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STELLAR POPULATIONS IN NEARBY LENTICULAR GALAXIES 1
O. K. Sil'chenko
Sternberg Astronomical Institute, Universitetskij Prospect 13, Moscow 119992; and Isaac Newton Institute of Chile,
Moscow Branch, Moscow 119017, Russia; olga@sai.msu.su
Received 2005 May 16; accepted 2005 December 7
ABSTRACT
We have obtained two­dimensional spectral data for a sample of 58 nearby S0 galaxies with the Multi­Pupil Fiber/
Field Spectrograph of the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences.
The Lick indices H#, Mg b, and hFei are calculated separately for the nuclei and for the bulges taken as the rings
between R ¼ 4 00 and 7 00 , and the luminosity­weighted ages, metallicities, and Mg/Fe ratios of the stellar populations
are estimated by comparing the data to single stellar population (SSP) models. Four types of galaxy environments
are considered: clusters, centers of groups, other places in groups, and the field. The nuclei are found to be on average
slightly younger than the bulges in any type of environment, and the bulges of S0 galaxies in sparse environments are
younger than those in dense environments. The effect can be partly attributed to the well­known age correlation with
the stellar velocity dispersion in early­type galaxies (in our sample the galaxies in sparse environments are on average
less massive than those in dense environments), but for the most massive S0 galaxies, with # # ¼ 170--220 km s #1 , the
age dependence on the environment is still significant at the confidence level of 1.5 #.
Subject headinggs: galaxies: elliptical and lenticular, cD --- galaxies: evolution --- galaxies: nuclei
1. INTRODUCTION
In the classical morphological sequence by Hubble (1936)
lenticular galaxies occupy an intermediate position between el­
liptical and spiral galaxies: they have a smooth and red appear­
ance like the elliptical galaxies, but they also have stellar disks,
almost as large as those of the spiral galaxies. The most popu­
lar hypothesis of S0 galaxy origin is that of their transforma­
tion from spiral galaxies by stopping global star formation and
removing or consuming the remaining gas ( Larson et al. 1980).
In distant (z # 0:5) clusters this transformation is now observed
directly: the number of lenticular galaxies in the clusters di­
minishes strongly with redshift ( Fasano et al. 2000); instead, one
can see ``passive spiral galaxies''---red spiral galaxies lacking
star formation---at the periphery (``infalling regions'') of the
intermediate­redshift clusters (Goto et al. 2003; Yamauchi &
Goto 2004). Many theoretical studies have been done to explain
in detail what physical mechanisms could be involved in the
process of spiral galaxy transformation into lenticular galaxies:
tidally induced collisions of disk gas clouds ( Byrd & Valtonen
1990), harassment ( Moore et al. 1996), ram pressure by the in­
tercluster medium (Quilis et al. 2000), etc. For the S0 galax­
ies in the field, the scheme of their transformation from spiral
galaxies is not so clear, but the common view is that some ex­
ternal action such as minor merger could produce the necessary
effect.
By reviewing the various mechanisms of secular evolution
that can transform a spiral galaxy into a lenticular one, we have
noticed that most of them result in gas concentration in the very
center of the galaxy, so a nuclear star formation burst seems an
unavoidable circumstance of S0 galaxy birth. If referring to S0
galaxy statistics in the clusters located between z ¼ 0 and 1, the
main epoch of S0 galaxy formation is z # 0:4--0.5, so the nu­
clear star formation bursts in the nearby S0 galaxies must not
be older than 5 Gyr. Indeed, in my spectral study of the cen­
tral parts of nearby galaxies in different types of environments
(Sil'chenko 1993) I found that #50% of nearby lenticular gal­
axies have strong absorption lines H# and H# in their nuclear
spectra, so they are of the ``E+A'' type, as it is currently called,
and are dominated by an intermediate­age stellar population.
In this respect the S0 galaxies have more resembled rather
early­type spiral galaxies than elliptical galaxies. Here I aim to
continue this study, with a larger sample and with panoramic
spectral data in order to separate the nuclei and their outskirts
( bulges), which is a substantial advantage with respect to aper­
ture spectroscopy.
Another crucial point of the present study, and also of a global
paradigm of galaxy formation, is environmental influence. The
current hierarchical assembly paradigm predicts a younger age
of galaxies in lower density environments (for the most recent
simulations see, e.g., Lanzoni et al. [2005] or De Lucia et al.
[2006]). Observational evidence concerning early­type galaxies
is controversial: some authors find differences of stellar pop­
ulation ages between the clusters and the field ( Terlevich &
Forbes 2002; Kuntschner et al. 2002; Thomas et al. 2005), and
some authors do not find any dependence of the stellar popula­
tion age on environment density ( Kochanek et al. 2000). In order
to check whether the mean ages of the stellar populations depend
on environment density monotonically, as the hierarchical par­
adigm predicts, in this work I consider four types of environment
separately: the cluster galaxies, the brightest (central ) galaxies
of groups, the second­ranked group members, and the field
galaxies.
2. SAMPLE
The sample of lenticular galaxies considered in this work con­
sists of 58 objects, mostly nearby and bright. It does not pretend
to be complete but rather representative. In the Lyon­Meudon
Extragalactic Database ( LEDA) we have found 122 galaxies in
total, among them 40 Virgo Cluster members, with the follow­
ing parameters: #3 # T # 0, v r < 3000 km s #1 , B 0
T
< 13:0,
# 2000:0 > 0, and without a bright active galactic nucleus (AGN )
1 Based on observations collected with the 6 m telescope ( BTA) at the
Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences
( RAS).
229
The Astrophysical Journal, 641:229--240, 2006 April 10
# 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.

TABLE 1
Our Sample of S0 Galaxies
Dates
Galaxy a Environment b Type c
# 0
d
( km s #1 )
v r
d
( km s #1 ) Green Red Reference
N0080........................ Group center SA0# 260 5698 1996 Aug, 2003 Oct . . . 1
N0474........................ Pair (R 0 )SA(s)0 0 164 2372 2003 Oct . . . . . .
N0524........................ Group center SA(rs)0+ 253 2379 1997 Oct 1996 Oct 2
N0676........................ Pair S0/a 140 e 1506 2003 Oct . . . . . .
N0936........................ Group center SB(rs)0+ 190 1430 2002 Oct, 2003 Oct 2002 Oct . . .
