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YOUNG STELLAR NUCLEI IN LENTICULAR GALAXIES: NGC 5574 AND NGC 7457 1
O. K. Sil'chenko 2
Sternberg Astronomical Institute, Universitetsky Pr. 13, Moscow State University, Moscow 119992, Russia; and Isaac Newton Institute of Chile, Moscow
Branch, Russia; olga@sai.msu.su
V. L. Afanasiev
Special Astrophysical Observatory, Russian Academy of Science, St. Zelenchukskaya, Karachi�Cherkesia, Nizhnij Arkhyz 369167, Russia; vafan@sao.ru
and
V. H. Chavushyan and J. R. Valdes
Instituto Nacional de Astrof� �sica, Optica y Electro � nica, A.P. 51 y 216, C.P. 72000, Puebla, Puebla 72840, Mexico; vahram@inaoep.mx, jvaldes@inaoep.mx
Received 2002 April 29; accepted 2002 June 10
ABSTRACT
The stellar population properties in the centers of NGC 5574 and NGC 7457 (two lenticular galaxies previ�
ously shown to have radially homogeneous blue optical�band colors) are studied by means of integral�field
spectroscopy. The compact nuclei of the galaxies appear to be chemically distinct: in NGC 5574 the nucleus
is distinguished by a higher than solar iron�to�magnesium ratio, and in NGC 7457 a drop of mean stellar met�
allicity by a factor of 2 exists between the nucleus and the bulge. Both galaxies demonstrate a rather young
mean luminosity�weighted age of the stellar populations in the bulges, not older than 5--7 Gyr; however, the
chemically distinct nuclei are still younger, 2--2.5 Gyr old. The kinematics of the stars in the center of NGC
5574 are probably a#ected by the influence of its global bar. The compact core of NGC 7457, with a radius of
about 1>5, shows a visible counterrotation; a combined analysis of the photometric and kinematical two�
dimensional maps allows us to conclude that the core rotates axisymmetrically, but its rotation plane is
inclined to the main symmetry plane of the galaxy. The stellar velocity dispersion in the centers of both gal�
axies is anomalously low: 60--80 km s #1 .
Subject headings: galaxies: evolution --- galaxies: individual (NGC 5574, NGC 7457) ---
galaxies: kinematics and dynamics --- galaxies: nuclei --- galaxies: structure
1. INTRODUCTION
A common view is that S0 galaxies are similar to ellipti�
cals with respect to their stellar populations, with both types
of galaxies possessing homogeneously old, metal�rich stars.
This opinion is mostly due to the fact that the mean inte�
grated colors of S0 and E galaxies are the same (Buta &Wil�
liams 1995). Therefore, when we undertook a spectral
survey of the central parts of 100 nearby galaxies some years
ago (Sil'chenko & Shapovalova 1989; Sil'chenko 1990) and
found that half of all lenticulars in our sample demonstrated
strong Balmer absorption lines in their spectra, similar to
early�type spirals rather than to ellipticals, this finding was
unexpected to us. Applying stellar population synthesis and
also comparing the data with Galactic globular cluster spec�
tra, we have shown that the mean luminosity�weighted stel�
lar ages in the centers of half of the nearby S0s observed by
us are really less than 5 Gyr (Sil'chenko 1993). Later, we
began to separate the stellar populations of the unresolved
nuclei from those of the surrounding bulges because their
properties (and so far, the evolution) often appeared to be
quite di#erent. We analyzed literature data on aperture
photometry and assembled a list of early�type galaxies
whose nuclei are significantly redder than the bulges (Sil'�
chenko 1994), suggesting that these are candidates to have
chemically distinct nuclei; the number of such candidates
reached 25%--30% of the S0 galaxies considered. Subsequent
two�dimensional spectroscopy of this sample has confirmed
that, indeed, most of them have chemically distinct nuclei
that are more metal rich by a factor of 2--10 than their near�
est outskirts. However, curiously, these chemically distinct,
metal�rich stellar nuclei, although redder, also appear to be
younger than the bulges: for example, the mean luminosity�
weighted stellar age in the nucleus in NGC 1023 is 7 Gyr,
and in the nucleus in NGC 7332 as low as 2 Gyr, although
their bulges have stellar populations as old as 15 Gyr (Sil'�
chenko 1999). Therefore, the red color in this case is a sig�
nature of a high mean metallicity and not of the
homogeneously old age of the stars. On the other hand,
among the lenticular galaxies with strong Balmer absorp�
tion lines in the integrated central�region spectra, there have
been a few with rather blue nuclei. A natural question is:
What is the di#erence between stellar populations in the red
and blue nuclei of lenticular galaxies? Are the blue ones
bluer due to lower age or due to lower metallicity, and do
any di#erences in stellar population properties exist between
the blue nuclei and their surrounding bulges? As a prelimi�
nary step toward answering these questions, we investigate
here two intermediate�luminosity S0 galaxies with blue
nuclei, NGC 5574 and NGC 7457. The global parameters of
the galaxies are given in Table 1.
NGC 5574 is a close neighbor of the intermediate�
luminosity elliptical NGC 5576 (with a separation of about
of 20 kpc); the pair as a whole is detected in the 21 cm line
(Lake & Schommer 1984), but no emission lines in the opti�
cal spectral range are seen, so no present star formation is
suspected. In addition, when we studied the central part of
2 Guest investigator of the UKAstronomy Data Centre.
1 Partly based on observations collected with the 6 m Bolshoi Teleskop
Azimutal'nij at the Special Astrophysical Observatory of the Russian
Academy of Sciences.
