Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://sn.sai.msu.ru/~sil/preprints/990234.ps.gz Äàòà èçìåíåíèÿ: Wed Jan 19 17:39:49 2000 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 19:42:17 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: dark nebula

THE ASTRONOMICAL JOURNAL, 119 : 126õ135, 2000 January
2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
(
YOUNG STELLAR NUCLEI IN THE LENTICULAR GALAXIES. II. NGC 72801
V. L. AFANASIEV
Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Russia ; vafan=sao.ru
AND
O. K. SILîCHENKO2
Sternberg Astronomical Institute, Moscow, 119899 Russia ; and Isaac Newton Institute, Chile, Moscow Branch ; olga=sai.msu.su
Received 1999 June 3 ; accepted 1999 October 1
ABSTRACT
We have undertaken bidimensional spectroscopy of the central part of the nearby lenticular galaxy
NGC 7280 with the Multi­Pupil Fiber Spectrograph of the 6 m telescope of the Special Astrophysical
Observatory. We ïnd a rather young stellar nucleus, with a mean population age of 1.5 ^ 0.5 Gyr,
which is more metal­rich than the bulge at RB 1 kpc by an order of magnitude. The chemically and
age­decoupled nucleus seems to be spatially resolved : the circumnuclear absorption index isolines rep­
resent ellipses elongated in P.A. B 100¡õ110¡. The same orientation, P.A. \ 103¡, is found for the elon­
gated circumnuclear stellar structure, revealed from the morphological analysis of the Hubble Space
Telescope (HST ) WFPC2 and NICMOS images of NGC 7280 and seen best of all at R\ 1@@. Taking
into account the stellar kinematics inside RB 2@@, we conclude that this structure is a circumnuclear
stellar disk inclined with respect to the global plane of the galaxy. Meanwhile, both photometric and
kinematical data in the radius range 2Aõ8A imply the existence of an intermediate­scale bar elongated in
P.A. B 60¡. The circumnuclear ionized gas is distributed and rotates in the plane orthogonal to the plane
of the circumnuclear stellar disk.
Key words : galaxies : nuclei õ galaxies : individual (NGC 7280) õ galaxies : evolution õ
galaxies : structure
1. INTRODUCTION
Integrated photometric and spectral properties of early­
type galaxies imply that their major stellar population is
very old, almost as old as the universe. But the center of a
galaxy is a special place ; the stellar nucleus may have a
particular evolution that di+ers from the evolution of the
whole galaxy. By analyzing statistics of photoelectric multi­
aperture photometry of nearby galaxies (Silîchenko 1994),
we have found that in at least 25%õ30% of all elliptical and
lenticular galaxies, the nuclei are distinguished from the
surrounding spheroids by a prominently red color. We have
interpreted this phenomenon as being due to a signiïcant
fraction of chemically distinct stellar nuclei in early­type
galaxies.
Chemically distinct galactic nuclei are still poorly
studied ; their relation to a global galactic structure is quite
unclear, and so any speculations on their origin are some­
what premature. However, there were some hints about the
relation between the chemically distinct nuclei in elliptical
galaxies and the presence of fast­rotating circumnuclear
stellar disks (Bender & Surma 1992 ; Surma & Bender 1995 ;
Scorza & Bender 1995 ; Forbes, Franx, & Illingworth 1995).
On the other hand, a correlation between the absorption­
line index Hb and the fourth Fourier coefficient has been
a 4
noted in elliptical galaxies, so that elliptical galaxies with
inner disks have younger (on average) stellar populations
(de Jong & Davies 1997). The resulting picture is consistent
with the occurrence of secondary star formation bursts in
the centers of some elliptical galaxies that produced
compact circumnuclear stellar disks whose mean age is
õõõõõõõõõõõõõõõ
1 Partly based on observations collected with the 6 m telescope at the
Special Astrophysical Observatory (SAO) of the Russian Academy of Sci­
ences (RAS).