N1023........................ Group center SB(rs)0# 204 637 1996 Oct . . . 3
N1161........................ Pair S0 185 e 1954 2003 Oct . . . . . .
N2300........................ Group member SA0+ 261 1938 2001 Sep . . . . . .
N2549........................ Group center SA(r)0+ 143 1039 2004 Oct 2002 Oct . . .
N2655........................ Group center SAB(s)0/a 163 1404 1999 Oct, 2000 Oct 2000 Oct 4
N2681........................ Group center (R 0 )SAB(rs)0/a 108 692 2001 Sep 2002 Mar . . .
N2685........................ Field (R)SB0+pec 94 883 1994 Oct . . . 5
N2732........................ Pair S0 154 1960 2000 Oct 2001 Sep 4
N2768........................ Group center S0 1/2 182 1373 2001 Jan 2000 Oct 4
N2787........................ Field SB(r)0+ 194 696 2000 Oct 2000 Oct 4
N2880........................ Field SB0# 136 1608 2001 Sep . . . . . .
N2911........................ Group center SA(s)0: 234 3183 1999 Dec 1998 Jan 4
N2950........................ Field (R)SB(r)0 0 182 1337 2003 Oct 2005 Oct . . .
N3065........................ Group center SA(r)0+ 160 2000 2001 Sep 2005 Oct . . .
N3098........................ Field S0 105 1311 2001 Jan . . . . . .
N3166........................ Group member SAB(rs)0/a 112 1345 2003 Mar 1998 Jan . . .
N3245........................ Group member SA(r)0 0 210 1358 2003 Mar . . . . . .
N3384........................ Group member SB(s)0# 148 704 1999 Dec . . . 6
N3412........................ Group member SB(s)0 0 101 841 2004 Mar . . . . . .
N3414........................ Group center S0pec 237 1414 2001 Jan 2002 Mar 4
N3607........................ Group center SA(s)0+ 224 935 2001 Apr 2002 Mar . . .
N3941........................ Group center SB(s)0 0 159 928 2003 Mar . . . . . .
N3945........................ Group member SB(rs)0+ 174 1259 2003 Mar . . . . . .
N4026........................ UMa Cluster S0 178 930 2003 Mar . . . . . .
N4036........................ Group center S0# 189 1445 1997 May, 1998 Jan 1998 Jan 7
N4111 ........................ UMa Cluster SA(r)0+ 148 807 2001 Jan 2002 Mar 4
N4125........................ Group center E6 pec 227 1356 2003 Mar 1998 Jan . . .
N4138........................ UMa Cluster SA(r)0+ 140 888 1998 Jan, 1999 Dec 1999 Dec 8
N4150........................ Group member SA(r)0+ 85 226 2001 Apr 2002 Mar . . .
N4179........................ Group center S0 157 1256 2003 Mar . . . . . .
N4233........................ Virgo Cluster S0 0 220 2371 2002 Apr 2002 Apr 4
N4350........................ Virgo Cluster SA0 181 1200 2001 Jan . . . . . .
N4379........................ Virgo Cluster S0# 108 1069 1999 Jun . . . . . .
N4429........................ Virgo Cluster SA(r)0+ 192 1106 1999 Jun 1997 May 9
N4526........................ Virgo Cluster SAB(s)0+ 264 448 2001 Apr 2002 Mar . . .
N4550........................ Virgo Cluster SB0 91 381 1998 Jan, 1999 Jun 1999 Jun 8
N4570........................ Virgo Cluster S0/E7 188 1730 2004 Mar . . . . . .
N4638........................ Virgo Cluster S0# 122 1164 2004 Mar . . . . . .
N4866........................ Pair SA(r)0+ 210 1988 2001 Apr . . . . . .
N5308........................ Group member S0# 211 2041 2003 Mar . . . . . .
N5422........................ Group member S0 165 1820 2003 Mar . . . . . .
N5574........................ Group member SB0#? 75 1659 1999 Jun . . . 10
N5866........................ Group member S0 3 159 672 1998 Aug 1996 May . . .
N6340........................ Group center SA(s)0/a 144 1198 1996 Aug, 1997 Oct 1996 Aug 2
N6548........................ Pair SB0 121 e 2179 2004 Oct . . . . . .
N6654........................ Pair (R 0 )SB(s)0/a 149 e 1821 2001 Sep . . . . . .
N6703........................ Field SA0# 180 2461 2003 Oct 2003 Oct . . .
N7013........................ Field SA(r)0/a 84 779 1996 Oct, 1998 Aug 1996 Aug 9
N7280........................ Field SAB(r)0+ 104 1844 1998 Aug 1998 Oct 11
N7332........................ Field S0 pec 124 1172 1996 Aug, 1997 Oct . . . 3
N7457........................ Field SA(rs)0#? 69 812 1999 Oct, 1999 Dec . . . 10
N7743........................ Field (R)SB(s)0+ 84 1710 2003 Oct . . . . . .
U11920...................... Field SB0/a 116 e 1145 2003 Oct . . . . . .
a Galaxy ID: N ¼ NGC, U ¼ UGC.
b From Guiricin et al. (2000).
c Hubble type from NED.
d From LEDA unless otherwise noted.
e From our observations.
References.---(1) Sil'chenko et al. 2003a; (2) Sil'chenko 2000; (3) Sil'chenko 1999; (4) Sil'chenko & Afanasiev 2004; (5) Sil'chenko 1998; (6) Sil'chenko et al. 2003b;
(7) Sil'chenko & Vlasyuk 2001; (8) Afanasiev & Sil'chenko 2002; (9) Sil'chenko & Afanasiev 2002; (10) Sil'chenko et al. 2002; (11) Afanasiev & Sil'chenko 2000.

or intense present star formation in a nucleus. For our sample,
from this list we have selected eight Virgo Cluster members and
42 other galaxies, half of the rest. A few galaxies are added to
broaden the luminosity range: NGC 5574, NGC 3065, and
NGC 7280 are fainter than B 0
T ¼ 13:0, and NGC 80 and NGC
2911 are very luminous but farther from us than 40 Mpc.