The Astrophysical Journal, 577:668--679, 2002 October 1
# 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
668

NGC 5576, we found a chemically distinct nucleus of a
rather low age: the luminosity�weighted mean stellar age in
the nucleus of NGC 5576 was found to be 3 Gyr, and a mar�
ginal iron overabundance was also detected (Sil'chenko
1997). Since faint outer stellar spiral arms are seen in NGC
5574 (Michard & Marchal 1994), one has to conclude that
there is an interaction between the two galaxies that may
a#ect the evolution of their central parts. We obtained mul�
tiaperture photoelectric data for NGC 5574 and measured
the same B#V colors in the nucleus and in the bulge (#0.8).
Both of these are rather blue for an intermediate�luminosity
lenticular (Sil'chenko 1989). Spectral investigation (Sil'�
chenko & Shapovalova 1989) has revealed strong Balmer
absorption lines up to H# in the central region of the galaxy,
so we are already prepared to find a young nucleus in NGC
5574, but the question of the bulge properties and also
that of the circumnuclear disk signatures remain to be
considered.
NGC 7457 is a visibly well�studied nearby lenticular gal�
axy. In particular, its surface photometry has been analyzed
more than once, and Kormendy (1977) has cited its surface
brightness profile as prototypical for an S0 galaxy. How�
ever, detailed consideration reveals some peculiarities even
here. For example, the galaxy is known as disk�dominated,
e.g., D=B � 1:6 is given by Burstein (1979), but Andredakis,
Peletier, & Balcells (1995), who have fitted galactic bulges
by Sersic's law with a free power 1=n and have claimed the
existence of a correlation between relative bulge luminosity
and n, have found an exclusive value of n � 6 for the bulge
of NGC 7457, which is even steeper than the profiles of pure
ellipticals. Peletier et al. (1999), from multiband photom�
etry, have concluded that the bulge of NGC 7457 is unusu�
ally blue for its luminosity and that its colors imply a mean
age of about 7 Gyr, whereas all the other bulges of their
sample galaxies are about 10 Gyr old. To complete the pic�
ture of the peculiarities in the `` prototypical S0 galaxy ''
NGC 7457, we must mention that although the galaxy lacks
any emission lines, particularly in the nucleus (Ho, Filip�
penko, & Sargent 1995), and any detectable H i (Chamar�
aux, Balkowski, & Fontanelli 1987), a rather large amount
of molecular gas is detected that is concentrated in the com�
pact circumnuclear disk with a scale length much shorter
than that of the global stellar disk (Taniguchi et al. 1994).
The layout of the paper is as follows. We report our
observations and other data that we use in x 2. The radial
variations of the stellar population properties are analyzed
in x 3, and in x 4 two�dimensional stellar velocity fields
obtained by integral�field spectroscopy are presented. Sec�
tion 5 gives the discussion and our conclusions.
2. OBSERVATIONS AND DATA REDUCTION
The spectral data that we analyze in this work were
obtained with two di#erent integral�field spectrographs.
Integral�field spectroscopy is a rather new approach that
was first proposed by Professor G. Courtes some 15 years
ago; for a description of the instrumental idea, see, e.g.,
Bacon et al. (1995). It allows us to simultaneously obtain a
set of spectra in a wide spectral range from an extended area
on the sky, for example, from the central part of a galaxy. A
two�dimensional array of microlenses provides a set of
micropupils that are put onto the entry of a spectrograph,
and having reduced the full set of spectra corresponding to
the individual spatial elements, we obtain a list of fluxes in
continuum and in emission lines, of line�of�sight velocities
for both stars and ionized gas, and of absorption�line equiv�
alent widths, which are usually expressed as indices in the
well�formulated Lick system (Worthey et al. 1994). This list
can be transformed into two�dimensional maps of the above
mentioned characteristics for the central part of the galaxy
that is studied. Besides the panoramic view benefits, such an
approach gives a unique opportunity to superpose various
two�dimensional distributions over each other without any
di#culties with positioning. In this work, we use the data in
the green spectral range, 4200--5600 A � . These spectra are
used to calculate the Lick indices H#, Mg b, Fe5270, and
Fe5335, which are suitable for determining the mean stellar
luminosity�weighted metallicity, age, and Mg=Fe ratio of
old populations (Worthey 1994), and are also used for
cross�correlation with spectra of template stars, usually of
K0 III--K3 III spectral type, to obtain a line�of�sight velocity
field for the stellar component and a map of stellar velocity
dispersion. The list of the exposures made for NGC 5574
and NGC 7457 with two di#erent two�dimensional spectro�
graphs is given in Table 2.
The most recent variant of the Multipupil Fiber Spectro�
graph (MPFS) became operational in the prime focus of the
6 m Bolshoi Teleskop Azimutal'nij (BTA) in 1998. 3 Com�
pared to the previous MPFS, in the new variant, the field of
view is increased and the common spectral range is larger
due to the use of fibers that transmit light from 16 # 15
TABLE 1
Global Parameters of the Galaxies
Parameter Database NGC 5574 NGC 7457
Type............................................................. NED SBO--?sp SA(rs)0--?