2 Guest Investigator of the RGO Astronomy Data Centre
younger and mean metallicity is higher than those of the
main galactic spheroids. The full set of these propertiesõ
higher nuclear metallicity, younger nuclear age, and photo­
metric signatures of circumnuclear disksõare directly
observed in the elliptical galaxies NGC 4365 (Bender &
Surma 1992 ; Surma & Bender 1995 ; Carollo et al. 1997a)
and NGC 4621 (Silîchenko 1997). Similar coincidence of the
chemically, morphologically, and dynamically decoupled
entities has also been found in the center of the lenticular
galaxy NGC 4816 (Mehlert et al. 1998).
We think that lenticular galaxies are more promising for
studying the connection between chemical distinctness,
young age, and diskiness of decoupled galactic cores. First,
their stellar nuclei are on average younger than the nuclei of
elliptical galaxies ; among nearby lenticulars, 50% show
prominent Balmer absorption lines Hc and/or Hd in their
nuclear spectra, implying a mean stellar age of less than 5
billion years (Silîchenko 1993a). Second, the same fraction,
50%, of nearby lenticulars appear to have separate circum­
nuclear stellar disks embedded in extended bulges (Seifert &
Scorza 1996). In the ïrst paper of this series (Silîchenko
1999a, hereafter Paper I) we studied two lenticular galaxies
with chemically decoupled nuclei, NGC 1023 and NGC
7332. We have detected intermediate­age stellar popu­
lations in the centers of these galaxies. The mean stellar age
in the nucleus of NGC 1023 is 7 billion years, and in the
nucleus of NGC 7332 it is 2.5 ^ 0.5 billion years. Moreover,
the intermediate­age stellar structures in the centers of these
lenticulars are not pointlike. An enhanced Hb absorption
line is observed up to 2A from the center in NGC 1023 and
up to 5A from the center in NGC 7332. The sizes of the
intermediate­age stellar structures coincide with the sizes of
embedded circumnuclear stellar disks, so for these galaxies
the connection between chemically and evolutionarily
decoupled cores and separate circumnuclear disks seems to
126

4800 5000 5200 5400
2000
4000
6000
8000
4800 5000 5200 5400
2000
4000
6000
8000
Wavelength, A
YOUNG NUCLEI IN LENTICULAR GALAXIES. II. 127
TABLE 1
GLOBAL PARAMETERS OF NGC 7280
NGC Parameter (Source) Value
Type (NED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SAB(r)0]
R 25 (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 kpc
B T 0 (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.64
M B (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [19.45
B[V (RC3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.85
U[B (RC3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.37
V r (radio) (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1846 ^ 14 km s~1
Distance (LEDA, H 0 \ 75 km s~1 Mpc~1) . . . . . . 26.3 Mpc
Inclination (LEDA) . . . . . . . . . . . . . 52¡
P.A. phot (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78¡
p 0 (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 km s~1
v m (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 km s~1
be quite probable. The target of the second stage of this
work, the lenticular galaxy NGC 7280, is rather gas­rich ;
about 109 neutral hydrogen has been detected in it
M _
(Biermann, Clarke, & Fricke 1979 ; Chamaraux, Balkowski,
& Fontanelli 1987). It possesses an irregular companion
galaxy (D28 kpc) from it, which makes some interaction
3@.7
with a gas accretion possible in the past. However, now no
traces of star­forming activity (namely, no Ha emission) are
observed in either the disk or the nucleus of NGC 7280
(Pogge & Eskridge 1987, 1993).
The global parameters of the galaxy are given in Table 1.
We report our observations and other data used in ° 2. The
photometric structure of the central part of NGC 7280 is
studied in ° 3, and the kinematical analysis is presented in
° 4. The radial variations of the stellar population properties
are analyzed in ° 5, and ° 6 gives our conclusions.
