and Pellegrini (1999, 2005), we
transform these magnitudes to those in the Johnson B-band using the filter transformations given
by Smith et al. (2002).
We also estimated, for each galaxy, the total dynamical mass as
Mdyn = A
σ2corrR50
Here G is the gravitational constant, A is a normalization constant, and σcorr is the velocity dis-
persion measured from the SDSS spectra corrected for aperture effects using
σcorr = σmeasured
Rfiber
R50/8
)0.04
, (4)
with Rfiber = 1.5 arcsec (Bernardi et al. 2003). The aperture correction is meant to give the velocity
dispersion within R50/8, and to make comparable galaxies at different distance but sampled with a
spectroscopic fiber of fixed size. Throughout this paper we adopt A = 5, which has been shown to
accurately reproduce the total dynamical masses inferred from more accurate modeling (Cappellari
et al. 20063). Note that Cappellari et al. have also shown that these dynamical masses are roughly
proportional to the stellar masses of early-type galaxies.
H06 cross-correlated their E/S0 sample with the FIRST radio survey (Becker, White & Helfand
1995), which yielded the 1.4GHz fluxes for 162 objects in the sample. In order to investigate the
relation between isophotal structure and soft X-ray properties, we also matched the H06 sample to
the ROSAT All Sky Survey Catalog (Voges et al. 1999). This yields ROSAT/PSPC count-rates in
the 0.1 – 2.4 keV energy band for 40 sample galaxies. We used the WebPIMMS tool4 to transform
the observed count-rates into astrophysical fluxes, corrected for Galactic extinction, assuming an
X-ray power-law spectrum with energy index αX = 1.5 (cf. Anderson et al. 2003). In addition,
we cross-identified the H06 sample with the spectroscopic catalogs released for DR4 by Kauffmann
et al. (2003a,b), and extracted, when available, the luminosity of the [OIII] λ5007 line corrected
for dust extinction (in ergs−1), the line-flux ratios [OIII]/Hβ and [NII]/Hα, and the S/N values
associated with the [OIII] and Hα fluxes.
3Cappellari et al. (2006) use a slightly different definition of σcorr in equation (2), namely that measured within
Re rather than R50/8. Given the weak dependence of the velocity dispersion on the enclosed radius, we estimate
that this difference results in an offset of ∼0.07 dex in Mdyn
4http://heasarc.gsfc.nasa.gov/Tools/w3pimms.html
– 7 –
Finally, in order to assess the environment of the sample galaxies, we cross-identified the
H06 sample with the SDSS group catalog of Weinmann et al. (2006; hereafter WBYM), which is
constructed using the halo-based group finder of Yang et al. (2005). This yields group (i.e. dark
matter halo) masses for a total of 431 galaxies, distributed over 403 groups. Of these, 350 are
‘central’ galaxies (defined as the brightest galaxy in its group) and 81 are ‘satellites’. As for the
groups, 83 have just a single member (the early-type galaxy in our sample), while 320 groups have
2 or more members. The fact that only 51 percent of the galaxies in the H06 sample are affiliated
with a group is due to the fact that the WBYM group catalog is based on the DR2, and to the fact
that not all galaxies can be assigned to a group (see Yang et al. 2005 for details).
3. The disky fraction across the sample
The main properties of the full H06 sample (comprising all 847 early-type galaxies) are summa-
rized in Figure 1. The sample spans about 3 orders of magnitude in MB (−17.8 > MB −5 log(h) >
−21.4) and a range of about 1.5 dex in dynamical mass (10.5 < log[Mdyn(h−1M⊙)] < 12) and 3 dex
in group (halo) mass (11.8 < log[Mgroup(h
−1M⊙)] < 15). As expected, the B-band magnitude is
well correlated with the dynamical mass, independent of whether the galaxy is a central galaxy or a
satellite, and independent of whether it is disky or boxy. The absolute magnitudes and dynamical
masses of satellite galaxies are clearly separated from those of the central galaxies when plotted as
function of the group (halo) mass. This simply reflects that centrals are defined as the brightest
(and, hence, most likely the most massive) group members. This clear segregation disappears when
the galaxies are split in disky and boxy systems (lower panels), indicating that there is no strong
correlation between morphology and group hierarchy.
The upper panels of Figure 2 show scatter plots of MB, Mdyn and Mgroup as function of the
isophotal parameter A4. They indicate that the fraction of disky systems (those with A4 > 0)
increases with decreasing luminosity and dynamical mass, in qualitative agreement with Bender et
al. (1989) and H06. In the case of Mgroup, a similar trend seems to be present, but only for the
central galaxies. In order to quantify these trends, we have computed the fraction, fdisky, of disky
galaxies as a function of MB , Mdyn and Mgroup. For each bin in absolute magnitude, dynamical
mass, or group mass, fdisky is defined as the number ratio between disky galaxies and the total
number of galaxies in that bin. Each bin contains at least ten disky galaxies. For comparison, the
disky fraction of the full H06 sample is 0.66.
The lower left-hand panel of Figure 2 plots fdisky as function of MB . The errorbars are
computed assuming Poisson statistics. The fraction of disky galaxies declines by a factor of about
1.6 from ∼ 0.8 at MB − 5 log(h) = −18.7 to ∼ 0.5 at MB − 5 log(h) = −20.8, and is well fitted by
fdisky(MB) = (0.61 ± 0.02) + (0.17 ± 0.03) [MB − 5 log(h) + 20] (5)
which is shown as the solid, grey line. Note that this relation should not be extrapolated to arbitrary
faint and/or bright magnitudes. Since 0 ≤ fdisky ≤ 1 it is clear that fdisky(MB) must flatten at
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both ends of the magnitude distribution. Apparently the magnitude range covered by our sample
roughly corresponds to the range in which the distribution transits (relatively slowly and smoothly)
from mainly disky to mainly boxy.
It has to be noted that the exact relation between fdisky and MB is somewhat sensitive to the
exact sample selection criteria, and equation (5) therefore has to be used with some care.
We have tested the robustness of the above relation by adding Gaussian deviates to each
measured value of A4, and then recomputing the best-fit relation between fdisky and MB . Figure
3 shows the slope and zero-point of this relation as function of the standard deviation of the
Gaussian deviates used (filled circles). The grey shaded horizontal bar represents the 1 σ interval
around the best-fit slope (left-hand panel) and the best-fit zero-point (right-hand panel). The grey
shaded vertical bar indicates the mean uncertainty on the observed A4 parameter obtained by
H06 (σ(A4) = 0.0012 ± 0.0008). Note that the best-fit slope and zero-point are extremely robust.
Adding an artificial error to the A4 measurements with an amplitude that is a factor five larger
than the average error quoted by H06 yields best-fit values that agree with those of equation (5)
at better than the 1σ errorbar on these parameters obtained from the fit.
The middle panel in the lower row of Figure 2 plots fdisky as function of Mdyn. As with the
luminosity, the disky fraction decreases smoothly with increasing dynamical mass, dropping from
∼ 0.80 at Mdyn = 6 × 1010h−1 M⊙ to ∼ 0.45 at Mdyn = 3 × 1011h−1 M⊙. The grey, dashed line
indicates the best-fit log-linear relation, which is given by
fdisky(Mdyn) = (0.73 ± 0.02) − (0.53 ± 0.08) log
1011h−1 M⊙
As for equation (5), the Gaussian-deviates test shows that this relation is robust against uncertain-
ties in the A4 measurements.
As is well-known from the morphology-density relation (e.g., Dressler 1980), early-type galaxies
preferentially reside in denser environments and hence in more massive halos (e.g, Croton et al. 2005;
Weinmann et al. 2006). It is interesting to investigate whether the halo mass also determines
whether the early-type galaxies are disky or boxy. We can address this using the WBYM group
catalog described in §2.3. The lower right-hand panel of Figure 2 plots the disky fraction of centrals
(crosses) and satellites (open triangles) as function of group mass. The fraction of disky centrals
decreases with increasing group (halo) mass, declining from ∼ 0.82 at Mgroup = 1.7 × 1012h−1 M⊙
to ∼ 0.54 at Mgroup = 5.0 × 1013h−1 M⊙. For the most massive groups, we have enough satellite
galaxies to also compute their disky fraction. Interestingly, these are larger (though only marginally
so) than those of central galaxies in groups of the same mass.
Although these results seem to suggest that group mass and group hierarchy (i.e., central vs.
satellite) play a role in determining the morphology of an early-type galaxy, they may also simply
be reflections of the fact that (i) satellite galaxies are fainter than central galaxies in the same
parent halo, (ii) fainter centrals typically reside in lower mass halos (cf. Figure 1), and (iii) fainter
galaxies have a larger fdisky. In order to discriminate between these options we proceed as follows.
– 9 –
Under the null-hypothesis that the isophotal structure of an early-type galaxy is only governed by
the galaxy’s absolute magnitude or dynamical mass, the predicted fraction of disky systems for a
given sub-sample is simply
fdisky,0 =
fdisky(Xi) (7)
where Xi is either MB − 5 log(h) or log(Mdyn) of the ith galaxy in the sample, and fdisky(X) is the
average relation between fdisk and X. The grey solid and dashed lines in the lower right-hand panel
of Figure 2 show the fdisky,0(Mgroup) thus obtained, using equations (4) and (5), respectively. These
are perfectly consistent with the observed trends (for both the centrals and the satellites). A possible
exception is the disky fraction of central galaxies in groups with Mgroup < 3.0× 1012h−1M⊙, which
is ∼ 2.5σ higher than predicted by the null-hypothesis. Overall, however, these results support
the null-hypothesis that the morphology of an early-type galaxy depends only on its luminosity or
dynamical mass: there is no significant indication that group mass and/or group hierarchy have a
direct impact on the morphology of early-type galaxies.
4. The disky fraction of active early-type galaxies
4.1. Defining different activity classes
In the standard unified model, AGN are distinguished in AGN of Type I when the central black
hole, its continuum emission and its broad emission-line region are viewed directly, and Type II, if
the central engine is obscured by a dusty circumnuclear medium. Our sample of early-type galaxies
does not contain any Type I AGN, simply because these systems are not part of the main galaxy
sample in the SDSS. However, the E/S0 sample of H06 is not biased against Type II AGN. In order
to identify these systems, one needs to be able to distinguish them from early-types with some
ongoing, or very recent, star formation, which also produces narrow emission lines. Since stars and
AGN produce different ionization spectra, one can discriminate between them by using line-flux
ratios. In particular, star formation and AGN activity can be fairly easily distinguished using the
so-called BPT diagram (after Baldwin, Phillips & Terlevich 1981; see also Veilleux & Osterbrock
1987), whose most common version involves the line-flux ratios [OIII]/Hβ and [NII]/Hα.
Figure 4 plots the BPT diagram for those sample galaxies whose [OIII] λ5007 and Hα lines
have been detected with a signal-to-noise ratio S/N ≥ 3. The solid curve was derived by Kauffmann
et al. (2003b) and separates star-forming galaxies from type II AGN, with the latter lying above the
curve. We follow Kauffmann et al. (2003b) and split the Type II AGN into Seyferts, LINERS, and
Transition Objects (TOs) according to their line-flux ratios: Type II Seyferts have log([OIII]/Hα) ≥
0.5 and log([NII]/Hα) ≥ −0.2, LINERS have log([OIII]/Hα) < 0.5 and log([NII]/Hα) ≥ −0.2, and
all galaxies with log([NII]/Hα) < −0.2 and laying above the curve are labelled TO. Kewley et al.
(2006) have recently studied in detail the properties of LINERs and type II Seyferts, and found
that LINERs and Seyferts form a continuous progression in the accretion rate L
, with LINERs
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dominating at low L
and Seyferts prevailing at high L
. The results obtained by Kewley et
al. suggest that most LINERs are AGN and require a harder ionizing radiation field together with
a lower ionization parameter than Seyferts.
In order to increase the statistics of our subsequent analysis, we have organized the 847 galaxies
in the H06 sample into 3 categories:
1. AGN: This class consists of 28 early-type galaxies with a Seyfert-like activity and 286 early-
type galaxies with a LINER-like activity.