Table 1 lists all the galaxies with some of their characteris­
tics such as morphological type, redshift, and central velocity
dispersion. The galaxies were sorted according to their environ­
ment type by using the Nearby Optical Galaxy ( NOG) group
catalog (Giuricin et al. 2000); we have only classified three
galaxies belonging to the Ursa Major Cluster following Tully
et al. (1996). Our sample includes 11 cluster galaxies from Virgo
and Ursa Major, 17 central (the brightest) galaxies of groups with
three members and more, 18 second­ranked group members to
which we have added paired galaxies, and 12 field lenticular
galaxies that are not mentioned in the NOG catalog at all.
All the galaxies of Table 1 have been observed with the integral­
field unit ( IFU ) Multi­Pupil Fiber / Field Spectrograph ( MPFS;
Afanasiev et al. 2001), 2 of the 6 m telescope of the Special As­
trophysical Observatory of the Russian Academy of Sciences
between 1994 and 2005. During these years the instrument was
modified more than once. We started with a field of view of
10 00 ; 12 00 , spatial element (pupil ) size of 1B3, spectral resolu­
tion of 5 8, and spectral range less than 600 8. Now we have
a field of view of 16 00 ; 16 00 , spatial element (pupil ) size of
1 00 , spectral resolution of 3.5 8, and spectral range of 1500 8.
Usually we observe two spectral ranges, the green one centered
on 5000 8 and the red one centered on the H# line. The optical
design was modified, too: two different schemes, a TIGER­like
one (for a description of the instrumental idea of the TIGER
[ Traitement Integral de Galaxies par l'Etude de leurs Raies]
mode of IFU, see Bacon et al. 1995) and that with fibers, were
used before and after 1998, respectively. We have described in
detail 23 of 58 lenticular galaxies in our previous papers (see the
references in Table 1), where one can find not only the char­
acteristics of the various versions of the MPFS but also two­
dimensional maps of Lick indices and kinematical parameters.
Here we consider only two discrete areas of every galaxy, the
unresolved nuclei and the wide rings, with R in ¼ 4 00 and R out ¼
7 00 , that we are treating as the ``bulges.'' The boundaries of the
rings have been selected as a compromise between the seeing
limitation (the seeing FWHMs are typically 2B5 at the 6 m
telescope) in order to avoid the influence of the nuclei on the
bulge measurements and the size of our field of view, which
causes incomplete azimuthal coverage at R > 7 00 . At our limit
distance, D ¼ 40 Mpc, the outer radius of the bulge areas, 7 00 ,
corresponds to a linear size of 1.35 kpc. The nuclei are pre­
sented by the integrated fluxes over the central spatial elements
within the maximum radius of 0.1 kpc from the centers.
The Lick indices H#, Mg b, Fe5270, and Fe5335 have been
measured for the nuclei and for the bulges of all the galaxies; fur­
thermore, we use the composite iron index hFei # ( Fe5270 ×
5335)/2. During all our observational runs we observed stan­
dard stars from Worthey et al. (1994) and calibrated our index
system onto the standard Lick one. The measured indices were
corrected for the stellar velocity dispersions; we calculated the
corrections by artificial broadening of the spectra of the standard
stars. We estimate the typical statistical accuracy in each of the
three indices (defined by the signal­to­noise ratio [S/N ], which
has been kept as 70--90 [ per angstrom] in the nuclei and #30 at
2 See http://www.sao.ru / hq/lsfvo/devices/mpfs/mpfs _ main.html.
TABLE 2
Comparison of Two Independent Index Determinations with the MPFS
H# (8) Mg b (8) hFei (8)
Galaxy ( NGC) Nucleus Bulge Nucleus Bulge Nucleus Bulge
80................................ 1.57 1.70 5.12 4.44 3.14 3.22
1.66 1.59 # 0.20 5.00 4.44 # 0.04 2.94 2.95 # 0.15
936.............................. 1.13 1.41 # 0.03 4.64 4.51 # 0.03 2.86 2.50 # 0.01
1.41 1.07 # 0.07 4.93 4.53 # 0.10 3.20 2.80 # 0.07
2655............................ 1.56 1.55 # 0.03 3.77 3.60 # 0.11 2.10 2.07 # 0.05
1.73 1.35 # 0.05 3.70 3.69 # 0.02 2.38 2.47 # 0.02
4036............................ 0.12 0.92 # 0.08 5.56 4.09 # 0.13 2.56 2.64 # 0.07
0.82 0.80 # 0.08 5.85 3.61 # 0.25 3.28 . . .
4138............................ 1.14 1.10 # 0.03 4.76 3.34 # 0.15 2.97 2.00 # 0.14
0.74 0.96 # 0.06 4.66 3.45 # 0.21 2.65 2.11 # 0.07
4550............................ 1.64 1.92 3.18 3.13 . . . . . .
1.64 1.41 # 0.03 3.20 3.14 # 0.05 2.53 1.95 # 0.08
6340............................ 1.05 0.86 4.65 3.06 2.92 2.10
1.56 1.24 4.49 3.18 2.76 2.12
7013............................ 1.63 2.03 3.84 3.32 2.99 . . .
1.58 2.15 # 0.09 3.78 3.27 # 0.05 3.00 2.35 # 0.09
7332............................ 2.10 1.54 # 0.10 3.67 2.54 # 0.20 2.92 2.05 # 0.16
2.24 1.65 # 0.10 3.80 2.77 # 0.12 2.80 2.23
7457............................ 1.93 2.27 2.72 3.37 2.49 2.24
1.99 2.21 # 0.05 2.92 2.98 # 0.06 2.44 2.26 # 0.07
TABLE 3
Mean Differences between Our Indices
and the Trager et al. Data
Line
Difference
(8)
Error a
(8)
H# .......................................... +0.07 #0.06
Mg b....................................... #0.05 #0.07
hFei ........................................ +0.12 #0.09
a Formal errors of the mean offsets of our index system
with respect to the Lick one.