R 25 (kpc) ...................................................... LEDA 5.0 7.4
B 0
T ................................................................ RC3 13.19 11.76
M B ............................................................... LEDA #18.5 #18.6
�B#V� 0
T ....................................................... RC3 0.82 0.83
�U#B� 0
T ....................................................... RC3 0.31 0.32
V r (km s #1 ) ................................................... NED 1659 812
Distance: H 0 = 75 km s #1 Mpc #1 (Mpc)....... LEDA 23 12.2
Inclination (deg)........................................... LEDA 59 70
PA phot (deg) ................................................. LEDA 63 130
3 See http://www.sao.ru/#gafan/devices/mpfs/mpfs_main.htm.
NGC 5574 AND NGC 7457 669

square elements of the galaxy image to the slit of the spec�
trograph, together with an additional 16 fibers that transmit
the sky background light, taken separately from the galaxy,
so that separate sky exposures are not necessary. The size of
one spatial element is approximately 1 00 # 1 00 ; a CCD TK
1024 # 1024 detector is used. The reciprocal dispersion is
1.35 A � per pixel, with a rather stable spectral resolution of
5 A � . To calibrate the wavelength scale, we use separate
exposures of a helium, neon, and argon hollow cathode
lamp; the internal accuracy of the fit to these calibration
spectra is typically 0.25 A � in the green. In addition, we mea�
sured the [O i] #5577 night sky emission line to check the
accuracy of our calibration and found no systematic veloc�
ity shift. To calibrate the new MPFS index system onto the
standard Lick one, we observed 15 stars from the list of
Worthey et al. (1994) during four observational runs and
calculated the linear regression formulae to transform our
index measurements into the Lick system; the rms scatters
of points near the linear dependencies are about 0.2 A � for all
four indices under consideration, thus being within the
observational errors of Worthey et al. (1994). To correct the
index measurements for the stellar velocity dispersion,
which is usually substantially nonzero in the centers of
early�type galaxies, we have smoothed the spectrum of the
standard star HD 97907 by a set of Gaussians of various
widths; the derived dependencies of index corrections on #
were approximated by fourth�order polynomials and
applied to the measured index values before their calibra�
tions into the Lick system. However, in both galaxies con�
sidered here, the stellar velocity dispersion is below 100 km
s #1 , so the correction for it is negligible.
The second two�dimensional spectrograph, the data of
which we use in this work, is a new instrument operated at
the 4.2 m William Herschel Telescope (WHT) on La Palma
named SAURON; for its detailed description, see Bacon et
al. (2001). We have taken the data for NGC 7457 obtained
with it in 1999 October from the open Isaac Newton Group
(ING) Archive of the UK Astronomy Data Centre. 4 The
field of view of this instrument is 41 00 # 33 00 , with a spatial
element size of 0>94 # 0>94. The sky background 2 from
the center of the galaxy was exposed simultaneously with
the target. The spectral range is 4800--5400 A � , and the recip�
rocal dispersion is 1.11--1.21 A � varying from the left to the
right edge of the frame. The comparison spectrum is a neon
arc, and the linearization is made using a second�order poly�
nomial fit with an accuracy of 0.07 A � . The index system is
checked by using 12 stars from the list of Worthey et al.
(1994) that were observed during the same observational
run. Linear regressions were fitted to the dependence of the
instrumental indices on the standard Lick indices (the rms
scatters of the points about the fitting lines were below 0.2
A � , as in the case of the MPFS data), and these fits were used
to transform our measurements into the standard Lick
system.
The exposure times for the galaxies observed with the
MPFS were chosen to be long enough to provide signal�to�
noise ratios of more than 60 (per angstrom) in the nucleus
and #30 near the edges of the frames; the statistical error
estimations made in the manner of Cardiel et al. (1998)
range from 0.1 A � for NGC 7457 and 0.2 A � for NGC 5574 in
the center to, correspondingly, 0.3 and 0.7 A � for the individ�
ual spatial elements in the outer parts. Besides constructing
two�dimensional maps of indices, in order to obtain a con�
stant level of accuracy along the radius, we co�added ele�
mentary spectra obtained with every two�dimensional
spectrograph in concentric rings centered onto the nuclei
and studied the radial dependencies of the stellar population
properties by comparing the azimuthally averaged observed
absorption�line indices with evolutionary synthesis models
of old stellar populations by Worthey (1994). We estimate
the typical accuracy of our azimuthally averaged indices as
better than 0.1 A � .
In addition to the two�dimensional spectral data, we have
obtained near�infrared (NIR) photometric data for the gal�
axies under consideration. The observations were made at
the 2.1 m telescope of the National Astronomical Observa�
tory of Mexico at San Pedro Martir, with the infrared cam�
era CAMILA. The camera is equipped with the detector
NICMOS 3, with a format of 256 # 256 pixels; a mode with
a scale of 0>85 (f =4:5) was used. The details of the photo�
metric observations are given in Table 3. Also, for NGC
7457 we have retrieved the NICMOS Hubble Space Tele�
scope (HST) data from the HST Archive (program ID
7450, PI R. F. Peletier). The galaxy was observed on 1997
July 6 with the NIC 2 camera, through the filters F110W
and F160W for 128 s each.
All the data, spectral and photometric, except the data
obtained with the MPFS, were reduced with the software
produced by V. V. Vlasyuk in the Special Astrophysical
TABLE 2
Two�dimensional Spectroscopy of NGC 5574 and NGC 7457
Date Galaxy
Exposure
(minutes) Configuration
Field
(arcsec 2 )
Spectral Range
(A � )
Seeing
(arcsec)
1999 Jun 6 ......... NGC 5574 60 BTA/MPFS + CCD, 1024 # 1024 16 # 15 4200--5600 1.7
1999 Dec 13....... NGC 7457 90 BTA/MPFS + CCD, 1024 # 1024 16 # 15 4200--5600 2.2
1999 Oct 11 ....... NGC 7457 (1st position) 120 WHT/SAURON + CCD, 2k # 4k 33 # 41 4800--5400 1.8
NGC 7457 (2st position) 60 WHT/SAURON + CCD, 2k # 4k 33 # 41 4800--5400 1.8
1999 Oct 12 ....... NGC 7457 (2st position) 60 WHT/SAURON + CCD, 2k # 4k 33 # 41 4800--5400 1.1
TABLE 3
Photometric Observations of NGC 5574 and NGC 7457
Date Galaxy Filter
Exposure Time
(minutes)
Seeing
(arcsec)
2000 Mar 20 .... NGC 5574 J 8 3.5
NGC 5574 H 6 2.9
NGC 5574 K 0 8 3.1
2001 Sep 7 ....... NGC 7457 J 18 1.6
NGC 7457 H 18 2.1
4 See http://archive.ast.cam.ac.uk/ingarch.
670 SIL'CHENKO ET AL. Vol. 577

Observatory (Vlasyuk 1993). Primary reduction of the data
obtained with the MPFS was done in IDL with software
created by one of the authors (V. L. A.). The Lick indices
were calculated with our own FORTRAN program as well
as by using the FORTRAN program of A. Vazdekis.