2. OBSERVATIONS AND DATA REDUCTION
In 1998 we undertook two­dimensional spectroscopy of
NGC 7280 with the Multi­Pupil Fiber Spectrograph
(MPFS) of the 6 m telescope of the Special Astrophysical
Observatory (Nizhnij Arkhyz, Russia). The blue­green spec­
tral range, 4250õ5600 was exposed on August 17, and the
A# ,
red spectral range, 5650õ7000 was exposed on October
A# ,
16. The exposure times were 60 minutes for each spectral
range (3 ] 20). A grating of 1200 grooves mm~1 was used,
which provided a reciprocal dispersion of 1.3 per pixel
A#
and a spectral resolution of 3 On both dates a seeing of
A# .
FWHM\ 2@@ was estimated.
These spectral observations have been made with the new
variant of the panoramic spectrophotometer, which began
to work at the prime focus of the 6 m telescope at the end of
1997. With respect to the previous variants of MPFS
(Afanasiev et al. 1990, 1996), the ïeld of view is now
increased, and the common spectral range is larger as a
result of using ïbers, which transmit light from 16 ] 16
square elements of the galaxy image to the slit of the
spectrograph (256 ïbers) together with the sky background
taken in o+ the galaxy itself (6 ïbers). The size of one
4@.5
spatial element is 1@@ ] 1@@. At the exit of the spectrograph a
1024 ] 1024 CCD registers all the 262 spectra simulta­
neously. The primary reduction of the data is made within
IDL. After bias­subtracting, ÿat­ïelding, and extracting
one­dimensional spectra from the CCD frame, we linearize
and analyze each spectrum individually. The one­element
spectral characteristics, such as ÿuxes in continuum or in
emission lines, redshift, and absorption­line indices are then
combined into two­dimensional arrays corresponding to
the galactic region under consideration, with the help of
software developed earlier at the Special Astrophysical
Observatory (SAO ; Vlasyuk 1993) and with our own
FORTRAN programs. To calculate absorption­line indices
and their errors, we have also used the FORTRAN
program of A. Vazdekis. As a result, we obtain two­
dimensional surface brightness distributions, velocity ïelds,
and maps of stellar population characteristics. In the blue­
green spectral range, we measure the absorption­line indices
Hb, Mg b, Fe j5270, and Fe j5335 in the popular Lick
system (Worthey et al. 1994) ; to check the consistency of
our measurements with the model indices calculated in this
system (Worthey 1994), we also observed stars from their
list (Worthey et al. 1994). In addition, we use our blue­green
spectra to derive a stellar velocity ïeld in the center of NGC
7280 by cross­correlating elementary galactic spectra with
the spectrum of a K­giant star, the brighter component of
the visual binary ADS 15470. In the red spectral range, we
have measured barycenter positions of the emission line
[N II] j6583 to derive a velocity ïeld of the ionized gas (Ha
emission is absent in NGC 7280). We have estimated the
best accuracy of our velocity measurements as 6 km s~1
from the night­sky line [O I] j6300 analysis. As for the
absorption­line index accuracy, we have made estimates
using the method of Cardiel et al. (1998) ; the typical error of
the indices varies from 0.15 in the nucleus to 0.6 for the
A# A#
individual elements at the edges of the area investigated. To
keep a constant level of accuracy along the radius, we
summed the spectra in concentric rings centered on the
nucleus and studied the radial dependencies of the
absorption­line indices by comparing them to synthetic
models of the old stellar populations of Worthey (1994) and
Tantalo, Chiosi, & Bressan (1998). We estimate the mean
accuracy of our azimuthally averaged indices as 0.1 To
A# .
give an impression on our data quality, the azimuthally
averaged spectra are displayed in Figure 1.
The photometric data involved in our analysis are taken
from the La Palma and Hubble Space Telescope (HST )
archives.
FIG. 1.õAzimuthally averaged spectra obtained with the MPFS for
NGC 7280. The corresponding radii are 0A, 1A.0, 2A.0, 3A.0, 4A.0, 5A.0, 6A.0, 7A.0,
and (top to bottom). The normalizations and shifts are arbitrary.