STELLAR POPULATIONS IN LENTICULAR GALAXIES 231

the edges of the frames) as 0.1 8. Some galaxies of the sample
have been observed twice. In Table 2 we show the raw index
measurements from two independent observational runs for each
of those objects; the uncertainties accompanying the bulge in­
dices partly reflect the index variations along the radii---we av­
erage four measurements at four R­values from 4 00 to 7 00 for each
galaxy and give here the errors of the means. The mean abso­
lute difference between two independent index measurements
is 0.20 8 for the nuclei and 0.18 8 for the bulges over Table 2. If
we analyze the three indices separately, we obtain the mean ab­
solute differences (the rms of the differences) of 0.22 8 (0.29 8)
for H#, 0.15 8 (0.19 8) for Mg b, and 0.22 8 (0.28 8) for the
composite iron index. These results mean that the accuracy of
the Mg b index corresponds to our expectations from the S/N
statistics, namely, that it is 0.1 8, and that the accuracy of H# and
hFei is somewhat worse, namely, that it is 0.15 8. Among our 58
galaxies, 28 objects have Lick index measurements through the
central aperture 2 00 ; 4 00 by Trager et al. (1998). The results of the
comparison of these standardized Lick indices with our mea­
surements for the nuclei are presented in Table 3 and in Figure 1.
The smallest scatter is found for H# and the largest one for hFei,
which is consistent with the fact that among the four indices, H#,
Mg b, Fe5270, and Fe5335, the errors quoted by Trager et al.
(1998) are the smallest for H# (0.24 8 on average over the com­
mon list) and the largest for Fe5335 (0.34 8 on average over
the common list). In general, our index system does not deviate
from the standard Lick one in any systematic way, so we can de­
termine the stellar population properties by comparing our indi­
ces to evolutionary synthesis models.
3. STELLAR POPULATION PROPERTIES IN THE NUCLEI
AND THE BULGES OF S0 GALAXIES
Tables 4 and 5 contain the measured Lick indices H#, Mg b,
and hFei # ( Fe5270 × 5335)/2 for the nuclei and for the bulges,
respectively, as well as the parameters of the stellar population---
luminosity­weighted age and metallicity---determined with these
indices as described below. Some galaxies have measurements
only for the nuclei or only for the bulges for various reasons;
for example, in NGC 5866 the nucleus is completely obscured
by dust, and in NGC 676 the bulge measurements are severely
contaminated by a bright star projected at 5 00 from the nucleus.
For the indices presented here, there are models based on evolu­
tionary synthesis of simple (one age and one metallicity) stellar
populations (see, e.g., Worthey 1994). These models allow esti­
mation of the luminosity­weighted mean metallicities and the
ages of the stellar populations by comparing the hydrogen­line
index H# to any metal­line index. We also consider the dura­
tion of the last major star­forming episode by comparing hFei to
Mg b. Chemical evolution models (see, e.g., Matteucci 1994)
show that because of the difference in the timescales of iron
and magnesium production by a stellar generation, the solar
Mg/Fe abundance ratio can be obtained only by very contin­
uous star formation, and brief star formation bursts, with # #
0:1 Gyr, would give significant magnesium overabundance, up
to ½Mg/Fe# ¼ ×0:3 to +0.4. In this work we use recent models
by Thomas et al. (2003) because these models are calculated for
several values of [Mg/Fe]: they allow us to estimate Mg/Fe ra­
tios of the stellar populations from Mg b and hFei#( Fe5270 ×
5335)/2 measurements.
Figure 2 presents the hFei versus Mg b diagrams for the bulges
and Figure 3 the same diagrams for the nuclei, for all four types
of environments. For some galaxies (e.g., NGC 2655 and NGC
2911) where the N i k5199 emission is significant, the Mg b
indices are corrected from this emission line according to the
prescription of Goudfrooij & Emsellem (1996). The model se­
quences for ½Mg/Fe# ¼ 0:0, +0.3, and +0.5 are well separated on
the diagrams of hFei versus Mg b, so we can estimate the mean
Mg/Fe ratios ``by eye.'' Surprisingly, the bulges of the group
central galaxies differ from those of the second­ranked group
members: the former have a mean ½Mg/Fe# # ×0:2, and the
latter have +0.1. As by definition the second­rank group galaxies
are less luminous than the central ones, this difference could be
attributed not to the environment density but to the galaxy mass
effect, at first glance. To check this, in Figure 4 we have plotted
the bulges only for the galaxies within the narrow stellar veloc­
ity dispersion range # # ¼ 145--215 km s #1 ; in this # # range
the central and second­rank group members of our sample have
the same mean # # of 172 km s #1 , but the difference between the
central group galaxies and the second­rank members still persists
in Figure 4. This tendency of the S0 galaxies in the centers of
groups to more resemble the cluster lenticular galaxies and of the
second­rank group members and the paired galaxies to be like
the field S0 galaxies is in general confirmed by the nucleus
distribution in the hFei versus Mg b diagrams ( Fig. 3), although
there are more ``outliers'' among the nuclei: evidently, the evo­
lution of nuclear stellar populations bears more individual fea­
tures than that of the bulges.
To break the age­metallicity degeneracy and simultaneously
determine the mean luminosity­weighted ages and the metal­
licities of the stellar populations, we compare the H# indices to
the combined metal­line index ½MgFe# # ( Mg bhFei) 1/2 by plot­
ting our data together with the models of Thomas et al. (2003);
Fig. 1.---Comparison of our measurements of the nuclear Lick indices with the aperture data of Trager et al. (1998) for 28 common galaxies. The straight lines are
the bisectors of the quadrants (``lines of equality'').