3. STELLAR POPULATIONS IN THE
CENTRAL REGIONS
3.1. NGC 5574
Figure 1 presents maps of the Lick indices H#, Mg b, and
hFei # �Fe5270 � Fe5335�=2 for the central part of NGC
5574. They are smoothed by a two�dimensional Gaussian
with FWHM � 2>4, except for the central 4 00 # 4 00 . Since we
knew about homogeneous color distribution in this object,
we did not expect to see a chemically distinct nucleus. How�
ever, although the whole index distribution looks rather flat,
the compact nucleus is somewhat outstanding. The iron
index has a maximum near the center, and this feature is
rather common, but the magnesium index shows a `` hole,''
or a shallow minimum, in the very nucleus, which is quite
unusual. One can say that an iron�overabundant nucleus is
detectable `` by eye.'' As usual, the hydrogen H# index qual�
itatively matches the hFei distribution: it also demonstrates
a maximum in the center. Thus, in the case of NGC 5574,
we verify that the nucleus can be distinguished from the
bulge by the properties of its stellar population and (by
implication) by a di#erent stellar evolutionary history.
Let us determine the characteristics of the stellar popula�
tion in the center of NGC 5574 by comparing its Lick indi�
ces with those calculated for simple stellar population
models by the evolutionary synthesis method. Figure 2
presents index�index diagrams in which we have plotted our
azimuthally averaged measurements and the models of
Worthey (1994). In the plot of Fe5270 versus Mg b, the
models calculated in the frame of the solar chemical pattern
�Mg=Fe# � 0, except for various global metallicities and
ages, occupy a narrow locus. The nucleus of NGC 5574
deviates significantly to the left of this locus, representing
the rare phenomenon of an iron�overabundant stellar sys�
tem. We note that most elliptical galaxies are magnesium
overabundant and are located to the right of the model
solar�ratio locus (Worthey, Faber, & Gonzalez 1992). The
iron overabundance can sometimes be seen in irregular gal�
axies, and it is usually treated as a consequence of multiple
discrete star formation bursts (Gilmore & Wyse 1991; Mar�
coni, Matteucci, & Tosi 1994). Curiously, the nucleus of the
neighboring elliptical galaxy NGC 5576 also seems to be
iron overabundant (Sil'chenko 1997).
Age diagnostic diagrams commonly compare the
hydrogen�line index, H# in our work, with one of the metal�
line indices. In the case of NGC 5574, there are di#culties
with the age determination in the nucleus, because the Wor�
they models are calculated for �Mg=Fe# � 0 and the nucleus
of the galaxy is iron overabundant. Comparing H# to Mg b
or to hFei (Fig. 2), we obtain slightly di#erent estimates of
the mean stellar age in the nucleus; however, the di#erence
is not very large: 3 or 2 Gyr, respectively. In any case, it
means that the nucleus of the lenticular galaxy NGC 5574 is
rather young, or `` poststarburst.'' Again, the mean age of
the nuclear stellar population in NGC 5576 is also 3 Gyr
(Sil'chenko 1997); are we dealing with synchronous multiple
nuclear star formation bursts?
The bulge beyond R # 2 00 has a solar magnesium�to�iron
ratio, but it is also rather young: not older than 3--5 Gyr,
with a global metallicity only slightly below the solar one.
Therefore, we must conclude that the central bulge of NGC
5574 has also been polluted by recent star formation, not
earlier than a few billion years ago.
3.2. NGC 7457
For this galaxy we have two sets of panoramic spectro�
scopic data that provide a unique opportunity to estimate
the external accuracy of our results. Figure 3 presents maps
of the Lick indices H#, Mg b, and hFei # �Fe5270 �
Fe5335�=2 obtained by using the MPFS data, and Figure 4
presents similar maps, except for the right panel, which is
for Fe5270 only, obtained by using the SAURON data
Fig. 1.---MPFS index maps for NGC 5574. A green (5000 A � ) continuum is overlain by isophotes.
No. 2, 2002 NGC 5574 AND NGC 7457 671

(more exactly, by using the first�position SAURON data,
with the galaxy positioned closer to the frame bottom to
avoid systematic distortion of H# at the frame top). We
knew from our earlier MPFS observations that NGC 7457
did not possess a chemically distinct nucleus (Sil'chenko,
Afanasiev, & Vlasyuk 1992). Indeed, the MPFS maps of all
three indices (Fig. 3) obtained under a seeing of more than
2 00 look very flat, with the nucleus quite indistinguishable
against the bulge background. However, the SAURON
maps (Fig. 4) obtained under seeing better than 2 00 demon�
strate marginal di#use metal�index enhancements near the
center and a quite definite H#�index maximum. We must
admit that the nucleus of NGC 7457 is also distinct, but
probably it is so faint and low�contrast that it can only be
detected under rather good spatial resolution. The high
signal�to�noise level of the SAURON data allows us to sug�
gest that the region of the enhanced H#�index is elongated
and extended roughly in the north�south direction, or close
to the isophote minor axis. Let us note that the circumnu�
clear enhancement of the H#�index in NGC 5574 also looks
extended (Fig. 1), but there the e#ect is seen near the limit of
our spatial resolution.