8A.0

128 AFANASIEV & SILîCHENKO Vol. 119
FIG. 2.õIsophotes in the central part of NGC 7280 obtained with the HST WFPC2 through F606W (left) and the HST NICMOS2 through F160W
(right).
The broadband R image of NGC 7280 was obtained on
1992 May 28 at the 1 m Jacobus Capteyn Telescope on La
Palma. The minimum exposure time was 10 minutes, and
the very center, inside is saturated. The seeing
RB 1A.5,
quality is estimated from the neighboring star measure­
ments as The central part of the galaxy has
FWHM\ 1A. 5.
been observed by the HST (PI, M. Stiavelli ; program 6359).
It has been exposed with the WFPC2 through the wide
optical­band ïlter F606W for 600 s (400 ] 200) on 1996
December 1, and with the NICMOS2 through the ïlter
F160W for 128 s on 1997 December 27. The spatial
resolution was and respectively. We have derived
0A. 1 0A.2,
morphological characteristics of the surface brightness dis­
tribution in NGC 7280 by analyzing these images ; the
program FITELL of V. V. Vlasyuk has been used for this
purpose.
3. STRUCTURE OF THE CENTRAL PART OF NGC 7280
Figure 2 presents a direct view of the central part of NGC
7280 in the optical (left) and near­infrared bands (right)
obtained with high spatial resolution at the HST . In the left
panel of Figure 2, the most prominent features are two
narrow dust lanes almost perpendicular to the major axis of
the continuum distribution ; they are located and to
0A.3 1A. 0
the west of the center and elongated in the north­south
direction. Carollo et al. (1997b) described the conïguration
of this dust complex as ```` dust lane wraps several times
around nucleus.îî In the near­infrared (Fig. 2, right) the dust
is invisible, making the turn of isophotes in the very center
quite evident. This turn can also be suspected from the left
panel of Figure 2 ; light excess in the westõeast direction
gives an appearance of an inclined compact circumnuclear
disk superposed on the smooth spheroidal brightness dis­
tribution.
This impression has been conïrmed by morphological
analysis of the isophotes. Figure 3 presents radial variations
of the major­axis position angle and ellipticity if the iso­
photes are approximated by ellipses. Orientation of the
global line of nodes of the galaxy is 78¡ according to RC3
and 74¡ according to Lu (1998) ; we have obtained P.A.
0 \
72¡ at our outermost point, RB 45@@. However, in the
central part of the galaxy the orientation of the isophotes is
quite di+erent : at R\ 1@@, P.A.(r) has a maximum, 103¡, that
is well measured both from the WFPC2 and from the
NICMOS data, and at RB 9@@, P.A.(r) has a minimum at
58¡. The latter feature is accompanied by a local maximum
of ellipticity, D0.4. In general, we note that the isophotes of
FIG. 3.õRadial variations of the isophote morphological character­
istics in the center of NGC 7280 according to the HST data (stars) and the
La Palma data (open circles).

No. 1, 2000 YOUNG NUCLEI IN LENTICULAR GALAXIES. II. 129
FIG. 4.õResidual brightness map of the central part of NGC 7280, obtained from the La Palma JKT R image by subtracting the pure ellipsoidal
brightness distribution. North is up, east is to the left ; the black spot in the center marks the nucleus position, and the light ellipse borders the area where the
model has been subtracted.
NGC 7280 are rather highly elongated even close to the
center : 1 [ b/a \ 0.3 is already achieved at R\ 1@@, and the
maximum ellipticity appropriate for a thin disk under the
inclination of NGC 7280, i \ 52¡ (see Table 1), would be
0.38 (indeed, 0.356 is measured by Lu 1998). Such behavior
of the ellipticity is rather strange in a lenticular galaxy with
a prominent bulge. Figure 4 shows a map of residual
R­band brightness after the subtraction of pure elliptical
isophotes with the parameters plotted in Figure 3. The cir­
cumnuclear substructure elongated in P.A. \ 103¡ cannot
be seen here because of the saturation of the central 2A on
the La Palma image, but one can clearly see the substruc­
ture with a radius of D9A elongated in P.A. B 60¡. The
substructure looks asymmetric ; the northeastern part is
brighter than the southwestern one. This seems natural if we
extrapolate circumnuclear dust skewness toward the west
(Fig. 2, left) onto the larger scale. In general, the substruc­
ture resembles a strong, although low­contrast, bar, which
is seen almost edge­on, and to the southwest of the nucleus,
where the receding edge of the bar is the nearest to us (see
° 4), we see it through the dust lane.