SIL'CHENKO
232

TABLE 4
Indices and Ages for the Nuclei of the S0 Galaxies
Galaxy a
Environment b
H# Mg b hFei
T c
(Gyr) [Z /H] c
EW([O iii] k5007)
(8)
T 0 d
(Gyr) [Z/H] 0 d
N0080......................... Group center 1.62 5.06 3.04 7 +0.4 0.08 6 +0.5
N0474......................... Group member ( pair) 1.70 4.55 3.14 4 +0.4 0.94 2 #+0.7
N0524......................... Group center 1.33 4.87 2.68 14 +0.1 0.46 10 +0.2
N0676......................... Group member (pair) 1.02 4.16 2.90 >15 0 2.0 3 +0.7
N0936......................... Group center 1.27 4.78 3.03 15 +0.2 0.79 5 +0.4
N1023......................... Group center 1.57 5.03 2.99 8 +0.4 0.12 7 +0.4
N1161......................... Group member (pair) 1.84 5.31 3.04 3 +0.7 0.06 4 +0.7
N2300......................... Group member 1.64 5.19 2.87 7 +0.4 0 7 +0.4
N2549......................... Group center 2.51 4.47 3.32 <2 #+0.7 0.20 <2 #+0.7
N2655......................... Group center 1.65 3.74 2.24 2 0 2.51 2 0
N2681......................... Group center 3.52 2.31 2.02 <2 #0 0.56 <2 #0
N2685......................... Field 1.75 3.59 2.58 4 +0.1 0.57 4 +0.1
N2732......................... Group member (pair) 1.88 3.55 2.71 7 0 0.46 3 +0.3
N2768......................... Group center 0.91 4.90 2.64 11 +0.2 0.91 15 +0.1
N2787......................... Field 0.61 5.25 2.12 >15 0 0.95 >15 0
N2880......................... Field 1.72 4.15 2.63 9 +0.1 0.1 8 +0.2
N2911......................... Group center #0.11 5.65 2.59 15 +0.1 2.38 >15 0
N2950......................... Field 2.66 4.67 3.23 <2 #+0.7 0.28 <2 #+0.7
N3065......................... Group center 0.42 4.16 2.42 . . . e
. . . e
2.36 6 +0.2
N3098......................... Field 1.79 3.65 2.20 10 #0.2 0.28 7 #0.1
N3166......................... Group member 2.36 3.68 2.94 <2 +0.7 0.57 <2 +0.7
N3245......................... Group member 0.67 4.52 2.96 6 +0.3 0.61 >15 +0.1
N3384......................... Group member 2.04 4.64 3.07 3 +0.7 0.05 3 +0.7
N3412......................... Group member 2.33 4.00 3.02 2 +0.7 0.23 <2 +0.7
N3414......................... Group center 0.82 5.21 2.74 13 +0.2 1.23 7 +0.4
N3607......................... Group center 0.93 5.24 2.78 12 +0.2 0.71 15 +0.2
N3941......................... Group center 1.69 4.61 3.26 4 +0.7 0.83 2 #+0.7
N3945......................... Group member 1.44 4.74 3.28 6 +0.5 0.30 7 +0.5
N4026......................... UMa Cluster 1.73 4.44 3.11 6 +0.4 0 6 +0.4
N4036......................... Group center 0.47 5.70 2.92 11 +0.3 1.42 10 +0.4
N4111 ......................... UMa Cluster 1.99 4.60 2.56 <2 +0.7 0.54 2 +0.6
N4125......................... Group center 1.31 4.66 3.14 7 +0.4 0.70 5 +0.5
N4138......................... UMa Cluster 0.94 4.71 2.81 12 +0.2 4.7 <2 #+0.7
N4150......................... Group member 2.65 2.51 1.60 2 #0.2 0.87 <2 #0.1
N4179......................... Group center 1.90 4.94 3.31 4 +0.7 0 4 +0.7
N4233......................... Virgo Cluster 1.06 4.80 3.00 15 +0.2 0.78 10 +0.3
N4350......................... Virgo Cluster 1.41 5.26 2.91 8 +0.4 0.17 10 +0.4
N4379......................... Virgo Cluster 1.51 4.36 2.45 15 0 0.14 13 0
N4429......................... Virgo Cluster 1.60 4.61 2.96 3 +0.7 0.25 6 +0.4
N4526......................... Virgo Cluster 1.62 4.75 2.78 3 +0.7 0.24 6 +0.4
N4550......................... Virgo Cluster 1.64 3.20 2.53 5 0 1.16 3 +0.2
N4570......................... Virgo Cluster 1.72 5.18 2.86 5 +0.4 0 5 +0.4
N4638......................... Virgo Cluster 2.01 4.75 3.42 3 +0.7 0.06 3 +0.7
N4866......................... Group member (pair) 1.28 4.60 2.85 8 +0.3 0.69 8 +0.3
N5308......................... Group member 1.48 5.14 2.92 11 +0.3 0 11 +0.3
N5422......................... Group member 1.41 4.85 3.28 12 +0.4 0.52 5 +0.5
N5574......................... Group member 2.78 2.48 2.47 2 0 0.25 <2 0
N6340......................... Group center 1.30 4.57 2.84 11 +0.2 0.33 13 +0.2
N6548......................... Group member (pair) 1.67 4.58 2.90 8 +0.3 0 8 +0.3
N6654......................... Group member (pair) 1.67 4.51 2.78 8 +0.3 0 8 +0.3
N6703......................... Field 1.49 4.34 3.14 12 +0.2 0.33 7 +0.3
N7013......................... Field 1.60 3.81 3.00 6 +0.2 1.08 2 +0.5
N7280......................... Field 2.61 3.57 3.10 <2 +0.7 0.07 <2 +0.7
N7332......................... Field 2.12 3.72 2.86 3 +0.3 0.25 2 +0.4
N7457......................... Field 1.96 2.82 2.46 8 #0.2 0.46 4 0
N7743......................... Field 2.21 3.21 2.26 <2 +0.7 6.51 <2 . . .
U11920....................... Field 1.60 4.44 3.12 9 +0.3 0.76 3 +0.7
a
Galaxy ID: N ¼ NGC, U ¼ UGC.
b
From Guiricin et al. (2000).
c Estimated with the H# index corrected from the emission through the equivalent width of the H# emission line.
d Estimated with the H# index corrected from the emission through the [O iii] k5007 equivalent width.
e We cannot correct this H# index from the emission through the H# equivalent width.

TABLE 5
Indices and Ages for the Bulges of the S0 Galaxies
Galaxy a
Environment b
H# Mg b hFei
T c
(Gyr) [Z/H] c
EW([O iii] k5007)
(8)
T d
(Gyr) [Z/H] 0 d
N0080......................... Group center 1.60 4.67 2.82 (10) (+0.2) 0.08 8 +0.3
N0474......................... Group member (pair) 1.74 4.18 2.99 (7) (+0.2) 0.35 4 +0.4
N0524......................... Group center 1.07 3.65 2.00 >15 0? 0 >15 0?