2. Emission-line (EL): This class consists of those galaxies that according to the BPT diagram
are star formers or transition objects, as well as those galaxies that lack one or both of the
BPT line-flux ratios, but that have an [OIII] emission line with a S/N ≥ 3. There are a total
of 383 early-type galaxies in the H06 sample that fall in this category.
3. Non-active (NA): These are the 150 galaxies that are not in the AGN or EL categories.
Therefore, these galaxies either have no emission lines at all, or have a detected [OIII] line
but with a S/N < 3. Among these, 43 objects (29 percent) show Hα emission with a S/N
≥ 3. Their presence could signal a problem with the spectroscopic pipeline, which failed to
properly measure the [OIII] line, or be real and due to an episode of star formation in its
early phases. In any case, their low S/N in [OIII] prevents us from classifying these galaxies
in one of the above two categories.
Given our aim to establish the presence/absence of a correlation between the AGN activity
and the disky/boxy morphology of the host early-type galaxy, the classification above is clearly
driven by the detection of the [OIII] line emission, which is commonly used as a proxy for the AGN
strength (cf. Kauffmann et al. 2003b, Kewley et al. 2006).
Along with these 3 categories which describe the galaxy activity in the optical, we have also
defined two additional activity classes: ‘FIRST’, which consists of the 162 sample galaxies with a
1.4GHz flux in the FIRST catalog (Becker et al. 1995), and ‘ROSAT’, containing the 40 sample
galaxies that have been detected in the ROSAT All Sky Survey (Voges et al. 1999). The soft
X-ray luminosities of these ROSAT galaxies span the range 41.3 < log[LX/(ergs
−1)] < 42.7 and
are consistent (though with large scatter), with the well known LX ∝ L2B relation (Trinchieri &
Fabbiano 1985; Canizares et al. 1987). This X-ray emission is therefore associated with a hot corona
surrounding the galaxy, rather than with X-ray binaries, and we can use it to indirectly probe the
environment where galaxies live. As shown by Bender et al. (1989) and O’Sullivan et al. (2001),
the LX ∝ L2B relation applies to X-ray luminosities between 10
40 and 1043 erg s−1. Our ROSAT
category is thus somehow incomplete at 40 < log[LX/(ergs
−1)] < 41 and the trends discussed below
for this class should be taken with some caution.
Table 1 lists the number of galaxies in each of these five activity classes. Note that the AGN,
EL and NA classes are mutually exclusive, but that a galaxy in each of these three classes can
– 11 –
appear also in the FIRST and ROSAT sub-samples. The vast majority of the galaxies detected by
FIRST or ROSAT reveal activity also in the optical, and are classified as either AGN or EL. The
radio and soft X-ray detections themselves, however, are not well correlated: only 12 percent of the
galaxies detected by FIRST have also been detected in soft X-rays.
Before computing fdisky for the galaxies in these various activity classes, it is useful to examine
how their respective distributions in MB, Mgroup and L[OIII] compare. This is shown in Figures 5,
6 and 7, respectively. While the luminosity distributions of the AGN and EL galaxies are in good
agreement with that of the full sample, the galaxies detected by ROSAT are on average about half
a magnitude brighter than the galaxies in the full sample. Also the non-active and radio galaxies
are brighter than average, though the differences are less pronounced. McMahon et al. (2002)
estimated a limiting magnitude of R ≃ 20 for the optical counterparts of FIRST sources at a 97
percent completeness level. Since the apparent magnitude limit of the H06 sample is brighter than
this limit, the FIRST subsample extracted from H06 is to be considered complete. Therefore, the
shift towards higher luminosities for the galaxies detected by FIRST with respect to the full sample
in Figure 5 is real rather than an artifact due to the depth of the different surveys.
Similar trends are present with respect to the group masses: whereas AGN and EL galaxies
have group masses that are very similar to those of the full sample, galaxies detected by ROSAT and
FIRST seem to prefer more massive groups. Somewhat surprisingly, the same applies to the class
of non-active galaxies. As for the luminosity of their [OIII] line plotted in Figure 7, AGN galaxies
tend to be brighter than EL and ROSAT galaxies, while the [OIII] luminosities of FIRST galaxies
are consistent with those of the EL and AGN galaxies combined (grey shaded histogram). In
agreement with Best et al. (2005), no correlation is found between the radio and [OIII] luminosities
of the sample galaxies in common with FIRST. Finally, it is worth emphasizing that the optical
activity defined in this paper occurs at log(L[OIII]/L⊙) ≥ 4.6; therefore, the class of non-active
galaxies may also contain weak AGN with [OIII] fluxes below this limit. Using the KS-test, we
have investigated whether the various distributions are consistent with being drawn from the same
parent distribution. We have found that only EL and ROSAT are consistent, in terms of their [OIII]
luminosity, with belonging to the same population, as well as the pairs (AGN,EL) and (NA,FIRST)
in terms of their absolute magnitude, the pair (AGN,EL) with respect to their dynamical mass,
and the pairs (AGN,EL), (NA,FIRST), (NA,ROSAT) and (FIRST,ROSAT) in terms of their group
halo mass.
Another aspect of defining different modes of activity is to study their actual frequency, i.e.
the fraction of galaxies sharing the same kind of activity (with respect to the full sample) as a
function of MB , Mdyn and environment. This is plotted in Figure 8, where the percentage of
NA, FIRST and ROSAT galaxies increases by a factor of about 4 towards higher luminosities and
larger dynamical masses (cf. Best et al. 2005, O’Sullivan et al. 2001), and by a factor of about 3
as their hosting group halo becomes more massive. EL and AGN galaxies define a far less clear
picture; EL galaxies seem to occur at any MB and Mgroup with a constant frequency, while their
fraction decreases by a factor of about 1.5 as Mdyn gets larger. The percentage of AGN galaxies
– 12 –
drops by a factor of about 2 at brighter MB values. It very weakly decreases in massive group
halos, and appears quite insensitive to Mdyn. As for the hierarchy inside a group, there is a weak
indication that EL, FIRST and ROSAT galaxies are preferentially associated with central galaxies,
while satellite galaxies are more frequently NA and AGN galaxies. Within the Poisson statistics,
however, none of these trends with group hierarchy is significant.
4.2. The relation between activity and morphology
A first glance at how morphology varies with activity is provided by Table 2, which lists
fdisky for the 5 classes defined in §4.1. As for Table 1, AGN, EL and NA galaxies are mutually
exclusive, while any of them can be included in the FIRST and ROSAT categories. In this case,
fdisky is derived from the pool of galaxies common to FIRST (ROSAT) and one of the optically
active sub-samples. The ROSAT galaxies are clearly biased towards boxy shapes as their fdisky
is systematically lower than ∼ 0.50. AGN and NA galaxies with or without radio emission are
generally disky (with fdisky > 0.60). The radio emission seems to make a difference in the case
of EL galaxies: while the full sub-sample of ELs is as disky as AGN and NA galaxies, those ELs
detected by FIRST are dominated by boxy systems with fdisky ≃ 0.45.
The upper panels of Figure 9 show scatter plots of the [OIII] luminosity, radio luminosity and
X-ray luminosity as function of A4. The lower panels plot the corresponding fractions of disky
systems. In the lower left-hand panel, fdisky is plotted as function of L[OIII] for both AGN (filled
squares) and EL (filled triangles) galaxies. This shows that both AGN and EL galaxies have a disky
fraction that is consistent with that of the full H06 sample (fdisky = 0.66), and with no significant
dependence on the actual [OIII] luminosity. The grey lines (solid for AGN and dotted for EL
galaxies) indicate the disky fractions predicted under the null-hypothesis that fdisky is a function
only of MB . These predictions are in excellent agreement with the data, suggesting that the (level
of) optical activity does not help in better predicting the disky/boxy morphology of an early-type
galaxy. The only possible exception is the sub-sample of EL galaxies with log(L[OIII]/L⊙) < 5.2
which has fdisky = 0.54, approximately 3σ lower than given by the null-hypothesis.
The relatively small number of sample galaxies detected by FIRST and ROSAT prevents us
from applying the above analysis as a function of radio and/or soft X-ray luminosity. Instead
we have determined fdisky separately for the sub-samples of galaxies detected and not-detected by
FIRST or ROSAT. The results are shown in the lower middle and lower right-hand panels of Figure
9. Clearly, the disky fraction of galaxies detected by ROSAT (fdisky = 0.48 ± 0.08) is significantly
lower than those with no detected soft X-ray flux (fdisky = 0.66 ± 0.02), in agreement with the
results of Bender et al. (1989) and Pellegrini (1999, 2005). The fact that galaxies detected by
ROSAT are more boxy is expected since they are significantly brighter than those with no soft
X-ray detection (cf. Figure 5). The grey lines, which correspond to fdisky,0(MB), indicate that this
explains most of the effect. Although it is intriguing that the disky fraction of ROSAT detections
is ∼ 1σ lower than predicted, a larger sample of early-type galaxies with soft X-ray detections is
– 13 –
needed to rule out (or confirm) the null-hypothesis. As for the galaxies detected by FIRST, there is
a weak indication that these galaxies have a somewhat lower fdisky: this finding is again in excellent
agreement with the predictions based on the null-hypothesis. Therefore, there is no indication that
the morphology of an early-type galaxy is directly related to whether the galaxy is active in the
radio or not.
To further test the null-hypothesis that the isophotal structure of early-type galaxies is entirely
dictated by their absolute magnitude or dynamical mass, we have derived fdisky of NA, EL and
AGN galaxies in bins of MB and Mdyn. The results are shown in Figure 10 (symbols), which shows
that the disky fraction of all three samples decreases with increasing luminosity and dynamical
mass. The grey solid and dashed lines indicate the predictions based on the null-hypothesis, which
have been computed using equations (5)–(7). Overall, these predictions are in excellent agreement
with the data, indicating that elliptical galaxies with ongoing star formation or with an AGN do
not have a significantly different morphology (statistically speaking) than other ellipticals of the
same luminosity or dynamical mass.
Finally, in Figure 11 we plot the disky fractions of NA, EL and AGN galaxies as function of
their group mass (upper panels) and group hierarchy (lower panels). For comparison, the grey
solid and dashed lines indicate the predictions based on the null-hypothesis. Although overall these
predictions are in good agreement with the data, there are a few noteworthy trends. At the massive
end (Mgroup >∼ 10
13h−1 M⊙) the disky fraction of AGN is higher than expected, while that of NA
galaxies is lower. The lower panels show that this mainly owes to the satellite galaxies in these
massive groups. Whereas the null-hypothesis accurately predicts the disky fractions of NA, EL and
AGN centrals, it overpredicts fdisky of NA satellites, while underpredicting that of AGN satellites
at the 3 σ level. These results clearly warrant a more detailed investigation with larger samples.
Note that only about half of the 847 galaxies in the H06 sample are also in our group catalog. A
future analysis based on larger SDSS samples and a more complete group catalog would sufficiently
boost the statistics to examine the trends identified here with higher confidence.
5. Discussion and conclusions
In spite of their outwardly bland and symmetrical morphology, early-type galaxies reveal a
far more complex structure, whose isophotes usually deviate from a purely elliptical shape. As
shown by Bender et al. (1989), these deviations correlate with other parameters; for example, boxy
early-type galaxies are on average brighter and bigger than disky galaxies and are supported by
anisotropic pressure. Early-type galaxies with disky isophotes, on the other hand, are consistent
with being isotropic oblate rotators. With the advent of large galaxy redshift surveys such as
the SDSS, it is now possible to collect large and homogeneous samples of early-type galaxies and
quantify these correlations in much greater detail. In addition, it also allows for a detailed study
of the relation between morphology and environment.
– 14 –
We have used a sample of 847 early-type galaxies imaged by the SDSS and analyzed by Hao et
al. (2006a) to study the fraction of disky galaxies (fdisky) as a function of their absolute magnitude
MB , their dynamical mass Mdyn and the mass of the dark matter halo Mgroup in which they are
located. Using the Hα, Hβ, [OIII] and [NII] emission lines in the SDSS spectra we have split the
sample in AGN galaxies, emission-line (EL) galaxies, and non-active (NA) galaxies (see Figure 4).