All of the above concern the stellar surface distribution.
However, using the MPFS data we have also constructed a
map of the surface brightness distribution for the emission
line [N II] j6583 (Fig. 5), hoping that it reÿects a distribu­
FIG. 5.õSurface brightness distribution for the emission line [N II]
j6583 (isolines) superimposed on the gray­scaled red continuum map,
which is in logarithmic units. The nitrogen brightness units are not cali­
brated ; within the peaks the counts are about 60, and the isoline step is 5.

130 AFANASIEV & SILîCHENKO Vol. 119
tion of the ionized gas. Although this map involves only the
central part of the galaxy, 16@@ ] 16@@, the distribution of the
ionized gas emission looks complex ; there is an overall
elongation in P.A. B 60¡, but inside this distribution there is
also compact circumnuclear substructure with a diameter of
D3A elongated perpendicularly to the major axis of contin­
uum isophotes (the separation of two emission brightness
peaks is statistically insigniïcant). Remembering that cir­
cumnuclear dust lanes are also aligned perpendicularly to
the continuum isophote major axis, we can suppose the
existence of the orthogonal circumnuclear di+use matter
disk or bar. To make a choice between them, we need kine­
matical data, which is discussed in the next section.
4. KINEMATICS OF THE IONIZED GAS AND STARS IN THE
CENTER OF NGC 7280
Figure 6 presents isovelocities for stars (left) and ionized
gas (right) superimposed on the continuum image of the
central part of NGC 7280. Even at ïrst glance, both velocity
ïelds look rather complex ; isovelocities are twisted beyond
the close vicinity of the nucleus, and this kinematical pecu­
liarity conïrms our hypothesis about the medium­scale bar
implied by the photometric data reviewed in the previous
section. However, in the very center, the regular behavior of
the stellar isovelocities allows us to suppose an axisym­
metric solid­body rotation. The gaseous isovelocities near
the center of NGC 7280 are almost aligned along the major
axis of the continuum isophotes, that is, perpendicular to
the stellar isovelocities. This fact, together with the nitrogen
emission brightness distribution (Fig. 5) and crossing dust
lanes (Fig. 2, left), strengthens our idea that the gaseous
dusty circumnuclear disk settles and rotates in the plane
orthogonal to the global plane of the galaxy.
Let us try to more precisely determine the parameters of
the stellar and gaseous rotation around the nucleus of NGC
7280. Under the assumption of plane axisymmetric rota­
tion, the azimuthal dependence of central line­of­sight
velocity gradients must be a cosine one,
dv r /dr \u sin i cos (P.A. [P.A. 0 ) ,
where u is the deprojected central angular rotation velocity,
i is the inclination of the rotation plane, and is the
P.A. 0
orientation of the line of nodes coinciding in the case of an
axisymmetrical ellipsoid (or a thin disk) with the photo­
metric major axis.
In axisymmetrically rotating stellar or gaseous systems
with moderate inclination, i \ 40¡õ60¡, ïtting of the
observed azimuthal dependence of the circumnuclear line­
of­sight velocity gradients by a cosine law is a powerful tool
for determining rotation plane orientation and for estimat­
ing angular rotation velocity. Figure 7 presents such depen­
dencies for the stars and ionized gas in the center of NGC
7280, measured at two di+erent distances from the nucleus.