To quantify the comparison of the MPFS and the SAU�
RON Lick�index data, we plot together azimuthally aver�
aged radial profiles of H#, Mg b, and Fe5270 measured
from one MPFS and two SAURON data sets for two di#er�
ent nucleus positions on the frame (Fig. 5). In preparing the
MPFS data, we co�added spectra in the rings; in preparing
the SAURON data, which have the higher signal�to�noise
ratios, we averaged the measured individual�element indices
in the rings so that the attached error bars were the errors of
the means. One can see a good agreement of all three data
sets except for H# at R < 5 00 ; since the estimate for the cen�
ter of NGC 7457 made by Trager et al. (1998) in the 2 00 # 4 00
aperture, H# � 2:47 # 0:19, supports the SAURON data
and since the two independent SAURON data sets made
for the two di#erent nucleus positions on the lens frame are
in agreement, we consider the SAURON data for H# as
more exact.
To determine stellar population characteristics in the
nucleus and in the bulge of NGC 7457, let us consider
together the index�index diagrams of Fe5270 versus Mg b
and H# versus Mg b (Fig. 6). If we take into account the
rather young mean age of the central stellar population in
NGC 7457, we can assure ourselves that both the nucleus
and the bulge have a close�to�solar magnesium�to�iron
ratio, and because of this, we can surely use the models of
Worthey (1994) to determine mean stellar ages. From Fig�
ure 6 (bottom) one can see that the nucleus, and only the
nucleus resolved by the SAURON, has a luminosity�
weighted age of the stellar population of less than 3 Gyr; this
`` young region '' is not pointlike but has a radius of some
2 00 . The metallicity of the nuclear stellar population is
approximately solar. The points corresponding to the bulge,
according to all three data sets, occupy a parameter�space
area between 3 and 8 Gyr and indicate a mean metallicity of
about half of solar. These values are consistent with the
results of Peletier et al. (1999), which are based on the
broadband colors B#I and I#H and on quite di#erent stel�
lar population models: they have obtained T � 4 Gyr and
Z # 0:02 for the nucleus and T � 7 Gyr and Z # 0:008 for
the bulge of NGC 7457. Curiously, such a combination of
parameters forces the optical colors, B#V in our previous
analysis (Sil'chenko 1994) and B#I in the measurements of
Peletier et al. (1999), to be similar in the nucleus and in the
bulge, but results in a prominent di#erence of the NIR color
I#H. In general, the broadband optical colors are highly
degenerative with respect to the e#ects of age and metallic�
ity: an increase of each of these parameters results in a simi�
lar degree of reddening; for a quantitative model estimate of
Fig. 2.---Index vs. index diagnostic diagrams for the azimuthally aver�
aged index measurements in NGC 5574. The data points (circles) are taken
along the radius of the galaxy with steps of 1 00 . As a reference, the model
equal�age sequences from Worthey (1994) for old stellar populations with
�Mg=Fe# � 0 are plotted by small squares connected with thin lines; the
metallicities for the Worthey models are +0.50, +0.25, 0.00, #0.22, #0.50,
#1.00,#1.50, and #2.00, if one follows the squares from right to left.
672 SIL'CHENKO ET AL. Vol. 577

the degeneracy degree, see Worthey (1994). Even using two�
color diagrams does not save the situation, because for
example, on the diagram of (U#B, B#V ), metal�poor old
globular clusters and dwarf ellipticals occupy the same area
as star�forming spiral galaxies with a solar metallicity
(Zasov & Sil'chenko 1983; Sil'chenko 1985). This degener�
acy may result in a situation when color gradients are absent
in a galaxy but the properties of the stellar population, and
some absorption�line indices, change along the radius. Just
such a case can be found in the well�studied dE galaxy M32,
in which the broadband colors are homogeneous but the
indices H#, Fe5270, and Fe5335 demonstrate significant
negative gradients, implying a younger and more metal�rich
nucleus than the whole galaxy (Davidge 1991). Therefore,
the same V#I colors of the circumnuclear disks and of their
surrounding spheroids found in the HST�based study of
kinematically distinct nuclei in elliptical galaxies by Carollo
et al. (1997) do not mean in fact similar stellar populations
in the nucleus and in the total galaxy: the bluing due to
younger age may be compensated by reddening due to
higher metallicity. In the particular case of NGC 7457, we
must reconsider the older diagnosis of Sil'chenko et al.
(1992) and conclude that the nucleus of this galaxy is prob�
ably chemically distinct, with a metallicity di#erence of a
factor of 2 between the nucleus and the bulge. However, in
what is rather atypical for a lenticular galaxy with a chemi�
cally distinct nucleus, the bulge of NGC 7457 is rather
young, with a mean age of the stellar population of 6--7 Gyr,
although it is still older than the nucleus, which is less than
3�Gyr old.
4. STELLAR KINEMATICS IN THE
CENTRAL REGIONS
Since the integral�field spectroscopy provides us with
two�dimensional line�of�sight velocity fields, we are now
Fig. 3.---MPFS index maps for NGC 7457. A green (5000 A � ) continuum is overlain by isophotes.
Fig. 4.---SAURON index maps for NGC 7457. A green (5000 A � ) continuum is overlain by isophotes.
No. 2, 2002 NGC 5574 AND NGC 7457 673

able to analyze both the characteristics of the rotation and
the degree of axisymmetry in the central structure compo�
nents. If we have an axisymmetric mass distribution and
rotation in circular orbits, the direction of the maximum
central line�of�sight velocity gradient (we call it the `` kine�
matical major axis '') should coincide with the line of nodes
as well as with the photometric major axis, whereas in the
case of a triaxial potential, the isovelocities should align
with the principal axis of the ellipsoid, and the kinematical
and photometric major axes should generally diverge, show�
ing turns with respect to the line of nodes in opposite direc�
tions (Monnet, Bacon, & Emsellem 1992; Moiseev &
Mustsevoy 2000). In the simple case of cylindrical (disklike)
rotation, we have a convenient analytical expression for the
azimuthal dependence of central line�of�sight velocity
gradients within the area of solid�body rotation:
dv r =dr � ! sin i cos�PA # PA 0 �, where ! is the deprojected
central angular rotation velocity, i is the inclination of the
rotation plane, and PA 0 is the orientation of the line of
nodes, coinciding in the case of an axisymmetric ellipsoid
(or a thin disk) with the photometric major axis. Therefore,
by fitting azimuthal variations of the central line�of�sight
velocity gradients with a cosine curve, we can determine the
orientation of the kinematical major axis by its phase and
the central angular rotation velocity by its amplitude. It is
our main tool of kinematical analysis.