In addition we also constructed two sub-samples of those ellipticals that have also been detected in
the radio (in FIRST) or in soft X-rays (with ROSAT), and we have analyzed the relations between
fdisky and the level of AGN activity in the optical and the radio, and the strength of soft X-ray
emission.
The fraction of disky galaxies in the full sample decreases strongly with increasing luminosity
and dynamical mass (see Figure 2). More quantitatively, fdisky decreases from ∼ 0.8 at MB −
5 log(h) = −18.6 (Mdyn = 6 × 1010h−1M⊙) to ∼ 0.5 at MB − 5 log(h) = −20.6 (Mdyn = 3 ×
1011h−1M⊙). This indicates a smooth transition between disky and boxy shapes, which is well
represented by a log-linear relation between fdisky and luminosity or dynamical mass (at least over
the ranges probed here). The relatively large sample allows us to measure these relations with a
good degree of accuracy that is robust against the uncertainties involved in the measurement of
the A4 parameter.
We have used these log-linear relations to test the null-hypothesis that the isophotal shape of
early-type galaxies depends only on their absolute magnitude or dynamical mass. The main result
of this paper is that the data is fully consistent with this simple ansatz, and that the correlations
seen among group mass, group hierarchy (central vs. satellite), soft X-ray emission, activity (both
in the optical and in the radio) and the disky/boxy morphology of an early-type galaxy reflect
the dependence of each of these properties on galaxy luminosity. In fact, the luminosity (mass)
dependence of fdisky predicts, with good accuracy, the following observed trends:
1. The variation of fdisky of central and satellite galaxies in the sample as a function of their
group halo mass (see Figure 2).
2. The constancy of fdisky of EL and AGN galaxies with respect to their [OIII] luminosity (see
Figure 9).
3. The decreasing fdisky of NA, EL and AGN galaxies with increasing MB and Mdyn (see Figure
4. The dependence of fdisky of NA, EL and AGN galaxies on their group halo mass and hierarchy
(see Figure 11).
5. The average value of fdisky among those sample galaxies detected by ROSAT and FIRST.
The fact that our null-hypothesis is also consistent with the fraction of disky radio-emitters con-
tradicts Bender et al. (1989), who wrote that “the isophotal shape is the second parameter besides
– 15 –
luminosity determining the occurance of radio activity in ellipticals”. We claim instead, using a
much larger, more homogeneous sample, that the radio activity is merely a reflection of the multi-
variate dependence of radio activity, luminosity and morphology. We have further checked this
result using an inverse approach, based on the fradio - MB relation. Briefly, we derived, for the full
sample, the fraction of radio galaxies (fradio) with respect to the total as a function of MB , and
obtained a log-linear relation whereby fradio smoothly increases from 0.06 at MB−5 log(h) = −18.7
to 0.34 at MB − 5 log(h) = −20.7. The fraction of radio galaxies among disky and boxy galaxies in
the full sample turns out to be fradio(disky) = 0.17 ± 0.02 and fradio(boxy) = 0.23 ± 0.02. Entering
the mean absolute magnitude of disky and boxy galaxies in the fradio - MB relation, we obtain radio
fractions of 0.18 and 0.21, respectively, well within 1 σ from the observed values.
Although the data is in good overall agreement with the null-hypothesis, there are a few weak
deviations at the 1 (3 at most) σ level (throughout errors have been computed assuming Poisson
statistics). First of all, emission line galaxies with log(L[OIII]/L⊙) < 5.2 have a disky fraction
that is ∼ 3σ lower than predicted by the null-hypothesis. Note however, that for higher [OIII]
luminosities, the null-hypothesis is in excellent agreement with the disky fraction of EL galaxies.
Another mild discrepancy between data and null-hypothesis regards the disky fraction of ellipticals
detected by ROSAT, which is ∼ 1σ lower than predicted. Finally, the disky fraction of NA and
AGN satellites in groups with Mgroup >∼ 10
13h−1 M⊙ are slightly too high and low, with respect
to the null-hypothesis, respectively. Whether these discrepancies indicate a true shortcoming of
the null-hypothesis, and thus signal that the isophotal shape of early-type galaxies depends on
additional parameters, requires a larger sample even. In the relatively near future, the final SDSS
should be able to roughly double the size of the sample used here, while a group catalog of this final
SDSS should increase the statistics regarding the environmental dependencies by an even larger
amount.
The relations between fdisky and MB (Mdyn) derived here provide a powerful test-bench for
theories of galaxy formation. In particular, they can be used to constrain the nature and the
merging history of the progenitors of present-day early-type galaxies. In a follow-up paper, we
will use semi-analytical models featuring AGN and supernova feedback in order to predict and
understand the observed log-linear relations in terms of the amount of cold gas in the progenitors
at the time of the last merger and their mass ratio (Kang et al., in prep).
AP acknowledges useful discussions with Sandra Faber and John Kormendy. We thank an
anonymous referee for his/her useful comments on the paper. Funding for the creation and distri-
bution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating
Institutions, the National Aeronautics and Space Administration, the National Science Foundation,
the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The
SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research
Consortium (ARC) for the Participating Institutions. The Participating Institutions are The Uni-
http://www.sdss.org/
– 16 –
versity of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The
Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-
Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New
Mexico State University, University of Pittsburgh, University of Portsmouth, Princeton University,
the United States Naval Observatory, and the University of Washington.
REFERENCES
Adelman-McCarthy, J.K., et al. 2006, ApJS, 162, 38
Anderson, S.F., et al. 2003, AJ, 126, 2209
Baldwin, J., Phillips, M., Terlevich, R. 1981, PASP, 93, 5
Barnes, J.E., Hernquist, L.E. 1991, ApJL, 370, L65
Barnes, J.E., Hernquist, L.E. 1996, ApJ, 471, 115
Becker, R.H., White, R.L., Helfand, D.J. 1995, ApJ, 450, 559
Bender, R. 1988, A&AS, 193, 7
Bender, R., Döbereiner, S., Möllenhoff, C., 1988, A&AS, 74, 385
Bender, R., Möllenhoff, C. 1987, A&A, 177, 71
Bender, R., Surma, P., Döbereiner, S., Möllenhoff, C., Madejsky, R. 1989, A&A, 217, 35
Bernardi, M., et al. 2003, AJ, 125, 1817
Best, P.N., Kauffmann, G., Heckman, T.M., Brinchmann, J., Charlot, S., Ivezić, Z., White, S.D.M.
2005, MNRAS, 362, 25
Blanton, M.R., et al. 2005, AJ, 129, 2562
Bower, R.G., Lucey, J.R., Ellis, R.S. 1992, MNRAS, 254, 601
Canizares, C.R., Fabbiano, G., Trinchieri, G. 1987, ApJ, 312, 503
Cappellari, M., et al. 2006, MNRAS, 366, 1126
Carter, D. 1987, ApJ, 312, 514
Cattaneo, A., Blaizot, J., Devriendt, J., Guiderdoni, B. 2005, MNRAS, 364, 407
Croton, D.J., et al. 2005, MNRAS, 356, 1155
Djorgovski, S., Davis, M. 1987, ApJ, 313, 59
– 17 –
Davies, R.L., Efstathiou, G., Fall, S.M., Illingworth, G., Schechter, P.L. 1983, ApJ, 266, 41
Dressler, A. 1980, ApJ, 236, 351
Dressler, A. 1987, ApJ, 317, 1
Ellis, R.S., Smail, I., Dressler, A., Couch, W.J., Oemler, A.Jr., Butcher, H., Sharples, R.M. 1997,
ApJ, 483, 582
Fabbiano, G. 1989, ARA&A, 27, 87
Faber, S.M., Jackson, R.E. 1976, ApJ, 204, 668
Faber, S.M., et al. 1997, AJ, 114, 1771
Ferrarese, L., van den Bosch, F.C., Ford, H.C., Jaffe, W., O’Connell, R.W. 1994, AJ, 108, 1598
Gebhardt, K., et al. 1996, AJ, 112, 105
Graham, A.W., Erwin, P, Caon, N., Trujillo, I. 2001, ApJ, 563, L11
Hao, C.N., Mao, S., Deng, Z.G., Xia, X.Y., Wu, H. 2006a, MNRAS, 370, 1339
Hao, C.N., Mao, S., Deng, Z.G., Xia, X.Y., Wu, H. 2006b, MNRAS, 373, 1264
Jaffe, W., Ford, H.C., O’Connell, R.W., van den Bosch, F.C., Ferrarese, L. 1994, AJ, 108, 1567
Jedrzejewski, R.I. 1987, MNRAS, 226, 747
Jesseit, R., Naab, Y., Burkert, A. 2005, MNRAS, 360, 1185
Kang, X., van den Bosch, F.C., Pasquali, A., 2007, preprint (arXiv:0704.0932)
Kauffmann, G., et al. 2003a, MNRAS, 341, 33
Kauffmann, G., et al. 2003b, MNRAS, 346, 1055
Kewley, L.J., Groves, B., Kauffmann, G., Heckman, T., 2006, MNRAS, 372, 961
Khochfar, S., Burkert, A. 2005, MNRAS, 359, 1379
Kuntschner, H., et al. 2006, MNRAS, 369, 497
Lauer, T.R. 1985, MNRAS, 216, 429
Lauer, T.R., et al. 1995, AJ, 110, 2622
Lauer, T.R., et al. 2005, AJ, 129, 2138
McMahon, R.G., White, R.L., Helfand, D.J., Becker, R.H. 2002, ApJ Supl.Ser., 143, 1
http://arxiv.org/abs/0704.0932
– 18 –
Mihos, J.C., Hernquist, L.E. 1994, ApJL, 425, L13
Mihos, J.C., Hernquist, L.E. 1996, ApJ, 464, 641
Milosavljević, M., Merritt D., Rest A., van den Bosch F.C. 2002, MNRAS, 331, 51
Morganti, R., et al. 2006, MNRAS, 371, 157
Naab, Y., Jesseit, R., Burkert, A. 2006, MNRAS, in press (astro-ph/0605155)
Nieto, J.-L., Capaccioli, M., Held, E.V. 1988, A&A, 195, 1
O’Sullivan, E., Forbes, D.A., Ponman, T.J. 2001, MNRAS, 328, 461
Pellegrini, S. 1999, A&A, 351, 487
Pellegrini, S. 2005, MNRAS, 364, 169
Ravindranath, S., Ho, L.C., Peng, C.Y., Filippenko, A.V., Sargent, W.L.W. 2001, AJ, 122, 653
Rest, A., et al. 2001, AJ, 121, 2431
Sarzi, M., et al. 2006, MNRAS, 366, 1151
Scorza, C., Bender, R. 1995, A&A, 293, 20
Scorza, C., van den Bosch, F.C. 1998, MNRAS, 300, 469
Serra, P., Trager, S.C., van der Hulst, J.M., Oosterloo, T.A., Morganti, R. 2006, A&A, 453, 493
Shioya, Y., Taniguchi, Y. 1993, PASJ, 43, 39
Smith, J.A. et al. 2002, AJ, 123, 2121
Springel, V. 2000, MNRAS, 312, 859
Stoughton, C., et al., 2002, AJ, 123, 485
Strauss, M.A., et al, 2002, AJ, 124, 1810
Toomre, A., Toomre, J. 1972, ApJ, 178, 623
Trager, S.C., Faber, S.M., Worthey, G., González, J.J. 2000, AJ, 119, 1645
Tran, H.D., Tsvetanov, Z., Ford, H.C., Davies, J., Jaffe, W., van den Bosch, F.C., Rest, A. 2001,
AJ, 121, 2928
Trinchieri, G., Fabbiano, G. 1985, ApJ, 296, 447
Veilleux, S., Osterbrock, D. 1987, ApJS, 63, 295
http://arxiv.org/abs/astro-ph/0605155
– 19 –
Visvanathan, N., Sandage, A. 1977, 216, 214
Voges, W., et al. 1999, A&A, 349, 389
Weinmann, S., van den Bosch, F.C., Yang, X., Mo, H.J. 2006, MNRAS, 366, 2
Yang, X., Mo, H.J., van den Bosch, F.C., Jing, Y.P. 2005, MNRAS, 356, 1293
This preprint was prepared with the AAS LATEX macros v5.2.