N0936......................... Group center 1.40 4.51 2.50 15 0 0.48 9 +0.2
N1023......................... Group center 1.43 3.94 2.69 (15) (#0.1) 0.06 15 #0.1
N1161......................... Group member (pair) 1.71 4.24 2.86 (8) (+0.2) 0 8 +0.2
N2300......................... Group member 1.50 4.85 2.84 (12) (+0.2) 0 12 +0.2
N2549......................... Group center 2.22 4.05 3.15 2 +0.7 0.20 2 +0.7
N2655......................... Group center 1.45 3.64 2.27 15 #0.2 0.78 7 #0.1
N2681......................... Group center 2.66 2.30 1.97 3 #0.2 0.42 2 #0.2
N2685......................... Field 1.41 2.63 2.37 (>15) (#0.3?) 0.82 9 #0.3
N2732......................... Group member (pair) 1.62 3.53 2.30 11 #0.2 0.60 6 0
N2768......................... Group center 1.24 4.21 2.48 13 0 0.70 11 0
N2787......................... Field 1.03 4.54 2.38 >15 #0.1? 0.48 >15 #0.1
N2880......................... Field 1.74 3.92 2.56 (9) (0) 0.12 8 +0.1
N2911......................... Group center 0.69 3.89 2.34 >15 ? 0.78 >15 <0
N2950......................... Field 2.13 4.42 3.02 3 +0.7 0.72 <2 >+0.7
N3065......................... Group center 1.54 3.94 2.51 15 #0.1 0.90 4 +0.2
N3098......................... Field 1.96 3.57 2.33 (6) (0) 0.32 4 0
N3166......................... Group member 2.54 3.37 2.66 2 +0.3 0.35 <2 +0.3
N3245......................... Group member 1.70 4.30 3.02 (8) (+0.3) 0.16 5 +0.4
N3384......................... Group member 1.71 4.00 2.87 (9) (+0.2) 0.18 7 +0.2
N3412......................... Group member 2.13 3.62 2.80 (3) (+0.3) 0.25 2 +0.3
N3414......................... Group center 1.08 4.70 2.47 15 0 1.05 9 +0.2
N3607......................... Group center 1.59 4.37 2.83 10 +0.2 0 10 +0.2
N3941......................... Group center 1.58 3.80 2.70 (13) (0) 0.87 3 +0.3
N3945......................... Group member 1.38 4.19 3.12 (15) (+0.1) 0.12 14 +0.1
N4026......................... UMa Cluster 1.66 4.09 2.89 (10) (+0.2) 0.59 4 +0.3
N4036......................... Group center 0.86 3.85 2.64 >15 ? 0.36 >15 <0
N4111 ......................... UMa Cluster 1.61 3.53 2.32 11 #0.2 0.34 6 0
N4125......................... Group center 1.61 4.64 3.13 6 +0.4 0.34 4 +0.4
N4138......................... UMa Cluster 1.03 3.40 2.06 8 #0.2 1.7 6 #0.1
N4150......................... Group member 2.22 2.51 1.87 5 #0.3 0.70 3 #0.2
N4179......................... Group center 1.69 4.35 3.04 (8) (+0.3) 0 8 +0.3
N4233......................... Virgo Cluster 1.63 4.19 2.78 11 +0.1 0 11 +0.1
N4350......................... Virgo Cluster 1.63 4.73 2.75 (9) (+0.3) 0 9 +0.3
N4379......................... Virgo Cluster 1.66 3.98 2.29 (12) (#0.1) 0.35 8 0
N4429......................... Virgo Cluster 1.43 4.52 2.69 15 +0.1 0.19 12 +0.1
N4526......................... Virgo Cluster 1.30 4.45 2.76 3 +0.6 0.06 >15 +0.1
N4550......................... Virgo Cluster 1.66 3.14 1.95 15 #0.3 0.70 6 #0.2
N4570......................... Virgo Cluster 1.64 4.74 2.91 (8) (+0.3) 0 8 +0.3
N4638......................... Virgo Cluster 2.13 4.17 3.20 (3) (+0.7) 0 3 +0.7
N4866......................... Group member (pair) 1.50 4.08 2.23 (15) (#0.2) 0.60 8 0
N5308......................... Group member 1.62 4.94 3.04 (7) (+0.4) 0.08 6 +0.5
N5422......................... Group member 1.62 4.78 3.16 (7) (+0.4) 0.73 3 +0.7
N5574......................... Group member 2.38 3.15 2.43 (3) (+0.2) 0.36 2 +0.2
N5866......................... Group member 1.73 3.62 2.95 7 +0.1 0.28 5 +0.2
N6340......................... Group center 1.05 3.12 2.11 >15 #0.3 0.62 >15 #0.3
N6548......................... Group member (pair) 1.91 4.71 3.02 (4) (+0.5) 0 4 +0.5
N6654......................... Group member (pair) 1.36 4.19 2.68 (#15) (0) 0.24 15 0
N6703......................... Field 1.92 4.45 3.16 3 +0.6 0.38 3 +0.7
N7013......................... Field 2.09 3.30 2.35 3 +0.1 0.60 2 +0.2
N7280......................... Field 1.87 3.01 2.72 7 #0.1 0.07 7 #0.1
N7332......................... Field 1.60 2.66 2.14 (15) (#0.3) 0.58 7 #0.2
N7457......................... Field 2.24 3.18 2.25 (4) (#0.1) 0.20 4 #0.1
N7743......................... Field 2.18 3.00 2.47 (4) (0) 1.48 <2 +0.3
U11920....................... Field 1.78 4.06 2.80 (8) (+0.2) 0.73 2 +0.5
Note.---The values of age and metallicity in parentheses are obtained without correcting the H# indices from emission.
a
Galaxy ID: N ¼ NGC, U ¼ UGC.
b From Guiricin et al. (2000).
c Estimated with the H# index corrected from the emission through the equivalent width of the H# emission line.
d Estimated with the H# index corrected from the emission through the [O iii] k5007 equivalent width.
234

Fig. 2.---hFei vs. Mg b diagrams for the bulge index measurements. The typical accuracy of the azimuthally averaged indices is 0.1--0.15 8. The simple stellar
population models of Thomas et al. (2003) for three different magnesium­to­iron ratios (0.0, +0.3, and +0.5; curve triads from top to bottom) and three different ages
(5, 8, and 12 Gyr; from top to bottom in every triad) are plotted as a reference. The small symbols along the model curves mark metallicities of +0.67, +0.35, 0.00,
#0.33, #1.35, and #2.25 ( from right to left).
235

Fig. 3.---Same as Fig. 2, but for the nucleus index measurements. The typical accuracy of the nuclear indices is 0.1--0.15 8.