4.1. NGC 5574
Figure 7 shows the line�of�sight velocity field and the map
of the velocity dispersion for the stellar component in the
center of NGC 5574 according to our MPFS data. The stel�
lar velocity field demonstrates a regular, quasi--solid�body
rotation, with ! sin i � 10 # 3 km s #1 arcsec #1 at R # 2 00
and 9 # 4 km s #1 arcsec #1 at R � 3 00 --5 00 . The stellar velocity
dispersion, being rather low over the whole field of view,
reveals a flat maximum in the center, elongated along the
major axis of the isophotes. To explain the kinematical
properties of the galactic central region, we should com�
pare the orientations of the kinematical and photometric
structures.
Fig. 6.---Index vs. index diagnostic diagrams for the azimuthally aver�
aged index measurements in NGC 7457. The data points are taken along
the radius of the galaxy with steps of 1 00 ; the MPFS data points (circles) are
connected by a dashed line in the direction of increasing radius, and the
SAURON data for the bulge are given as a homogeneous point cloud (tri�
angles, open squares), whereas the nucleus is marked by `` nuc.'' As a refer�
ence, the model equal�age sequences from Worthey (1994) for old stellar
populations with �Mg=Fe# � 0 are plotted by small filled squares connected
with thin lines; the metallicities for the Worthey models are +0.50, +0.25,
0.00, and #0.22 for the 2 Gyr model, +0.25, 0.00, and #0.22 for the 3 Gyr
model, 0.00, #0.22, and #0.50 for the 8 Gyr model, and #0.22 and #0.50
for the 12 Gyr and 17 Gyr models, respectively, if one follows the small
filled squares from right to left.
Fig. 5.---Comparison of two SAURON sets and one MPFS set of azimu�
thally averaged index measurements for NGC 7457. The MPFS spectra are
co�added and the two�dimensional SAURON index measurements are azi�
muthally averaged in concentric rings centered on the nucleus.
674 SIL'CHENKO ET AL. Vol. 577

To avoid problems with dust�e#ect uncertainty, in this
work we use NIR photometric data. Figure 8 presents radial
variations of the major�axis position angle measured from
the JHK images obtained with CAMILA. For the outer�
most part, we check our measurements by using the Digi�
tized Sky Survey (DSS) image of NGC 5574. Although
diverging at R < 5 00 because of bad seeing quality, all three
photometric data sets reveal a slight turn of isophotes by
some 5 # in the inner part of the galaxy, at R < 20 00 ; let us
remember that NGC 5574 possesses a bar (see Table 1)
aligned almost exactly with the line of nodes, and the kine�
matical major axis, determined by fitting azimuthal varia�
tions of the line�of�sight velocity gradients by a cosine law,
also turns, but in the opposite sense: at R # 3>5, we obtain
in two radial bins a kinematical major axis of PA 0 � 240 # ,
or 60 # in Figure 8, which di#ers by 3 # from the line�of�nodes
orientation. Although the deviations by 3 # --5 # are close to
the limit of our determination accuracy, the whole picture---
the turns of the photometric and kinematical major axes by
the same angle in the opposite sense with respect to the line
of nodes---is quite consistent with the probable bar influ�
ence on the stellar kinematics in the center of the galaxy.
Another feature that can be attributed to the bar influence is
the lower than usual stellar velocity dispersion (for stellar
velocity dispersion statistics in S0s, see Kormendy & Illing�
worth 1983) and its maximum area elongation along the bar
direction: the latter e#ect has been theoretically predicted by
Vauterin & Dejonghe (1997) and observationally demon�
strated by Moiseev (2001).
4.2. NGC 7457
Concerning the whole picture of the stellar kinematics in
the center of NGC 7457, we can compare our MPFS data
with those from SAURON, but as for subtle details, the bet�
ter spatial resolution of the SAURON data is crucial in
revealing a much more complex dynamical structure of the
galaxy than has been thought before. Figure 9 presents three
di#erent measurements of the stellar velocity field, one from
MPFS and two from SAURON,with di#erent nucleus posi�
tions on the frame, smoothed by a two�dimensional Gaus�
sian with FWHM � 2>35. All three pictures are quite
consistent with each other: we observe large�scale quasi--
solid�body rotation: rather slow, ! sin i � �4:8 # 0:4� km
s #1 arcsec #1 , but very regular. However, the small�scale pat�
tern of the isovelocities is somewhat wavy, and in particular,
both SAURON velocity maps, although being smoothed,
nevertheless reveal a strange isovelocity turn in the very
nucleus. An attentive examination of the central part of the
unsmoothed kinematical maps (Fig. 10) gives an explana�
tion for this disturbance: NGC 7457 has a counterrotating
core. More exactly, the probable turn of the kinematical
major axis in the center of the galaxy is about 130 # --140 # ,
but it is very di#cult to measure it with more confidence
because of the extreme compactness of the counterrotating
component: its radius is less than 2 00 . For the two di#erent
positions of the nucleus on the lens array, we obtain
PA 0 � 152 # for the kinematic axis of position 2, when the
nucleus is within a single lens and the spatial resolution is
the best, and PA 0 � 185 # for the kinematic axis of position
1, when the nucleus is between two neighboring lenses. The
corresponding angular rotation velocity estimates are
16 # 7 and 6 # 4 km s #1 arcsec #1 ; the former estimate,
obtained under a seeing quality of 1 00 , implies a rather fast
core rotation. Another interesting feature in Figure 10
related to the first one is a significant minimum of the stellar
velocity dispersion in the nucleus of NGC 7457. An exami�
nation of the picture from the MPFS (Fig. 10 [left]) confirms
the minimum of the stellar velocity dispersion in the
nucleus. We think that in this particular case, the increase of
the stellar velocity dispersion with the radius can be
Fig. 7.---Line�of�sight velocity field for the stellar component (isolines
superposed over the continuum image) and stellar velocity dispersion map
(gray scale, with the green continuum isophotes superposed) in the center
of NGC 5574.