– 20 –
Fig. 1.— The distributions in MB , Mdyn and Mgroup for the full sample, split between central and
satellite galaxies (grey crosses and open triangles respectively, in the top panels) and between disky
and boxy galaxies (open triangles and grey crosses respectively, in the bottom panels).
– 21 –
Fig. 2.— Top panels: the distributions of MB , Mdyn and Mgroup as a function of the isophotal
parameter A4 for the full sample, also split between central (grey crosses) and satellite (open
triangles) galaxies. Bottom panels: the fraction of disky galaxies as a function of MB and Mdyn
for the full sample. The fraction of disky galaxies is also shown per bin of group halo mass Mgroup
for central (crosses) and satellite (open triangles) galaxies. The errorbars are at the 1 σ level, and
were computed assuming Poisson statistics. The grey solid and dashed lines in the left hand-side
and middle panels are the best fits to the fractions of disky galaxies across the full sample. The
same lines in the right hand-side panel represent the predicted fractions of disky galaxies from the
working null-hypothesis.
– 22 –
Fig. 3.— Impact of individual A4 measurement errors: the slope and the zero-point of the log-
linear correlation between fdisky and MB (equation 4) are shown as a function of the standard
deviation of the Gaussian used to simulate errors on the observed A4 values. The grey shaded
areas indicate the best-fitting slope (0.17 ± 0.03) and zero-point (0.61 ± 0.02) in equation 4, and
the mean uncertainty on the observed A4 parameter (0.0012 ± 0.0008) as measured by H06.
– 23 –
-1 -0.5 0 0.5 1
Liners
Seyferts
Fig. 4.— The BPT diagram for the galaxies in the full sample, whose [OIII] λ5007 and Hα emission
lines were detected with a S/N ratio larger than 3. These objects have been split among Seyfert
galaxies of type 2, LINERs, Transition Objects (TO) and star-forming (SB) according to Kauffmann
et al. (2003b).
– 24 –
Fig. 5.— The distributions of the full sample (grey shaded area) and the 5 different activity classes
defined in §4.1 in absolute magnitude MB (in AB system). Each distribution is normalized by the
size of the sample from where it was extracted.
– 25 –
Fig. 6.— As in Figure 5, but for the group halo mass Mgroup.
– 26 –
Fig. 7.— As in Figure 5, but for the luminosity in the [OIII] line. Here, the grey shaded histogram
refers to emission-line and AGN galaxies together.
– 27 –
-19 -20 -21
ROSAT
FIRST
11 11.5
12 12.5 13 13.5 14
Group hierarchy
Fig. 8.— The fraction of galaxies in the 5 different activity classes with respect to the full sample
as a function of MB , Mdyn, Mgroup and split between centrals and satellites.
– 28 –
Fig. 9.— Top panels: the distribution of the luminosities in the [OIII] line, at 1.4 GHz and in
the soft X-rays, as a function of the isophotal parameter A4. Emission-line and AGN galaxies
are represented with grey filled triangles and black filled squares respectively. Bottom panels: the
fraction of disky galaxies among emission-line (triangles) and AGN (squares) galaxies as a function
of the luminosity in the [OIII] line (left hand-side panel). The grey solid and dotted lines trace the
predictions from the working null-hypothesis in MB . The fraction of disky galaxies for the galaxies
detected and non-detected by FIRST and ROSAT are shown in the middle and right hand-side
panels, together with the predictions from equations (4) and (5). The errorbars are at the 1 σ level,
and were computed assuming Poisson statistics.
– 29 –
Fig. 10.— The fraction of disky galaxies for non-active (black filled circles), emission-line (black
filled triangles) and AGN (black filled squares) galaxies as a function of MB and Mdyn. The grey
solid and dashed lines represent the predictions from equation (5), i.e. the working null-hypothesis.
The errorbars are at the 1 σ level, and were computed assuming a Poisson statistics.
– 30 –
Fig. 11.— As in Figure 10, but splitting the non-active, emission-line and AGN galaxies between
centrals and satellites.
– 31 –
Table 1. Activity Classes
AGN EL NA FIRST ROSAT
AGN 314 −− −− 91 17
EL −− 383 −− 53 16
NA −− −− 150 18 7
FIRST 91 53 18 162 22
ROSAT 17 16 7 22 40
Note. — The number of sample galaxies in the
five different activity classes. Note that the AGN,
EL and NA classes are mutually exclusive.
– 32 –
Table 2. Fraction of disky galaxies across the activity classes
AGN EL NA FIRST ROSAT
AGN 0.69 −− −− 0.65 0.47
EL −− 0.64 −− 0.45 0.50
NA −− −− 0.63 0.61 0.43
FIRST 0.65 0.45 0.61 0.58 0.54
ROSAT 0.47 0.50 0.43 0.54 0.47
	Introduction
	Data
	Sample Selection
	Isophotal Analysis
	Additional data
	The disky fraction across the sample
	The disky fraction of active early-type galaxies
	Defining different activity classes
	The relation between activity and morphology
	Discussion and conclusions

  We study the dependence of the isophotal shape of early-type galaxies on
their absolute B-band magnitude, their dynamical mass, and their nuclear
activity and environment, using an unprecedented large sample of 847 early-type
galaxies identified in the SDSS by Hao et al (2006). We find that the fraction
of disky galaxies smoothly decreases with increasing luminosity. The large
sample allows us to describe these trends accurately with tight linear
relations that are statistically robust against the uncertainty in the
isophotal shape measurements. There is also a host of significant correlations
between the disky fraction and indicators of nuclear activity (both in the
optical and in the radio) and environment (soft X-rays, group mass, group
hierarchy). Our analysis shows however that these correlations can be
accurately matched by assuming that the disky fraction depends only on galaxy
luminosity or mass. We therefore conclude that neither the level of activity,
nor group mass or group hierarchy help in better predicting the isophotal shape
of early-type galaxies.

Introduction
Early-type galaxies form a remarkably homogeneous class of objects with a well-defined Funda-
mental Plane and with tight relations between colour and magnitude, between colour and velocity
dispersion, and between the amount of α-element enhancement and velocity dispersion (e.g., Faber
& Jackson 1976; Visvanathan & Sandage 1977; Dressler 1987; Djorgovski & Davis 1987; Bower et
al. 1992; Ellis et al. 1997). They have old stellar populations, though sometimes with a younger
component (Trager et al. 2000; Serra et al. 2006; Kuntschner et al. 2006), contain little ionized and
cold gas (Sarzi et al. 2006; Morganti et al. 2006), and are preferentially located in massive dark
matter halos (e.g., Dressler 1980; Weinmann et al. 2006).
http://arxiv.org/abs/0704.0931v1
– 2 –
Ever since the seminal work by Davies et al. (1983), however, it has become clear that early-
type galaxies encompasses two distinct families. Davies et al. showed that bright ellipticals typically
have little rotation, such that their flattening must originate from anisotropic pressure. This is
consistent with bright ellipticals being in general triaxial. Low luminosity ellipticals, on the other
hand, typically have rotation velocities that are consistent with them being oblate isotropic rotators.
With the advent of CCD detectors, it quickly became clear that these different kinematic classes also
have different morphologies. Although ellipticals have isophotes that are ellipses to high accuracy,
there are small deviations from perfect ellipses (e.g., Lauer 1985; Carter 1987; Bender & Möllenhoff
1987). In particular, bright, pressure-supported systems typically have boxy isophotes, while the
lower luminosity, rotation-supported systems often reveal disky isophotes (e.g., Bender 1988; Nieto
et al. 1988). With the high angular resolution of the Hubble Space Telescope it has become clear
that both types have different central surface brightness profiles as well. The bright, boxy ellipticals
typically have luminosity profiles that break from steep outer power-laws to shallow inner cusps
(often called ‘cores’). The fainter, disky ellipticals, on the other hand, have luminosity profiles that
lack a clear break and have a steep central cusp (e.g., Jaffe et al. 1994; Ferrarese et al. 1994; Lauer
et al. 1995; Gebhardt et al. 1996; Faber et al. 1997; Rest et al. 2001; Ravindranath et al. 2001;
Lauer et al. 2005).
The isophotal shapes of early-type galaxies have also been found to correlate with the radio
and X-ray properties of elliptical galaxies (Bender et al. 1989; Pellegrini 1999). Objects which
are radio-loud and/or bright in soft X-ray emission generally have boxy isophotes, while disky
ellipticals are mostly radio-quiet and faint in soft X-rays. As shown in Pellegrini (2005), the soft
X-ray emission of power-law (and hence disky) ellipticals is consistent with originating from X-ray
binaries. Ellipticals with a central core (which are mainly boxy), however, often have soft X-ray
emission in excess of what may be expected from X-ray binaries. This emission originates from
a corona of hot gas which is distributed beyond the optical radius of the galaxy (e.g., Trinchieri
& Fabbiano 1985, Canizares et al. 1987; Fabbiano 1989). In terms of the radio and hard X-ray
emission, thought to originate from active galactic nuclei (AGN), it is found that those ellipticals
with the highest luminosities in radio and/or hard X-rays are virtually always boxy (Bender et
al. 1989; Pellegrini 2005). This is consistent with the results of Ravindranath et al. (2001), Lauer
et al. (2005) and Pellegrini (2005), all of whom find a somewhat higher fraction of ellipticals with
optical AGN activity (i.e., nuclear line emission) among cored galaxies.
The above mentioned trends between isophotal shape and galaxy properties have mainly been
based on relatively small, somewhat heterogenious samples of relatively few objects ( <∼ 100). Re-
cently, however, Hao et al. (2006a, hereafter H06) compiled a sample of 847 nearby, early-type
galaxies from the Sloan Digital Sky Survey (SDSS) for which they measured the isophotal shapes.
Largely in agreement with the aforementioned studies they find that (i) more luminous galaxies
are on average rounder and are more likely to have boxy isophotes (ii) disky ellipticals favor field
environments, while boxy ellipticals prefer denser environments, and (iii) disky ellipticals tend to
lack powerful radio emission, although this latter trend is weak.
– 3 –
The prevailing idea as to the origin of this disky-boxy dichotomy is that it reflects the galaxy’s
assembly history. Within the standard, hierarchical formation picture, in which ellipticals are
formed via mergers, the two main parameters that determine whether an elliptical will be boxy
and cored or disky and cuspy are the progenitor mass ratio and the progenitor gas mass fractions.
Pure N -body simulations without gas show that the isophotal shapes of merger remnants depend
sensitively on the progenitor mass ratio: major mergers create ellipticals with boxy isophotes,
while minor mergers mainly result in systems with disky isophotes (Khochfar & Burkert 2005,
Jesseit et al. 2005). As shown by Naab et al. (2006), including even modest amounts of gas
has a dramatic impact on the isophotal shape of equal-mass merger remnants. The gas causes a
significant reduction of the fraction of box and boxlet orbits with respect to collisionless mergers,
and the remnant appears disky rather than boxy. Therefore, it seems that the massive, boxy
ellipticals can only be produced via dry, major mergers. The cores in these boxy ellipticals are
thought to arise from the binding energy liberated by the coalescence of supermassive binary black
holes during the major merger event (e.g., Faber et al. 1997; Graham et al. 2001; Milosavljević et
al. 2002). When sufficient gas is present, however, dissipation and sub-sequent star formation may
regenerate a central cusp. Alternatively, the gas may serve as an energy sink for the binding energy
of the black hole binary, leaving the original stellar cusp largely intact. Thus, following Lauer
et al. (2005), we may summarize this picture as implying that power-laws reflect the outcome of
dissipation and concentration, while cores owe to mixing and scattering.