Fig. 4.---Same as Fig. 2, but only for the group galaxies with # # within the range of 145--215 km s #1 .

earlier we checked that this diagram is insensitive to the Mg/Fe
ratio. However, we have one serious problem here: the absorption­
line index H# could be contaminated by emission, especially in
the nuclear spectra. To correct for the emission the H# indices
that we have measured we have used data on equivalent widths
of H# emission lines because H# emission lines are always
much stronger than H# emission lines and because an H# ab­
sorption line is not deeper than an H# absorption line in spec­
tra of stellar populations of any age while in intermediate­age
population spectra it is much shallower. The emission­line in­
tensity ratio, H# / H#, has been well studied both empirically and
theoretically. The minimum value of this ratio, 2.5, is known for
the case of radiative excitation by young stars ( Burgess 1958).
For other types of excitation this ratio is higher. We have no pure
H ii--type nuclei in our sample, so here we use the formula
EW(H# emis ) ¼ 0:25EW(H# emis ): this mean relation is obtained
by Stasinska & Sodre ’ (2001) for a quite heterogeneous sample of
nearby emission­line galaxies. The data on EW(H# emis ) for the
nuclei we take mainly from Ho et al. (1997). The bulge H# in­
dices were corrected for the emission by using H# equivalent
widths obtained with the red MPFS spectra for about one­half of
the sample (28 objects; see Table 1). Unlike Ho et al. (1997),
who obtained EW(H# emis ) by subtracting a pure absorption­line
template from the observed spectra, we applied a multicompo­
nent Gaussian analysis to the combinations of the H# absorption
and emission lines, which was effective due to mostly different
velocity dispersions of stars and gas clouds in the galaxies under
consideration. From the rest, 16 galaxies have negligible emis­
sion lines in the bulge spectra [EW(½O iii#) # 0:3 8; see Table 5],
and for the others the age estimates obtained by using the H#
indices ``corrected through the H# '' ( Table 5) are indeed only
upper limits. To correct in some way all the bulge spectra, we
have also used the well­known approach that involves the [O iii]
k5007 emission line; Trager et al. (2000) recommend using the
statistical correction #H# ¼ 0:6EW(½O iii#k5007), although they
note that individual ratios H# /½O iii# could vary between 0.33
and 1.25 within their sample of elliptical galaxies. In Figure 5 we
compare the corrections obtained two different ways for the nu­
clei. If we exclude two galaxies with extremely strong emission
in the centers---NGC 4138 and NGC 7743---statistically, the two
types of corrections are indistinguishable; however, the accu­
racy of the [O iii] measurement is not very high due to strong
underlying absorption lines of Ti i, and the weak emission lines
[O iii] with equivalent widths of EW # 0:3 8 are evidently ar­
tifacts. To summarize this analysis, we conclude that while for
mutual comparisons of the age distributions we must take only
the age estimates corrected through [O iii] because this correc­
tion can be made for all galaxies of the sample, for the individual
galaxies having the red spectra the estimates made with the H#
indices corrected through H# are more reliable because the H#
emission is stronger and the ratio of the Balmer emission lines
depends only on the excitation mechanism, unlike the ratio of
H# to [O iii], which also depends on the metallicity of the gas.
Figure 6 presents the diagrams of H# versus [MgFe] for the
nuclei (top) and for the bulges (bottom) of the galaxies of all
types of environments with their H# indices corrected through
H# (left) and with their H# indices corrected through [O iii]
(right). By inspecting these diagrams, we determine the ages and
the metallicities by eye, which provides an accuracy of #0.1 dex
in metallicity and 1 Gyr for the ages less than 8 Gyr and #2 Gyr
for older stellar systems, which match our accuracy of the Lick
indices. Directly in the diagrams one can see that the range of the
ages of the nuclei is very wide: they may be as young as 1 Gyr
old and as old as 15 Gyr old. The bulges are on average older
than the nuclei, and in the bottom plots one can see a segregation
of the galaxies according to their type of environment: most of
the bulges of the group centers and the cluster galaxies are older
than 5 Gyr, whereas some of the group members and the field
lenticular galaxies have bulges as young as 2--3 Gyr old. The
metallicity ranges seem to be similar for the bulges in all types
of environments: their [Z/H] are confined between ##0.3 and
#+0.4. By applying formal Gaussian fittings to the metallicity
distributions, we obtain the mean metallicity for the bulges in
dense environments to be #0.04 and that for the bulges in sparse
environments to be #0.13, with the same rms of 0.5 dex. The
nuclei seem to be on average more metal­rich: only three nuclei
in the galaxies of sparse environments have metallicities less
than the solar value.
Kuntschner et al. (2002) have already reported the differ­
ence between the stellar population characteristics of early­type
galaxies in dense and sparse environments. Their measurements
were aperture spectroscopy, and their samples were the Fornax
Cluster as an example of dense environments and galaxies with­
out more than two neighbors within the search radius of 1.3 Mpc
as an example of sparse environments; the latter sample is prob­
ably close to our combination of the field plus paired galaxies.
They have found that the E/S0 galaxies in sparse environments
are younger than the E/S0 galaxies in the cluster by 2--3 Gyr,
and our result for the S0 galaxies is quite the same. But they have
also found an anticorrelation between age and metallicity, the
younger galaxies in sparse environments being on average more
metal­rich ( by 0.2 dex) than the older galaxies in the cluster,
while if we see any metallicity difference, it should be in the op­
posite sense.
In Figure 7 we plot cumulative distributions of the ages: the
number of galaxies not older than T versus log T (in gigayears).
We have united the samples of the brightest group S0 galaxies
and the cluster galaxies into a ``dense environment'' sample and
the group second­ranked members and the field S0 galaxies into
a ``sparse environment'' sample. The effect of environment is seen
both for the nuclei and for the bulges: in sparse environments the
Fig. 5.---Comparison of the H# index corrections from the emission obtained
two different ways: through H# equivalent widths and through [O iii] equivalent
widths as described in the text. The straight line is the bisector of the quadrant
(``line of equality'').