No. 2, 2002 NGC 5574 AND NGC 7457 675

explained by a superposition of two quasi�counterrotating
stellar subsystems. A very similar e#ect is observed in NGC
4550, a lenticular galaxy with well�known counterrotating
large�scale cold stellar disks (Rix et al. 1992): when it was
observed with a low spectral resolution, its central region
seemed not to be rotating at all, with a small central velocity
dispersion sharply rising along the radius. As for NGC
7457, here it would be even more di#cult to separate coun�
terrotating stellar subsystems than in NGC 4550, because
NGC 7457 is not edge�on as NGC 4550 is, and the line�of�
sight velocity separation of the components in it is smaller
and therefore hardly measurable.
To check the validity of our velocity measurements in
NGC 7457, we have simulated one�dimensional (e.g., long�
slit) cross sections of the maps of Figures 9 and 10 at
PA � 130 # and have compared them with the published
long�slit data of Simien & Prugniel (1997). In Figure 11 it is
evident that the agreement is excellent for both the line�of�
sight velocities and the stellar velocity dispersion measure�
ments. Even some hints of the nonmonotonic behavior,
which was clearly seen in the two�dimensional maps, can be
seen here too, although an accuracy of 15 km s #1 for indi�
vidual positions along the long slit in the data of Simien &
Prugniel (1997) is quite insu#cient to make conclusions
based only on the one�dimensional data.
Finally, Figure 12 presents a comparison of the orienta�
tions of the photometric and kinematical major axes in the
center of NGC 7457. Bearing in mind that the line of nodes
of the galaxy is at PA � 126 # (from our JH photometry for
the outermost parts of the galaxy), one must admit that all
the NIR photometry, both from the HST and ground�
based, indicates strong geometrical distortions inside
R # 3 00 . In the very center, R < 1 00 , isophotes turn in a posi�
tive sense, up to PA # 160 # , and farther from the nucleus,
over a small radius range of 1 00 --2 00 , they turn in the negative
sense, by some 6 # --8 # with respect to the line of nodes. Curi�
ously, the radial variations of the kinematical major axis
orientation repeat this trend exactly, but with some shift
outward. Whether this shift is due only to lower spatial reso�
lution (at least, with respect to the HST data) or whether
there is a physical reason for such a relation between the
photometric and kinematical characteristics cannot be
answered in this work. However, we can surely state that the
`` counterrotating '' stellar subsystem in the center of NGC
Fig. 9.---Line�of�sight velocity fields for the stellar component (isolines superposed over the continuum image) in the center of NGC 7457 according to
MPFS data and to two data sets from SAURON.Themaps are smoothed by a two�dimensional Gaussian with # x;y � 1 00 .
Fig. 8.---Comparison of the orientations of the isophote (photometric) major axis and the kinematical major axis (see the text) in NGC 5574
676 SIL'CHENKO ET AL. Vol. 577

7457 rotates axisymmetrically, although its rotation plane is
inclined with respect to the global symmetry plane of the
galaxy.
5. CONCLUSIONS AND DISCUSSION
We have studied the stellar populations in the centers of
two lenticular galaxies previously known to have radially
homogeneous blue optical�band colors: NGC 5574 and
NGC 7457. Despite our expectations, the compact nuclei of
the galaxies appear to be chemically distinct: in NGC 5574
the nucleus is distinguished by a higher than solar iron�to�
magnesium ratio, and in NGC 7457 a drop of the global
metallicity by a factor of 2 exists between the nucleus and
the bulge. An unusual (at least, as far as we know yet)
property of both galaxies is a rather young mean stellar
luminosity�weighted age of the stellar populations in their
bulges, not older than 5--7 Gyr, but the chemically distinct
nuclei are still younger, 2--2.5 Gyr old. The kinematics of
the stars in the center of NGC 5574 are probably responsive
to the influence of its global bar. The compact core of NGC
7457, with a radius of about 1>5, demonstrates a visible
counterrotation; a combined analysis of the photometric
and kinematical two�dimensional maps allows us to con�
clude that this core rotates axisymmetrically, but its rota�
tion plane is inclined to the main symmetry plane of the
galaxy. The stellar velocity dispersion in the centers of both
galaxies is anomalously low: 60--80 km s #1 .
Of the two galaxies under consideration, NGC 7457 has
been much more intensively studied than NGC 5574. Now,
with our new understanding of the central structure of
NGC 7457, we can comment on some long�standing puzzles
related to this galaxy. As we mention above, NGC 7457 has
an unusually low measured central velocity dispersion: #60
km s #1 in the very nucleus and 80--90 km s #1 to the north
and to the south of the nucleus. This fact was noted for the
first time by Kormendy & Illingworth (1983), who tried to
derive an L versus # 0 relation for the bulges of S0--Sbc gal�
axies. Among the 25 SA0--SAbc galaxies of their sample, 23
Fig. 11.---Comparison of the simulated one�dimensional cross sections
at PA � 130 # of our kinematic maps for NGC 7457 with the published
long�slit data of Simien & Prugniel (1997). Top: Relative stellar line�of�sight
velocity. Bottom: Stellar velocity dispersion.