But what about the correlation between isophotal shape and AGN activity? It is tempting
to believe that this correlation simply derives from the fact that both isophotal shape and AGN
activity may be related to mergers. After all, it is well known that mergers can drive nuclear inflows
of gas, which produce starbursts and feed the central supermassive black hole(s) (Toomre & Toomre
1972, Barnes & Hernquist 1991,1996, Mihos & Hernquist 1994,1996, Springel 2000, Cattaneo et
al. 2005). However, since the onset of such AGN activity requires wet mergers, this would predict
a higher frequency of AGN among disky ellipticals, contrary to the observed trend. Another
argument against mergers being responsible for the AGN-boxiness correlation is that the time scale
for merger-induced AGN activity is relatively short ( <∼ 10
8 yrs) compared to the dynamical time
in the outer parts of the merger remnant. This implies that active ellipticals should reveal strongly
distorted isophotes, which is not the case.
An important hint may come from the strong correlation between the presence of dust (either
clumpy, filamentary, or in well defined rings and disks) and the presence of optical emission line
activity (Tran et al. 2001; Ravindranath et al. 2001; Lauer et al. 2005). Although this suggests
that this dust is (related to) the actual fuel for the AGN activity, many questions remain. For
instance, it is unclear whether the origin of the dust is internal (shed by stellar winds) or external
(see Lauer et al. 2005 for a detailed discussion). In addition, it is not clear why the presence of dust,
and hence the AGN activity, would be more prevalent in boxy ellipticals. One option is that boxy
ellipticals are preferentially central galaxies (as opposed to satellites), so that they are more efficient
at accreting external gas (and dust). This is consistent with the fact that boxy ellipticals (i) are, on
– 4 –
average, brighter, (ii) reside in dense environments (Shioya & Taniguchi 1993; H06), and (iii) more
often contain hot, soft X-ray emitting halos. Another, more benign possibility, is that the relation
between morphology and AGN activity is merely a reflection of the fact that both morphology and
AGN activity depend on the magnitude of the galaxy (or on its stellar or dynamical mass). In this
case, AGN activity is only indirectly related to the morphology of its host galaxy.
In this paper we use the large data set of H06 to re-investigate the correlations between
morphology and (i) luminosity, (ii) dynamical mass, and (iii) emission line activity in the optical,
where we discriminate between AGN activity and star formation. In addition, we also examine to
what extent morphology correlates with X-ray emission (using data from ROSAT), with 1.4GHz
radio emission (using data from FIRST), and with the mass of the dark matter halo in which the
galaxy resides (using a SDSS galaxy group catalog). The outline of this paper is as follows. In § 2
we describe the data of H06; in § 3 we present the fraction of disky galaxies across the full sample
as a function of galaxy luminosity, dynamical mass and environment. In § 4 we split the sample
galaxies according to their activity in the optical, radio and X-rays, and investigate how the disky-
boxy morphology correlates with these various levels of ‘activity’. Finally, in § 5 we summarize
and discuss our findings. Throughout this paper we adopt a ΛCDM cosmology with Ωm = 0.3,
ΩΛ = 0.7, and H0 = 100h kms
−1 Mpc−1. Magnitudes are given in the AB system.
2. Data
2.1. Sample Selection
In order to investigate the interplay among AGN activity, morphology and environment for
early-type galaxies, we have analyzed the sample of H06, which consists of 847 galaxies in the SDSS
DR4 (Adelman-McCarthy et al. 2006) classified as ellipticals (E) or lenticulars (S0). As described
in H06, these objects are selected to be at z < 0.05, in order to ensure sufficient spatial resolution
to allow for a meaningful measurement of the isophotal parameters. In addition, the galaxies are
selected to have an observed velocity dispersion between 200 km s−1 and 420 km s−1 (where the
upper limit corresponds to the largest velocity dispersion that can be reliably measured from the
SDSS spectra), and are not allowed to be saturated. Note that, for the median sample distance,
the fiber radius of 1.5 arcsec corresponds to about 30% of the sample mean effective radius. From
all galaxies that obey these criteria, early-types have been selected by H06 using visual inspection.
Galaxies with prominent dust lanes have been excluded from the final sample in order to reduce
the effects of dust on the isophotal analysis.
– 5 –
2.2. Isophotal Analysis
Isophotes are typically parameterized by their corresponding surface brightness, I0, their semi-
major axis length, a, their ellipticity, ǫ, and their major axis position angle, θ0. In addition, since
isophotes are not perfectly elliptical, it is common practice to expand the angular intensity variation
along the best fit ellipse, δI(θ), in a Fourier series:
δI(θ) =
A′n cosn(θ − θ0) +B
n sinn(θ − θ0)
(e.g., Carter 1987; Jedrzejewski 1987; Bender & Möllenhoff 1987). Only the terms with n = 3 and
n = 4 are usually computed, as the data is often too noisy to reliably measure higher-order terms
(but see Scorza & Bender 1995 and Scorza & van den Bosch 1999). Note that, by definition, the
terms with n = 0, 1, and 2 are equal to zero within the errors. If the isophote is perfectly elliptical,
then A′n and B
n are also equal to zero for n ≥ 3. Non-zero A′3 and B′3 express deviations from
a pure ellipse that occur along the observed isophote every 120o. Typically, such deviations arise
from the presence of dust features or clumps. The most important Fourier coefficient, however, is
A′4, which quantifies the deviations taking place along the major and minor axes. Isophotes with
A′4 < 0 have a ‘boxy’ shape, while those with a positive A
4 parameter are ‘disk’-shaped.
For each of the 847 E/S0 galaxies in their sample H06 measured the isophotal parameters
using the IRAF1 task ELLIPSE. In particular, for each galaxy they provide the ellipticity, ǫ, the
position angle of the major axis, θ0, and the third and fourth order Fourier coefficients A3 and A4,
which are equal to A′3 and A
4, respectively, divided by the semi-major axis length and the local
intensity gradient. All the available parameters are intensity-weighted averages over the radial
interval 2Rs < R < 1.5R50. Here Rs is the seeing radius (typically lower than 1.5 arcsec, Stoughton
et al. 2002) and R50 is the Petrosian half-light radius
2. The Petrosian radius is defined as the
radius at which the ratio of the local surface brightness to the mean interior surface brightness is
0.2 (cf. Strauss et al. 2002). Therefore, R50 is the radius enclosing half of the flux measured within
a Petrosian radius and can be used as a proxy for the galaxy effective radius Re. In what follows,
we refer to galaxies with A4 ≤ 0 and A4 > 0 as ‘boxy’ and ‘disky’, respectively.
In their seminal papers on the isophotal shapes of elliptical galaxies, Bender & Möllenhoff
(1987), Bender et al. (1988) and Bender et al. (1989) define alternative structural parameters,
an/a and bn/a, which are related to the An and Bn parameters defined here as
1− ǫAn
1− ǫBn (2)
(Bender et al. 1988; Hao et al. 2006b).
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Asssociation of
Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation
2These data are publicly available at http://www.jb.man.ac.uk/∼smao/isophote.html
http://www.jb.man.ac.uk/~smao/isophote.html
– 6 –
2.3. Additional data
For all galaxies in the H06 sample we determined the absolute magnitudes in the SDSS g, r and
i bands, corrected for Galactic extinction, and K-corrected to z = 0, using the luminosity distances
corrected for Virgo-centric infall of the Local Group (LG) following Blanton et al. (2005). In order
to allow for a comparison with the samples of Bender et al. (1989) and Pellegrini (1999, 2005), we
transform these magnitudes to those in the Johnson B-band using the filter transformations given
by Smith et al. (2002).
We also estimated, for each galaxy, the total dynamical mass as
Mdyn = A
σ2corrR50
Here G is the gravitational constant, A is a normalization constant, and σcorr is the velocity dis-
persion measured from the SDSS spectra corrected for aperture effects using
σcorr = σmeasured
Rfiber
R50/8
)0.04
, (4)
with Rfiber = 1.5 arcsec (Bernardi et al. 2003). The aperture correction is meant to give the velocity
dispersion within R50/8, and to make comparable galaxies at different distance but sampled with a
spectroscopic fiber of fixed size. Throughout this paper we adopt A = 5, which has been shown to
accurately reproduce the total dynamical masses inferred from more accurate modeling (Cappellari
et al. 20063). Note that Cappellari et al. have also shown that these dynamical masses are roughly
proportional to the stellar masses of early-type galaxies.
H06 cross-correlated their E/S0 sample with the FIRST radio survey (Becker, White & Helfand
1995), which yielded the 1.4GHz fluxes for 162 objects in the sample. In order to investigate the
relation between isophotal structure and soft X-ray properties, we also matched the H06 sample to
the ROSAT All Sky Survey Catalog (Voges et al. 1999). This yields ROSAT/PSPC count-rates in
the 0.1 – 2.4 keV energy band for 40 sample galaxies. We used the WebPIMMS tool4 to transform
the observed count-rates into astrophysical fluxes, corrected for Galactic extinction, assuming an
X-ray power-law spectrum with energy index αX = 1.5 (cf. Anderson et al. 2003). In addition,
we cross-identified the H06 sample with the spectroscopic catalogs released for DR4 by Kauffmann
et al. (2003a,b), and extracted, when available, the luminosity of the [OIII] λ5007 line corrected
for dust extinction (in ergs−1), the line-flux ratios [OIII]/Hβ and [NII]/Hα, and the S/N values
associated with the [OIII] and Hα fluxes.
3Cappellari et al. (2006) use a slightly different definition of σcorr in equation (2), namely that measured within
Re rather than R50/8. Given the weak dependence of the velocity dispersion on the enclosed radius, we estimate
that this difference results in an offset of ∼0.07 dex in Mdyn
4http://heasarc.gsfc.nasa.gov/Tools/w3pimms.html
– 7 –
Finally, in order to assess the environment of the sample galaxies, we cross-identified the
H06 sample with the SDSS group catalog of Weinmann et al. (2006; hereafter WBYM), which is
constructed using the halo-based group finder of Yang et al. (2005). This yields group (i.e. dark
matter halo) masses for a total of 431 galaxies, distributed over 403 groups. Of these, 350 are
‘central’ galaxies (defined as the brightest galaxy in its group) and 81 are ‘satellites’. As for the
groups, 83 have just a single member (the early-type galaxy in our sample), while 320 groups have
2 or more members. The fact that only 51 percent of the galaxies in the H06 sample are affiliated
with a group is due to the fact that the WBYM group catalog is based on the DR2, and to the fact
that not all galaxies can be assigned to a group (see Yang et al. 2005 for details).
3. The disky fraction across the sample
The main properties of the full H06 sample (comprising all 847 early-type galaxies) are summa-
rized in Figure 1. The sample spans about 3 orders of magnitude in MB (−17.8 > MB −5 log(h) >
−21.4) and a range of about 1.5 dex in dynamical mass (10.5 < log[Mdyn(h−1M⊙)] < 12) and 3 dex
in group (halo) mass (11.8 < log[Mgroup(h
−1M⊙)] < 15). As expected, the B-band magnitude is
well correlated with the dynamical mass, independent of whether the galaxy is a central galaxy or a
satellite, and independent of whether it is disky or boxy. The absolute magnitudes and dynamical
masses of satellite galaxies are clearly separated from those of the central galaxies when plotted as
function of the group (halo) mass. This simply reflects that centrals are defined as the brightest
(and, hence, most likely the most massive) group members. This clear segregation disappears when
the galaxies are split in disky and boxy systems (lower panels), indicating that there is no strong
correlation between morphology and group hierarchy.
The upper panels of Figure 2 show scatter plots of MB, Mdyn and Mgroup as function of the
isophotal parameter A4. They indicate that the fraction of disky systems (those with A4 > 0)
increases with decreasing luminosity and dynamical mass, in qualitative agreement with Bender et
al. (1989) and H06. In the case of Mgroup, a similar trend seems to be present, but only for the
central galaxies. In order to quantify these trends, we have computed the fraction, fdisky, of disky
galaxies as a function of MB , Mdyn and Mgroup. For each bin in absolute magnitude, dynamical
mass, or group mass, fdisky is defined as the number ratio between disky galaxies and the total
number of galaxies in that bin. Each bin contains at least ten disky galaxies. For comparison, the
disky fraction of the full H06 sample is 0.66.