First, one can make sure once more that outside R[ 2@@ the
evident noncircular motions disturb regular rotation, par­
ticularly gas rotation ; the azimuthal dependence for R\
demonstrates characteristic ```` waves îî around the
2A. 0õ3A.1
best­ït cosinusoid, revealing the presence of higher­order
harmonics in the velocity ïeld. However, inside RB 2@@
(almost unresolved scale in our observations) the cosinu­
soids are rather good. The corresponding best­ït formulae
are
dv r /dr \ [20 cos (P.A. [ 262¡) ] 2.0] km s~1 arcsec~1
for the stars, and
dv r /dr \ [64 cos (P.A. [ 9¡) ] 1.9] km s~1 arcsec~1
for the gas, under the assumption that our measured sys­
temic velocity is 1860 km s~1. One can see that the rotation
plane of the ionized gas is indeed roughly orthogonal to the
rotation plane of the stars. We have already argued that the
gas rotation looks circular because the emission­line bright­
ness distribution and the dust lanes are aligned in the same
position angle as the ```` dynamical major axis îî of the gas.
But the same argument must be also applied to the stellar
FIG. 6.õIsovelocities in the centers of NGC 7280 for the stellar component (left) and the ionized gas (right). The isovelocities are superimposed on the
gray­scaled continuum distributions.

No. 1, 2000 YOUNG NUCLEI IN LENTICULAR GALAXIES. II. 131
FIG. 7.õAzimuthal dependencies of the central line­of­sight velocity gradients for the stars (left) and ionized gas (right) in NGC 7280 in two radial bins ;
solid lines show the best­ït cosinusoids.
rotation. Its dynamical major axis, P.A. \ 82¡, coincides
exactly with the photometric major axis at the correspond­
ing distance from the center (Fig. 3), and this is a signature
of axisymmetric rotation : in the presence of a triaxial poten­
tial, the dynamical and photometric major axes must turn
in opposite directions with respect to the line of nodes
(Monnet, Bacon, & Emsellem 1992). In the meantime, in the
center of NGC 7280 the dynamical and photometric major
axes are turned in the same direction, by D]10¡ with
respect to the line of nodes ; this means that we deal with a
compact (R ¹ 200 pc) circumnuclear stellar disk inclined
with respect to the global galactic plane. If we refer to the
high­resolution photometric data of HST (Fig. 3), the tilt of
the circumnuclear stellar disk may reach at least 30¡.
It is interesting to compare the angular rotation velocities
of the stars and ionized gas. The amplitude of the cosinu­
soid for the stars is lower than the amplitude for the ionized
gas by a factor of 3 (Fig. 7). If we assume that we see the
circumnuclear gaseous disk edge­on, while the circumnu­
clear stellar disk is seen more open, e.g., under i \ 52¡, just
as the global disk, then the di+erence in the angular rotation
velocities would be reduced only slightly, by a factor of 2.5.
The stellar velocity dispersion in the center of NGC 7280 is
too low (see Table 1) to support a slowly rotating bulge. We
can propose only one possible explanation for the visible
slow rotation of the stellar component : in the presence of a
global bar, a noticeable counterrotating stellar component
may exist (for theoretical evidence, see Wozniak & Pfenni­
ger 1997 ; for observational evidences for NGC 2841, see
Afanasiev & Silîchenko 1999 ; for NGC 7331, see Prada et
al. 1996 and Silîchenko 1999b). Under our moderate spec­
tral resolution, we cannot separate kinematical substruc­
tures with velocities close enough km s~1) ;
(*v r ¹ 200
instead, we measure line­of­sight velocities artiïcially
lowered by the presence of an unresolved counterrotating
stellar component.
Considering the above conditions, we would like to use
the rotation velocity of the ionized gas as a more reliable
estimator of the mass contained inside the radius of solid­
body rotation. If we take this radius to be equal to 2A (which
is in reality an upper limit), we obtain km s~1 at
v rot \ 128
R\ 2@@ and a corresponding mass within this radius of
D109 This value of the rotation velocity achieved at
M _ .
only 0.26 kpc from the center is almost exactly equal to the
half­width of the H I 21 cm line (Chamaraux et al. 1987).