STELLAR POPULATIONS IN LENTICULAR GALAXIES 237

Fig. 6.---Age­diagnostic diagrams for the stellar populations in the nuclei (top) and circumnuclear regions (bottom) of the galaxies under consideration; the H# index
measurements are corrected from the emission contamination by using H# in the left plots and by using [O iii] in the right plots, as described in the text. The typical
accuracy of the indices is 0.1 8 for the combined metal­line index and 0.15 8 for H#. The stellar population models of Thomas et al. (2003) for ½Mg/Fe# ¼ ×0:3 and five
different ages (2, 5, 8, 12, and 15 Gyr; curves from top to bottom) are plotted as a reference; the dashed lines crossing the model curves mark the metallicities of +0.67,
+0.35, 0.00, and #0.33 ( from right to left). In the top right plot the nucleus of NGC 7743, which has H# corr > 6 8, is omitted.

stellar populations are, on average, younger. The estimates of
the median ages are the following: 3.7 and 6 Gyr for the nuclei
of the galaxies in sparse and dense environments, respectively,
and 4.8 and 8.3 Gyr for the bulges.
4. DISCUSSION
It is a little bit surprising that according to my results, the
``dense'' type of environment must be ascribed not only to the
clusters but also to the centers of groups: the first­ranked and
the second­ranked S0 galaxies of the groups have very different
properties of their central stellar population. However, this con­
clusion is close to the recent finding by Proctor et al. (2004) that
the early­type galaxies of Hickson compact groups more resem­
ble the cluster galaxies than the field ones. I think it gives us a
hint that the dynamical effect of close neighbors could play the
main role in evolution rate, not the mass of the common dark
halo.
Recently, some evidence has been published (Caldwell et al.
2003; Nelan et al. 2005) that the ages of the stellar populations in
early­type galaxies are correlated with the central stellar velocity
dispersion. In our sample, the galaxies in dense environments are
on average more massive than those in sparse environments, so
one might suggest that the age difference found above could be
due to the mass difference and not to the environment influence.
To check this effect, I have plotted the bulge age estimates ver­
sus the central stellar velocity dispersion in Figure 8. Indeed, the
correlation is present, implying that the more massive bulges
are older; the slope of the regression log T versus log # #;0 is
1:76 # 0:65 for the dense environments and 1:30 # 0:43 for the
sparse ones with the correlation coefficients of 0.53 and 0.55,
respectively. Following Caldwell et al. (2003), we have calcu­
lated the mean ages of the bulges within narrow ranges of stellar
velocity dispersion (when the age estimate has only the low limit
of 15 Gyr, I have ascribed the value of 16 Gyr to it). These
estimates are given in Table 6 (compare with those in Caldwell
et al. [2003]: 7.4 Gyr, rms 4.2 Gyr, in the range of # # ¼ 100--
160 km s #1 , and 9.9 Gyr, rms 4.2 Gyr, for # # > 160 km s #1 ). One
can see from Table 6 that the ages of the bulges are the same in
different types of environments for the lower bins of # # , 105--
145 and 145--185 km s #1 , but in the highest bin, 185--225 km s #1 ,
the ages are dramatically different, the massive bulges in dense
environments being much older than the massive bulges in
sparse environments. By inspecting Figure 8, we notice that the
separation between the bulges in different types of environments
starts from about # # ¼ 170 km s #1 . Taking seven galaxies in
dense environments and seven galaxies in sparse environments
with # # in the range of 170--215 km s #1 , we obtain hT i ¼
9:7 # 1:3 Gyr, rms 3.2 Gyr, for the former and hTi ¼ 6:6 #
1:5 Gyr, rms 3.7 Gyr, for the latter subsample, so the difference
is 3:1 # 2:0 Gyr. The application of the Student T­test to this
double subsample of the massive bulges shows that the mean age
of the massive bulges in dense environments is larger than the
Fig. 7.---Cumulative age distributions: the number of objects younger than
the abscissa, which is log T in gigayears, vs. log T . Top: Stellar nuclei of the gal­
axies. Bottom: Bulges taken in the rings between R ¼ 4 and 7 00 .
Fig. 8.---Relation between the bulge age estimates obtained in this work and
central stellar velocity dispersions: the regression straight lines fitting the depen­
dences of log T on log # are shown here by power­law functions and plotted as a
solid line for the dense environment galaxies and as a dashed line for the sparse
environment galaxies.
TABLE 6
Mean Ages of the Bulges within Fixed Stellar
Velocity Dispersion Ranges
Dense Environments Sparse Environments
Range of # #
( km s #1 ) N gal
hT i
(Gyr) rms N gal
hT i
(Gyr) rms
105--145 ................. 6 6.2 # 2.2 4.9 6 4.5 # 1.0 2.3
145--184 ................. 8 6.5 # 1.0 2.6 9 6.6 # 1.6 4.5
185--225 ................. 7 11.6 # 1.2 2.8 5 8.6 # 1.9 3.9
STELLAR POPULATIONS IN LENTICULAR GALAXIES 239

mean age of the massive bulges in sparse environments with
a probability higher than 0.85 (the hypothesis of hTi dense #
hTi sparse is rejected at the significance level of 0.14).
5. CONCLUSIONS
By considering the stellar population properties in the nuclei
and the bulges of nearby lenticular galaxies in various types of
environment, I have found certain differences between the nu­
clei and the bulges, as well as between the galaxies in dense and
sparse environment. The nuclei are on average younger than the
bulges in any types of environment, and both the nuclei and the
bulges of S0 galaxies in sparse environments are younger than
those in dense environments. The results of the consideration
of the Mg/Fe ratios suggest that the main star formation epoch
could be more brief in the centers of the galaxies in dense
environments.
I am grateful to the astronomers of the Special Astrophysi­
cal Observatory of RAS V. L. Afanasiev, A. N. Burenkov, V. V.
Vlasyuk, S. N. Dodonov, and A. V. Moiseev for supporting the
MPFS observations at the 6 m telescope. The 6 m telescope is
operated under the financial support of the Science Ministry of
Russia (registration No. 01­43); we also thank the Programme
Committee of the 6 m telescope for allocating the observational
time. During the data analysis we have used the Lyon­Meudon
Extragalactic Database ( LEDA) supplied by the LEDA team at
the CRAL­Observatoire de Lyon ( France) and the NASA / IPAC
Extragalactic Database ( NED), which is operated by the Jet Pro­
pulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administra­
tion. The study of the young nuclei in lenticular galaxies was
supported by the grant of the Russian Foundation for Basic
Researches 01­02­16767.
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