The lower left-hand panel of Figure 2 plots fdisky as function of MB . The errorbars are
computed assuming Poisson statistics. The fraction of disky galaxies declines by a factor of about
1.6 from ∼ 0.8 at MB − 5 log(h) = −18.7 to ∼ 0.5 at MB − 5 log(h) = −20.8, and is well fitted by
fdisky(MB) = (0.61 ± 0.02) + (0.17 ± 0.03) [MB − 5 log(h) + 20] (5)
which is shown as the solid, grey line. Note that this relation should not be extrapolated to arbitrary
faint and/or bright magnitudes. Since 0 ≤ fdisky ≤ 1 it is clear that fdisky(MB) must flatten at
– 8 –
both ends of the magnitude distribution. Apparently the magnitude range covered by our sample
roughly corresponds to the range in which the distribution transits (relatively slowly and smoothly)
from mainly disky to mainly boxy.
It has to be noted that the exact relation between fdisky and MB is somewhat sensitive to the
exact sample selection criteria, and equation (5) therefore has to be used with some care.
We have tested the robustness of the above relation by adding Gaussian deviates to each
measured value of A4, and then recomputing the best-fit relation between fdisky and MB . Figure
3 shows the slope and zero-point of this relation as function of the standard deviation of the
Gaussian deviates used (filled circles). The grey shaded horizontal bar represents the 1 σ interval
around the best-fit slope (left-hand panel) and the best-fit zero-point (right-hand panel). The grey
shaded vertical bar indicates the mean uncertainty on the observed A4 parameter obtained by
H06 (σ(A4) = 0.0012 ± 0.0008). Note that the best-fit slope and zero-point are extremely robust.
Adding an artificial error to the A4 measurements with an amplitude that is a factor five larger
than the average error quoted by H06 yields best-fit values that agree with those of equation (5)
at better than the 1σ errorbar on these parameters obtained from the fit.
The middle panel in the lower row of Figure 2 plots fdisky as function of Mdyn. As with the
luminosity, the disky fraction decreases smoothly with increasing dynamical mass, dropping from
∼ 0.80 at Mdyn = 6 × 1010h−1 M⊙ to ∼ 0.45 at Mdyn = 3 × 1011h−1 M⊙. The grey, dashed line
indicates the best-fit log-linear relation, which is given by
fdisky(Mdyn) = (0.73 ± 0.02) − (0.53 ± 0.08) log
1011h−1 M⊙
As for equation (5), the Gaussian-deviates test shows that this relation is robust against uncertain-
ties in the A4 measurements.
As is well-known from the morphology-density relation (e.g., Dressler 1980), early-type galaxies
preferentially reside in denser environments and hence in more massive halos (e.g, Croton et al. 2005;
Weinmann et al. 2006). It is interesting to investigate whether the halo mass also determines
whether the early-type galaxies are disky or boxy. We can address this using the WBYM group
catalog described in §2.3. The lower right-hand panel of Figure 2 plots the disky fraction of centrals
(crosses) and satellites (open triangles) as function of group mass. The fraction of disky centrals
decreases with increasing group (halo) mass, declining from ∼ 0.82 at Mgroup = 1.7 × 1012h−1 M⊙
to ∼ 0.54 at Mgroup = 5.0 × 1013h−1 M⊙. For the most massive groups, we have enough satellite
galaxies to also compute their disky fraction. Interestingly, these are larger (though only marginally
so) than those of central galaxies in groups of the same mass.
Although these results seem to suggest that group mass and group hierarchy (i.e., central vs.
satellite) play a role in determining the morphology of an early-type galaxy, they may also simply
be reflections of the fact that (i) satellite galaxies are fainter than central galaxies in the same
parent halo, (ii) fainter centrals typically reside in lower mass halos (cf. Figure 1), and (iii) fainter
galaxies have a larger fdisky. In order to discriminate between these options we proceed as follows.
– 9 –
Under the null-hypothesis that the isophotal structure of an early-type galaxy is only governed by
the galaxy’s absolute magnitude or dynamical mass, the predicted fraction of disky systems for a
given sub-sample is simply
fdisky,0 =
fdisky(Xi) (7)
where Xi is either MB − 5 log(h) or log(Mdyn) of the ith galaxy in the sample, and fdisky(X) is the
average relation between fdisk and X. The grey solid and dashed lines in the lower right-hand panel
of Figure 2 show the fdisky,0(Mgroup) thus obtained, using equations (4) and (5), respectively. These
are perfectly consistent with the observed trends (for both the centrals and the satellites). A possible
exception is the disky fraction of central galaxies in groups with Mgroup < 3.0× 1012h−1M⊙, which
is ∼ 2.5σ higher than predicted by the null-hypothesis. Overall, however, these results support
the null-hypothesis that the morphology of an early-type galaxy depends only on its luminosity or
dynamical mass: there is no significant indication that group mass and/or group hierarchy have a
direct impact on the morphology of early-type galaxies.
4. The disky fraction of active early-type galaxies
4.1. Defining different activity classes
In the standard unified model, AGN are distinguished in AGN of Type I when the central black
hole, its continuum emission and its broad emission-line region are viewed directly, and Type II, if
the central engine is obscured by a dusty circumnuclear medium. Our sample of early-type galaxies
does not contain any Type I AGN, simply because these systems are not part of the main galaxy
sample in the SDSS. However, the E/S0 sample of H06 is not biased against Type II AGN. In order
to identify these systems, one needs to be able to distinguish them from early-types with some
ongoing, or very recent, star formation, which also produces narrow emission lines. Since stars and
AGN produce different ionization spectra, one can discriminate between them by using line-flux
ratios. In particular, star formation and AGN activity can be fairly easily distinguished using the
so-called BPT diagram (after Baldwin, Phillips & Terlevich 1981; see also Veilleux & Osterbrock
1987), whose most common version involves the line-flux ratios [OIII]/Hβ and [NII]/Hα.
Figure 4 plots the BPT diagram for those sample galaxies whose [OIII] λ5007 and Hα lines
have been detected with a signal-to-noise ratio S/N ≥ 3. The solid curve was derived by Kauffmann
et al. (2003b) and separates star-forming galaxies from type II AGN, with the latter lying above the
curve. We follow Kauffmann et al. (2003b) and split the Type II AGN into Seyferts, LINERS, and
Transition Objects (TOs) according to their line-flux ratios: Type II Seyferts have log([OIII]/Hα) ≥
0.5 and log([NII]/Hα) ≥ −0.2, LINERS have log([OIII]/Hα) < 0.5 and log([NII]/Hα) ≥ −0.2, and
all galaxies with log([NII]/Hα) < −0.2 and laying above the curve are labelled TO. Kewley et al.
(2006) have recently studied in detail the properties of LINERs and type II Seyferts, and found
that LINERs and Seyferts form a continuous progression in the accretion rate L
, with LINERs
– 10 –
dominating at low L
and Seyferts prevailing at high L
. The results obtained by Kewley et
al. suggest that most LINERs are AGN and require a harder ionizing radiation field together with
a lower ionization parameter than Seyferts.
In order to increase the statistics of our subsequent analysis, we have organized the 847 galaxies
in the H06 sample into 3 categories:
1. AGN: This class consists of 28 early-type galaxies with a Seyfert-like activity and 286 early-
type galaxies with a LINER-like activity.
2. Emission-line (EL): This class consists of those galaxies that according to the BPT diagram
are star formers or transition objects, as well as those galaxies that lack one or both of the
BPT line-flux ratios, but that have an [OIII] emission line with a S/N ≥ 3. There are a total
of 383 early-type galaxies in the H06 sample that fall in this category.
3. Non-active (NA): These are the 150 galaxies that are not in the AGN or EL categories.
Therefore, these galaxies either have no emission lines at all, or have a detected [OIII] line
but with a S/N < 3. Among these, 43 objects (29 percent) show Hα emission with a S/N
≥ 3. Their presence could signal a problem with the spectroscopic pipeline, which failed to
properly measure the [OIII] line, or be real and due to an episode of star formation in its
early phases. In any case, their low S/N in [OIII] prevents us from classifying these galaxies
in one of the above two categories.
Given our aim to establish the presence/absence of a correlation between the AGN activity
and the disky/boxy morphology of the host early-type galaxy, the classification above is clearly
driven by the detection of the [OIII] line emission, which is commonly used as a proxy for the AGN
strength (cf. Kauffmann et al. 2003b, Kewley et al. 2006).
Along with these 3 categories which describe the galaxy activity in the optical, we have also
defined two additional activity classes: ‘FIRST’, which consists of the 162 sample galaxies with a
1.4GHz flux in the FIRST catalog (Becker et al. 1995), and ‘ROSAT’, containing the 40 sample
galaxies that have been detected in the ROSAT All Sky Survey (Voges et al. 1999). The soft
X-ray luminosities of these ROSAT galaxies span the range 41.3 < log[LX/(ergs
−1)] < 42.7 and
are consistent (though with large scatter), with the well known LX ∝ L2B relation (Trinchieri &
Fabbiano 1985; Canizares et al. 1987). This X-ray emission is therefore associated with a hot corona
surrounding the galaxy, rather than with X-ray binaries, and we can use it to indirectly probe the
environment where galaxies live. As shown by Bender et al. (1989) and O’Sullivan et al. (2001),
the LX ∝ L2B relation applies to X-ray luminosities between 10
40 and 1043 erg s−1. Our ROSAT
category is thus somehow incomplete at 40 < log[LX/(ergs
−1)] < 41 and the trends discussed below
for this class should be taken with some caution.
Table 1 lists the number of galaxies in each of these five activity classes. Note that the AGN,
EL and NA classes are mutually exclusive, but that a galaxy in each of these three classes can
– 11 –
appear also in the FIRST and ROSAT sub-samples. The vast majority of the galaxies detected by
FIRST or ROSAT reveal activity also in the optical, and are classified as either AGN or EL. The
radio and soft X-ray detections themselves, however, are not well correlated: only 12 percent of the
galaxies detected by FIRST have also been detected in soft X-rays.
Before computing fdisky for the galaxies in these various activity classes, it is useful to examine
how their respective distributions in MB, Mgroup and L[OIII] compare. This is shown in Figures 5,
6 and 7, respectively. While the luminosity distributions of the AGN and EL galaxies are in good
agreement with that of the full sample, the galaxies detected by ROSAT are on average about half
a magnitude brighter than the galaxies in the full sample. Also the non-active and radio galaxies
are brighter than average, though the differences are less pronounced. McMahon et al. (2002)
estimated a limiting magnitude of R ≃ 20 for the optical counterparts of FIRST sources at a 97
percent completeness level. Since the apparent magnitude limit of the H06 sample is brighter than
this limit, the FIRST subsample extracted from H06 is to be considered complete. Therefore, the
shift towards higher luminosities for the galaxies detected by FIRST with respect to the full sample
in Figure 5 is real rather than an artifact due to the depth of the different surveys.
Similar trends are present with respect to the group masses: whereas AGN and EL galaxies
have group masses that are very similar to those of the full sample, galaxies detected by ROSAT and
FIRST seem to prefer more massive groups. Somewhat surprisingly, the same applies to the class
of non-active galaxies. As for the luminosity of their [OIII] line plotted in Figure 7, AGN galaxies
tend to be brighter than EL and ROSAT galaxies, while the [OIII] luminosities of FIRST galaxies
are consistent with those of the EL and AGN galaxies combined (grey shaded histogram). In
agreement with Best et al. (2005), no correlation is found between the radio and [OIII] luminosities
of the sample galaxies in common with FIRST. Finally, it is worth emphasizing that the optical
activity defined in this paper occurs at log(L[OIII]/L⊙) ≥ 4.6; therefore, the class of non-active
galaxies may also contain weak AGN with [OIII] fluxes below this limit. Using the KS-test, we
have investigated whether the various distributions are consistent with being drawn from the same
parent distribution. We have found that only EL and ROSAT are consistent, in terms of their [OIII]
luminosity, with belonging to the same population, as well as the pairs (AGN,EL) and (NA,FIRST)
in terms of their absolute magnitude, the pair (AGN,EL) with respect to their dynamical mass,
and the pairs (AGN,EL), (NA,FIRST), (NA,ROSAT) and (FIRST,ROSAT) in terms of their group
halo mass.