This means that the mass is strongly concentrated in the
center. Statistically, the central angular rotation velocity of
the ionized gas of 500 km s~1 kpc~1 and the central mass
concentration of D109 are quite typical for disk gal­
M _
axies with dynamically decoupled nuclei (Afanasiev,
Silîchenko, & Zasov 1989) ; the nontypical feature is the
conïnement of rotation to the circumnuclear polar ring.
5. PROPERTIES OF THE STELLAR POPULATION IN THE
CENTER OF NGC 7280
Figure 8 shows two­dimensional distributions of the
absorption­line indices Mg b (top), SFeT 4 (Fe j5270]Fe
j5335)/2 (middle), and Hb (bottom). The metal­line indices,
as well as Hb, are strongly peaked in the nucleus of the
galaxy. However, the region of enhanced absorption lines
seems to be spatially resolved ; the central isolines of all
three indices represent ellipses elongated in P.A. B 100¡õ
110¡. The Hb distribution has one additional peak to the
southwest of the center ; being elongated in P.A. \ 60¡/240¡,
it may have originated from a site of recent star formation
at the receding edge of the global bar. The Mg b map
demonstrates a rather lumpy central structure ; the compact
spots are statistically signiïcant, but we have no idea as to

132 AFANASIEV & SILîCHENKO Vol. 119
FIG. 8.õTwo­dimensional distributions in the central part of NGC
7280 for the Lick index Mg b, in (top), the Lick index SFeT, in (middle),
A# A#
and the Lick index Hb, in (bottom). The isolines representing the
A#
absorption­line indices are superimposed on the gray­scaled green contin­
uum map. The statistical errors of the indices are about 0.1õ0.15 in the
A#
center and about 0.6 at the edges ; the outer parts of the maps at R[ 3@@
A#
were smoothed by a Gaussian with (seeing quality) for a
FWHM\ 2A. 5
better presentation.
their possible origin. However, the circumnuclear ellipses in
distributions of all three indices are quite similar : evidently,
we see a metal­enriched and rather young elongated cir­
cumnuclear stellar structure. Its orientation, P.A. B 100¡õ
110¡, coincides exactly with the orientation of the
photometric circumnuclear stellar disk (see Fig. 3), but the
photometric disk is more compact than the chemically and
age­decoupled structure ; the radius of the former is 1A,
while the latter feature extends up to RB 3@@. One can argue
that the spatial resolutions of the HST data presented in
Figure 3 and the MPFS data presented in Figure 8 are too
di+erent. However, the continuum isophotes plotted in
Figure 8 are obtained under the same resolution as the
index maps, and at R\ 2@@õ3@@ they look elongated in
P.A. B 80¡, an orientation quite di+erent from that of the
index isolines. We can propose the following explanation
for this di+erence : while the surface brightness of the cir­
cumnuclear disk (and its contribution to the total surface
brightness) drops rapidly with the radius, the stellar popu­
lation properties in the disk must become more extreme,
that is, the age must fall and the metallicity must increase, to
keep a more shallow radial change of the integrated stellar
properties. This means that the secondary star formation
burst ïnished earlier in the nucleus and later in the circum­
nuclear disk. Other examples of such sequences of the sec­
ondary star formation bursts in galactic centers are the S0
galaxy NGC 1023 (Silîchenko 1999a) and the Sb galaxy
NGC 7331 (Silîchenko 1999b).
To determine physical characteristics of the stellar popu­
lation in the center of NGC 7280, we must compare the
observed absorption­line indices to model results. In doing
this there are two difficulties. First, the accuracy of index
measurements drops rapidly with the distance from the
center. If the random error of 0.15 in the nuclear indices
A#
allows us to determine stellar population parameters with a
sufficient reliability, corresponding roughly to a metallicity
accuracy of 0.1 dex and an age accuracy of 1 Gyr (if its
absolute value is near 2õ3 Gyr), increasing the typical error
even to 0.3 makes any e+orts in this direction quite inef­
A#
fective. Therefore, we cannot construct useful two­
dimensional maps of the stellar population parameters.