Fig. 10.---Unsmoothed kinematic maps for the center of NGC 7457. Left: Velocity dispersion measurements from MPFS. Middle: Velocity dispersion
measurements from SAURON, position 2. Right: Stellar line�of�sight velocity field from SAURON, position 2. The cross marks the photometric center.
No. 2, 2002 NGC 5574 AND NGC 7457 677

obeyed a very tight relation, L / # 7:8
0 , and only two, includ�
ing NGC 7457, were strongly o#; the gap between NGC
7457 and the main cloud of points was twice as large as the
scatter around the relation. The authors could not explain
this anomaly. The same year, Dressler & Sandage (1983)
studied the rotation of lenticular galaxies. Again, NGC 7457
appeared to fall o# the mean relation between luminosities
and rotation velocities for the lenticular galaxies: its formal
estimate of M=L made from the measured v max and M B
(from the Tully�Fisher diagram) was found to be about 1,
less than the mean for Sc galaxies. The authors thought it to
be improbable and concluded that since `` the observed rota�
tion value for galaxies such as NGC 7457. . . is not su#cient
to account for the masses of these galaxies,'' these galaxies,
including NGC 7457, were supported by `` internal velocity
dispersions.'' However, Kormendy & Illingworth (1983)
had just found that the velocity dispersion of NGC 7457 is
too low for its luminosity! This raises the question: Where is
the kinetic energy of the galaxy? The observations reported
here suggest not one, but two solutions for this problem.
The first solution is based on an analogy with NGC 4550.
NGC 4550 was also thought to be nonrotating, with a cen�
tral velocity dispersion of 86 # 7 km s #1 (Tonry & Davis
1981), until two counterrotating stellar disks were found in
this galaxy (Rubin, Graham, & Kenney 1992; Rix et al.
1992). Since now we know that NGC 7457 has a counterro�
tating core, we can suggest that some part of the stars at
larger radii may counterrotate too, visibly diminishing the
rotation velocity obtained from the spectra integrated along
the line of sight. The second possibility is that because the
bulge stellar population in NGC 7457 is significantly
younger than stellar populations in most other bulges
(Peletier et al. 1999), it must really demonstrate a lower
M=L B than the bulk of other lenticulars. The two e#ects
together may perhaps explain the unusual kinematics of the
galaxy.
However, whereas these previous puzzles are probably
solved, new puzzles arise. The bulges in NGC 5574 and
NGC 7457 have rather young stellar populations. A natural
conclusion is that they have been recently formed by secular
evolution. Indeed, NGC 5574 has a well�known bar; in x 4
we have seen that the central stellar kinematics of the galaxy
support the dynamical importance of the bar in this galaxy.
NGC 7457, although classified as SA0, is also not com�
pletely axisymmetric: it demonstrates variations of the iso�
phote major axis position angle, and Michard & Marchal
(1994) have noted the possible presence of a bar in this gal�
axy. Both galaxies possess some gas: H i is detected in NGC
5574 (Haynes et al. 1997) and CO in NGC 7457 (Taniguchi
et al. 1994). Thus, there is material for the secular evolution
of the bulges, there is a mechanism for inducing circumnu�
clear star formation, and consequently, such evolution is
highly probable. However, a common view is that secular
evolution produces exponential�profile bulges (see, e.g.,
Courteau, de Jong, & Broeils 1996, and references therein),
and both NGC 5574 and NGC 7457 possess classical de
Vaucouleurs profiles. For NGC 7457, it has been estab�
lished more than once, starting from the photographic
photometry of Burstein (1979), and for NGC 5574, there is
a nice bulge�disk decomposition by Baggett, Baggett, &
Anderson (1998). We ourselves have checked this with our
photometric data; indeed, the de Vaucouleurs' law is the
best for their bulges. On the other hand, Aguerri, Balcells, &
Peletier (2001) have argued that a de Vaucouleurs profile of
a bulge can only be the product of a dense satellite merger.
These considerations thus raise a new possibility: our data
for NGC 5574 and NGC 7457 may provide the first direct
evidence for merger�driven evolution in disk galaxies.
We thank S. N. Dodonov and the postgraduate student
of the Special Astrophysical Observatory A. V. Moiseev for
supporting the observations at the 6 m telescope. The 6 m
telescope is operated under the financial support of the Sci�
ence Ministry of Russia (registration number 01�43); we
also thank the Programme Committee of the 6 m telescope
for allocating observational time. During the data analysis,
we used the Lyon�Meudon Extragalactic Database (LEDA)
supplied by the LEDA team at the CRAL�Observatoire de
Lyon (France) and the NASA/IPAC Extragalactic Data�
base (NED), which is operated by the Jet Propulsion Labo�
ratory, California Institute of Technology, under contract
with the National Aeronautics and Space Administration.
The work is partially based on data taken from the ING
Fig. 12.---Comparison of the orientations of the isophote (photometric) major axis and the kinematical major axis (see the text) in NGC 7457. Besides our
data, the results of K�photometry by peletier & Balcell (1997) are also plotted.
678 SIL'CHENKO ET AL. Vol. 577

Archive of the UKAstronomy Data Centre and on observa�
tions made with the NASA/ESA HST, obtained from the
data archive at the Space Telescope Science Institute. STScI
is operated by the Association of Universities for Research
in Astronomy, Inc., under NASA contract NAS 5�26555.
The study of the young nuclei in lenticular galaxies is sup�
ported by the grant of the Russian Foundation for Basic
Researches 01�02�16767 and by the Federal Scientific�Tech�
nical Program contract of the Science Ministry of Russia
40.022.1.1.1101.
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