Another aspect of defining different modes of activity is to study their actual frequency, i.e.
the fraction of galaxies sharing the same kind of activity (with respect to the full sample) as a
function of MB , Mdyn and environment. This is plotted in Figure 8, where the percentage of
NA, FIRST and ROSAT galaxies increases by a factor of about 4 towards higher luminosities and
larger dynamical masses (cf. Best et al. 2005, O’Sullivan et al. 2001), and by a factor of about 3
as their hosting group halo becomes more massive. EL and AGN galaxies define a far less clear
picture; EL galaxies seem to occur at any MB and Mgroup with a constant frequency, while their
fraction decreases by a factor of about 1.5 as Mdyn gets larger. The percentage of AGN galaxies
– 12 –
drops by a factor of about 2 at brighter MB values. It very weakly decreases in massive group
halos, and appears quite insensitive to Mdyn. As for the hierarchy inside a group, there is a weak
indication that EL, FIRST and ROSAT galaxies are preferentially associated with central galaxies,
while satellite galaxies are more frequently NA and AGN galaxies. Within the Poisson statistics,
however, none of these trends with group hierarchy is significant.
4.2. The relation between activity and morphology
A first glance at how morphology varies with activity is provided by Table 2, which lists
fdisky for the 5 classes defined in §4.1. As for Table 1, AGN, EL and NA galaxies are mutually
exclusive, while any of them can be included in the FIRST and ROSAT categories. In this case,
fdisky is derived from the pool of galaxies common to FIRST (ROSAT) and one of the optically
active sub-samples. The ROSAT galaxies are clearly biased towards boxy shapes as their fdisky
is systematically lower than ∼ 0.50. AGN and NA galaxies with or without radio emission are
generally disky (with fdisky > 0.60). The radio emission seems to make a difference in the case
of EL galaxies: while the full sub-sample of ELs is as disky as AGN and NA galaxies, those ELs
detected by FIRST are dominated by boxy systems with fdisky ≃ 0.45.
The upper panels of Figure 9 show scatter plots of the [OIII] luminosity, radio luminosity and
X-ray luminosity as function of A4. The lower panels plot the corresponding fractions of disky
systems. In the lower left-hand panel, fdisky is plotted as function of L[OIII] for both AGN (filled
squares) and EL (filled triangles) galaxies. This shows that both AGN and EL galaxies have a disky
fraction that is consistent with that of the full H06 sample (fdisky = 0.66), and with no significant
dependence on the actual [OIII] luminosity. The grey lines (solid for AGN and dotted for EL
galaxies) indicate the disky fractions predicted under the null-hypothesis that fdisky is a function
only of MB . These predictions are in excellent agreement with the data, suggesting that the (level
of) optical activity does not help in better predicting the disky/boxy morphology of an early-type
galaxy. The only possible exception is the sub-sample of EL galaxies with log(L[OIII]/L⊙) < 5.2
which has fdisky = 0.54, approximately 3σ lower than given by the null-hypothesis.
The relatively small number of sample galaxies detected by FIRST and ROSAT prevents us
from applying the above analysis as a function of radio and/or soft X-ray luminosity. Instead
we have determined fdisky separately for the sub-samples of galaxies detected and not-detected by
FIRST or ROSAT. The results are shown in the lower middle and lower right-hand panels of Figure
9. Clearly, the disky fraction of galaxies detected by ROSAT (fdisky = 0.48 ± 0.08) is significantly
lower than those with no detected soft X-ray flux (fdisky = 0.66 ± 0.02), in agreement with the
results of Bender et al. (1989) and Pellegrini (1999, 2005). The fact that galaxies detected by
ROSAT are more boxy is expected since they are significantly brighter than those with no soft
X-ray detection (cf. Figure 5). The grey lines, which correspond to fdisky,0(MB), indicate that this
explains most of the effect. Although it is intriguing that the disky fraction of ROSAT detections
is ∼ 1σ lower than predicted, a larger sample of early-type galaxies with soft X-ray detections is
– 13 –
needed to rule out (or confirm) the null-hypothesis. As for the galaxies detected by FIRST, there is
a weak indication that these galaxies have a somewhat lower fdisky: this finding is again in excellent
agreement with the predictions based on the null-hypothesis. Therefore, there is no indication that
the morphology of an early-type galaxy is directly related to whether the galaxy is active in the
radio or not.
To further test the null-hypothesis that the isophotal structure of early-type galaxies is entirely
dictated by their absolute magnitude or dynamical mass, we have derived fdisky of NA, EL and
AGN galaxies in bins of MB and Mdyn. The results are shown in Figure 10 (symbols), which shows
that the disky fraction of all three samples decreases with increasing luminosity and dynamical
mass. The grey solid and dashed lines indicate the predictions based on the null-hypothesis, which
have been computed using equations (5)–(7). Overall, these predictions are in excellent agreement
with the data, indicating that elliptical galaxies with ongoing star formation or with an AGN do
not have a significantly different morphology (statistically speaking) than other ellipticals of the
same luminosity or dynamical mass.
Finally, in Figure 11 we plot the disky fractions of NA, EL and AGN galaxies as function of
their group mass (upper panels) and group hierarchy (lower panels). For comparison, the grey
solid and dashed lines indicate the predictions based on the null-hypothesis. Although overall these
predictions are in good agreement with the data, there are a few noteworthy trends. At the massive
end (Mgroup >∼ 10
13h−1 M⊙) the disky fraction of AGN is higher than expected, while that of NA
galaxies is lower. The lower panels show that this mainly owes to the satellite galaxies in these
massive groups. Whereas the null-hypothesis accurately predicts the disky fractions of NA, EL and
AGN centrals, it overpredicts fdisky of NA satellites, while underpredicting that of AGN satellites
at the 3 σ level. These results clearly warrant a more detailed investigation with larger samples.
Note that only about half of the 847 galaxies in the H06 sample are also in our group catalog. A
future analysis based on larger SDSS samples and a more complete group catalog would sufficiently
boost the statistics to examine the trends identified here with higher confidence.
5. Discussion and conclusions
In spite of their outwardly bland and symmetrical morphology, early-type galaxies reveal a
far more complex structure, whose isophotes usually deviate from a purely elliptical shape. As
shown by Bender et al. (1989), these deviations correlate with other parameters; for example, boxy
early-type galaxies are on average brighter and bigger than disky galaxies and are supported by
anisotropic pressure. Early-type galaxies with disky isophotes, on the other hand, are consistent
with being isotropic oblate rotators. With the advent of large galaxy redshift surveys such as
the SDSS, it is now possible to collect large and homogeneous samples of early-type galaxies and
quantify these correlations in much greater detail. In addition, it also allows for a detailed study
of the relation between morphology and environment.
– 14 –
We have used a sample of 847 early-type galaxies imaged by the SDSS and analyzed by Hao et
al. (2006a) to study the fraction of disky galaxies (fdisky) as a function of their absolute magnitude
MB , their dynamical mass Mdyn and the mass of the dark matter halo Mgroup in which they are
located. Using the Hα, Hβ, [OIII] and [NII] emission lines in the SDSS spectra we have split the
sample in AGN galaxies, emission-line (EL) galaxies, and non-active (NA) galaxies (see Figure 4).
In addition we also constructed two sub-samples of those ellipticals that have also been detected in
the radio (in FIRST) or in soft X-rays (with ROSAT), and we have analyzed the relations between
fdisky and the level of AGN activity in the optical and the radio, and the strength of soft X-ray
emission.
The fraction of disky galaxies in the full sample decreases strongly with increasing luminosity
and dynamical mass (see Figure 2). More quantitatively, fdisky decreases from ∼ 0.8 at MB −
5 log(h) = −18.6 (Mdyn = 6 × 1010h−1M⊙) to ∼ 0.5 at MB − 5 log(h) = −20.6 (Mdyn = 3 ×
1011h−1M⊙). This indicates a smooth transition between disky and boxy shapes, which is well
represented by a log-linear relation between fdisky and luminosity or dynamical mass (at least over
the ranges probed here). The relatively large sample allows us to measure these relations with a
good degree of accuracy that is robust against the uncertainties involved in the measurement of
the A4 parameter.
We have used these log-linear relations to test the null-hypothesis that the isophotal shape of
early-type galaxies depends only on their absolute magnitude or dynamical mass. The main result
of this paper is that the data is fully consistent with this simple ansatz, and that the correlations
seen among group mass, group hierarchy (central vs. satellite), soft X-ray emission, activity (both
in the optical and in the radio) and the disky/boxy morphology of an early-type galaxy reflect
the dependence of each of these properties on galaxy luminosity. In fact, the luminosity (mass)
dependence of fdisky predicts, with good accuracy, the following observed trends:
1. The variation of fdisky of central and satellite galaxies in the sample as a function of their
group halo mass (see Figure 2).
2. The constancy of fdisky of EL and AGN galaxies with respect to their [OIII] luminosity (see
Figure 9).
3. The decreasing fdisky of NA, EL and AGN galaxies with increasing MB and Mdyn (see Figure
4. The dependence of fdisky of NA, EL and AGN galaxies on their group halo mass and hierarchy
(see Figure 11).
5. The average value of fdisky among those sample galaxies detected by ROSAT and FIRST.
The fact that our null-hypothesis is also consistent with the fraction of disky radio-emitters con-
tradicts Bender et al. (1989), who wrote that “the isophotal shape is the second parameter besides
– 15 –
luminosity determining the occurance of radio activity in ellipticals”. We claim instead, using a
much larger, more homogeneous sample, that the radio activity is merely a reflection of the multi-
variate dependence of radio activity, luminosity and morphology. We have further checked this
result using an inverse approach, based on the fradio - MB relation. Briefly, we derived, for the full
sample, the fraction of radio galaxies (fradio) with respect to the total as a function of MB , and
obtained a log-linear relation whereby fradio smoothly increases from 0.06 at MB−5 log(h) = −18.7
to 0.34 at MB − 5 log(h) = −20.7. The fraction of radio galaxies among disky and boxy galaxies in
the full sample turns out to be fradio(disky) = 0.17 ± 0.02 and fradio(boxy) = 0.23 ± 0.02. Entering
the mean absolute magnitude of disky and boxy galaxies in the fradio - MB relation, we obtain radio
fractions of 0.18 and 0.21, respectively, well within 1 σ from the observed values.
Although the data is in good overall agreement with the null-hypothesis, there are a few weak
deviations at the 1 (3 at most) σ level (throughout errors have been computed assuming Poisson
statistics). First of all, emission line galaxies with log(L[OIII]/L⊙) < 5.2 have a disky fraction
that is ∼ 3σ lower than predicted by the null-hypothesis. Note however, that for higher [OIII]
luminosities, the null-hypothesis is in excellent agreement with the disky fraction of EL galaxies.
Another mild discrepancy between data and null-hypothesis regards the disky fraction of ellipticals
detected by ROSAT, which is ∼ 1σ lower than predicted. Finally, the disky fraction of NA and
AGN satellites in groups with Mgroup >∼ 10
13h−1 M⊙ are slightly too high and low, with respect
to the null-hypothesis, respectively. Whether these discrepancies indicate a true shortcoming of
the null-hypothesis, and thus signal that the isophotal shape of early-type galaxies depends on
additional parameters, requires a larger sample even. In the relatively near future, the final SDSS
should be able to roughly double the size of the sample used here, while a group catalog of this final
SDSS should increase the statistics regarding the environmental dependencies by an even larger
amount.
The relations between fdisky and MB (Mdyn) derived here provide a powerful test-bench for
theories of galaxy formation. In particular, they can be used to constrain the nature and the
merging history of the progenitors of present-day early-type galaxies. In a follow-up paper, we
will use semi-analytical models featuring AGN and supernova feedback in order to predict and
understand the observed log-linear relations in terms of the amount of cold gas in the progenitors
at the time of the last merger and their mass ratio (Kang et al., in prep).
AP acknowledges useful discussions with Sandra Faber and John Kormendy. We thank an
anonymous referee for his/her useful comments on the paper. Funding for the creation and distri-
bution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating
Institutions, the National Aeronautics and Space Administration, the National Science Foundation,
the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The
SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research
Consortium (ARC) for the Participating Institutions. The Participating Institutions are The Uni-
http://www.sdss.org/
– 16 –
versity of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The
Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-
Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New
Mexico State University, University of Pittsburgh, University of Portsmouth, Princeton University,
the United States Naval Observatory, and the University of Washington.
REFERENCES
Adelman-McCarthy, J.K., et al. 2006, ApJS, 162, 38
Anderson, S.F., et al. 2003, AJ, 126, 2209
Baldwin, J., Phillips, M., Terlevich, R. 1981, PASP, 93, 5
Barnes, J.E., Hernquist, L.E. 1991, ApJL, 370, L65
Barnes, J.E., Hernquist, L.E. 1996, ApJ, 471, 115
Becker, R.H., White, R.L., Helfand, D.J. 1995, ApJ, 450, 559
Bender, R. 1988, A&AS, 193, 7
Bender, R., Döbereiner, S., Möllenhoff, C., 1988, A&AS, 74, 385
Bender, R., Möllenhoff, C. 1987, A&A, 177, 71
Bender, R., Surma, P., Döbereiner, S., Möllenhoff, C., Madejsky, R. 1989, A&A, 217, 35
Bernardi, M., et al. 2003, AJ, 125, 1817
Best, P.N., Kauffmann, G., Heckman, T.M., Brinchmann, J., Charlot, S., Ivezić, Z., White, S.D.M.
2005, MNRAS, 362, 25
Blanton, M.R., et al. 2005, AJ, 129, 2562
Bower, R.G., Lucey, J.R., Ellis, R.S. 1992, MNRAS, 254, 601
Canizares, C.R., Fabbiano, G., Trinchieri, G. 1987, ApJ, 312, 503
Cappellari, M., et al. 2006, MNRAS, 366, 1126
Carter, D. 1987, ApJ, 312, 514
Cattaneo, A., Blaizot, J., Devriendt, J., Guiderdoni, B. 2005, MNRAS, 364, 407
Croton, D.J., et al. 2005, MNRAS, 356, 1155
Djorgovski, S., Davis, M. 1987, ApJ, 313, 59
– 17 –
Davies, R.L., Efstathiou, G., Fall, S.M., Illingworth, G., Schechter, P.L. 1983, ApJ, 266, 41
Dressler, A. 1980, ApJ, 236, 351
Dressler, A. 1987, ApJ, 317, 1
Ellis, R.S., Smail, I., Dressler, A., Couch, W.J., Oemler, A.Jr., Butcher, H., Sharples, R.M. 1997,
ApJ, 483, 582
Fabbiano, G. 1989, ARA&A, 27, 87
Faber, S.M., Jackson, R.E. 1976, ApJ, 204, 668
Faber, S.M., et al. 1997, AJ, 114, 1771
Ferrarese, L., van den Bosch, F.C., Ford, H.C., Jaffe, W., O’Connell, R.W. 1994, AJ, 108, 1598
Gebhardt, K., et al. 1996, AJ, 112, 105
Graham, A.W., Erwin, P, Caon, N., Trujillo, I. 2001, ApJ, 563, L11
Hao, C.N., Mao, S., Deng, Z.G., Xia, X.Y., Wu, H. 2006a, MNRAS, 370, 1339
Hao, C.N., Mao, S., Deng, Z.G., Xia, X.Y., Wu, H. 2006b, MNRAS, 373, 1264
Jaffe, W., Ford, H.C., O’Connell, R.W., van den Bosch, F.C., Ferrarese, L. 1994, AJ, 108, 1567
Jedrzejewski, R.I. 1987, MNRAS, 226, 747
Jesseit, R., Naab, Y., Burkert, A. 2005, MNRAS, 360, 1185
Kang, X., van den Bosch, F.C., Pasquali, A., 2007, preprint (arXiv:0704.0932)
Kauffmann, G., et al. 2003a, MNRAS, 341, 33
Kauffmann, G., et al. 2003b, MNRAS, 346, 1055
Kewley, L.J., Groves, B., Kauffmann, G., Heckman, T., 2006, MNRAS, 372, 961
Khochfar, S., Burkert, A. 2005, MNRAS, 359, 1379
Kuntschner, H., et al. 2006, MNRAS, 369, 497
Lauer, T.R. 1985, MNRAS, 216, 429
Lauer, T.R., et al. 1995, AJ, 110, 2622
Lauer, T.R., et al. 2005, AJ, 129, 2138
McMahon, R.G., White, R.L., Helfand, D.J., Becker, R.H. 2002, ApJ Supl.Ser., 143, 1
http://arxiv.org/abs/0704.0932
– 18 –
Mihos, J.C., Hernquist, L.E. 1994, ApJL, 425, L13
Mihos, J.C., Hernquist, L.E. 1996, ApJ, 464, 641
Milosavljević, M., Merritt D., Rest A., van den Bosch F.C. 2002, MNRAS, 331, 51
Morganti, R., et al. 2006, MNRAS, 371, 157
Naab, Y., Jesseit, R., Burkert, A. 2006, MNRAS, in press (astro-ph/0605155)
Nieto, J.-L., Capaccioli, M., Held, E.V. 1988, A&A, 195, 1
O’Sullivan, E., Forbes, D.A., Ponman, T.J. 2001, MNRAS, 328, 461
Pellegrini, S. 1999, A&A, 351, 487
Pellegrini, S. 2005, MNRAS, 364, 169
Ravindranath, S., Ho, L.C., Peng, C.Y., Filippenko, A.V., Sargent, W.L.W. 2001, AJ, 122, 653
Rest, A., et al. 2001, AJ, 121, 2431
Sarzi, M., et al. 2006, MNRAS, 366, 1151
Scorza, C., Bender, R. 1995, A&A, 293, 20
Scorza, C., van den Bosch, F.C. 1998, MNRAS, 300, 469
Serra, P., Trager, S.C., van der Hulst, J.M., Oosterloo, T.A., Morganti, R. 2006, A&A, 453, 493
Shioya, Y., Taniguchi, Y. 1993, PASJ, 43, 39
Smith, J.A. et al. 2002, AJ, 123, 2121
Springel, V. 2000, MNRAS, 312, 859
Stoughton, C., et al., 2002, AJ, 123, 485
Strauss, M.A., et al, 2002, AJ, 124, 1810
Toomre, A., Toomre, J. 1972, ApJ, 178, 623
Trager, S.C., Faber, S.M., Worthey, G., González, J.J. 2000, AJ, 119, 1645
Tran, H.D., Tsvetanov, Z., Ford, H.C., Davies, J., Jaffe, W., van den Bosch, F.C., Rest, A. 2001,
AJ, 121, 2928
Trinchieri, G., Fabbiano, G. 1985, ApJ, 296, 447
Veilleux, S., Osterbrock, D. 1987, ApJS, 63, 295
http://arxiv.org/abs/astro-ph/0605155
– 19 –
Visvanathan, N., Sandage, A. 1977, 216, 214
Voges, W., et al. 1999, A&A, 349, 389
Weinmann, S., van den Bosch, F.C., Yang, X., Mo, H.J. 2006, MNRAS, 366, 2
Yang, X., Mo, H.J., van den Bosch, F.C., Jing, Y.P. 2005, MNRAS, 356, 1293
This preprint was prepared with the AAS LATEX macros v5.2.
– 20 –
Fig. 1.— The distributions in MB , Mdyn and Mgroup for the full sample, split between central and
satellite galaxies (grey crosses and open triangles respectively, in the top panels) and between disky
and boxy galaxies (open triangles and grey crosses respectively, in the bottom panels).
– 21 –
Fig. 2.— Top panels: the distributions of MB , Mdyn and Mgroup as a function of the isophotal
parameter A4 for the full sample, also split between central (grey crosses) and satellite (open
triangles) galaxies. Bottom panels: the fraction of disky galaxies as a function of MB and Mdyn
for the full sample. The fraction of disky galaxies is also shown per bin of group halo mass Mgroup
for central (crosses) and satellite (open triangles) galaxies. The errorbars are at the 1 σ level, and
were computed assuming Poisson statistics. The grey solid and dashed lines in the left hand-side
and middle panels are the best fits to the fractions of disky galaxies across the full sample. The
same lines in the right hand-side panel represent the predicted fractions of disky galaxies from the
working null-hypothesis.
– 22 –
Fig. 3.— Impact of individual A4 measurement errors: the slope and the zero-point of the log-
linear correlation between fdisky and MB (equation 4) are shown as a function of the standard
deviation of the Gaussian used to simulate errors on the observed A4 values. The grey shaded
areas indicate the best-fitting slope (0.17 ± 0.03) and zero-point (0.61 ± 0.02) in equation 4, and
the mean uncertainty on the observed A4 parameter (0.0012 ± 0.0008) as measured by H06.
– 23 –
-1 -0.5 0 0.5 1
Liners
Seyferts
Fig. 4.— The BPT diagram for the galaxies in the full sample, whose [OIII] λ5007 and Hα emission
lines were detected with a S/N ratio larger than 3. These objects have been split among Seyfert
galaxies of type 2, LINERs, Transition Objects (TO) and star-forming (SB) according to Kauffmann
et al. (2003b).
– 24 –
Fig. 5.— The distributions of the full sample (grey shaded area) and the 5 different activity classes
defined in §4.1 in absolute magnitude MB (in AB system). Each distribution is normalized by the
size of the sample from where it was extracted.
– 25 –
Fig. 6.— As in Figure 5, but for the group halo mass Mgroup.
– 26 –
Fig. 7.— As in Figure 5, but for the luminosity in the [OIII] line. Here, the grey shaded histogram
refers to emission-line and AGN galaxies together.
– 27 –
-19 -20 -21
ROSAT
FIRST
11 11.5
12 12.5 13 13.5 14
Group hierarchy
Fig. 8.— The fraction of galaxies in the 5 different activity classes with respect to the full sample
as a function of MB , Mdyn, Mgroup and split between centrals and satellites.
– 28 –
Fig. 9.— Top panels: the distribution of the luminosities in the [OIII] line, at 1.4 GHz and in
the soft X-rays, as a function of the isophotal parameter A4. Emission-line and AGN galaxies
are represented with grey filled triangles and black filled squares respectively. Bottom panels: the
fraction of disky galaxies among emission-line (triangles) and AGN (squares) galaxies as a function
of the luminosity in the [OIII] line (left hand-side panel). The grey solid and dotted lines trace the
predictions from the working null-hypothesis in MB . The fraction of disky galaxies for the galaxies
detected and non-detected by FIRST and ROSAT are shown in the middle and right hand-side
panels, together with the predictions from equations (4) and (5). The errorbars are at the 1 σ level,
and were computed assuming Poisson statistics.
– 29 –
Fig. 10.— The fraction of disky galaxies for non-active (black filled circles), emission-line (black
filled triangles) and AGN (black filled squares) galaxies as a function of MB and Mdyn. The grey
solid and dashed lines represent the predictions from equation (5), i.e. the working null-hypothesis.
The errorbars are at the 1 σ level, and were computed assuming a Poisson statistics.
– 30 –
Fig. 11.— As in Figure 10, but splitting the non-active, emission-line and AGN galaxies between
centrals and satellites.
– 31 –
Table 1. Activity Classes
AGN EL NA FIRST ROSAT
AGN 314 −− −− 91 17
EL −− 383 −− 53 16
NA −− −− 150 18 7
FIRST 91 53 18 162 22
ROSAT 17 16 7 22 40
Note. — The number of sample galaxies in the
five different activity classes. Note that the AGN,
EL and NA classes are mutually exclusive.
– 32 –
Table 2. Fraction of disky galaxies across the activity classes
AGN EL NA FIRST ROSAT
AGN 0.69 −− −− 0.65 0.47
EL −− 0.64 −− 0.45 0.50
NA −− −− 0.63 0.61 0.43
FIRST 0.65 0.45 0.61 0.58 0.54
ROSAT 0.47 0.50 0.43 0.54 0.47
	Introduction
	Data
	Sample Selection
	Isophotal Analysis
	Additional data
	The disky fraction across the sample
	The disky fraction of active early-type galaxies
	Defining different activity classes
	The relation between activity and morphology
	Discussion and conclusions