Instead, we prefer to average index measurements in con­
centric rings with the center in the nucleus and a width of 1A,
and to analyze radial dependencies of the age and metal­
licity. The accuracy of the azimuthally averaged indices is
better than 0.1 except at the outermost point, where it is
A# ,
0.2 The second difficulty is that most stellar population
A# .
models are calculated for solar element ratios, while in real
galaxies these ratios may be nonsolar. For example, ellip­
tical galaxies are known to be magnesium­overabundant
with respect to the iron abundance (Worthey, Faber, &
Gonzalez 1992). There have also been claims that the bright
lenticulars are similar to ellipticals of the same luminosity
and have [Mg/Fe] [ 0 (Fisher, Franx, & Illingworth 1996) ;
however, our preliminary analysis shows evidence for a
rather solar magnesium­to­iron ratio in all disk galaxies,
including lenticulars (Silîchenko 1993b). In any case, we
prefer to check the magnesium­to­iron ratio of any individ­
ual galaxy. In Paper I, we have ascertained that the bulges
in NGC 1023 and NGC 7332 demonstrate solar Mg/Fe,
while the nuclei are both chemically decoupled and appear
to be di+erent : the younger nucleus in NGC 7332 has
[Mg/Fe] \ 0, and the older nucleus in NGC 1023 is
magnesium­overabundant, [Mg/Fe] B]0.3. The situation

No. 1, 2000 YOUNG NUCLEI IN LENTICULAR GALAXIES. II. 133
FIG. 9.õDiagram (SFeT vs. Mg b) for NGC 7280. The measurements
are azimuthally averaged and taken along the radius with the step of 1A. 0.
The ages of the Worthey (1994) models are given in Gyr.
in NGC 7280 can be clariïed with the help of Figure 9 ; on
the SFeT versus Mg b diagram, we compare the indices
observed in NGC 7280 in the radius range 0Aõ8A with the
stellar population models of Worthey (1994), calculated
under the assumption of a solar magnesium­to­iron ratio.
One can see that in the center of NGC 7280, despite of the
overall metallicity gradient, the solar Mg/Fe ratio is con­
served everywhere. This conclusion remains valid for the
nucleus of NGC 7280 even if it is younger than 5 Gyr.
Therefore, the results of an analysis of Figure 9 assure us
that we can safely use models with a solar magnesium­to­
iron ratio, particularly the popular models of Worthey
(1994), to determine the age of the stellar population at the
center of NGC 7280.
Figure 10 presents a set of age­diagnostics diagrams, in
which we compare our azimuthally averaged measurements
with the models of old stellar populations : the Hb versus
Mg b diagram with the models of Worthey (1994) (top),
Hb versus [MgFe]4 (Mg b SFeT)1@2 with the models of
Worthey (1994) (middle), and Hb versus SFeT with the
models of Tantalo et al. (1998) for [Mg/Fe] \ 0 (bottom).
All three comparisons give consistent results. The nucleus of
NGC 7280 is very young ; the mean age of the stellar popu­
lation is 1.5 Gyr. The mean stellar age increases radially
outward ; between R\ 2@@ and R\ 5@@ it oscillates
(remaining roughly constant?) between 5 and 8 Gyr, and at
R[ 5@@ it suddenly rises to 12õ17 Gyr. The behavior of the
mean metallicity anticorrelates with that of the mean age.
The stellar population in the nucleus is superõmetal­rich,
[m/H] º]0.5. Moving along the radius, the metallicity
decreases ; in the intermediate­age ```` belt îî at R\ 2@@õ5@@ it is
roughly solar, and in the old ```` bulge îî at R\ 6@@õ8@@ it falls
below [0.5. The full range of the metallicity variations
between the center and R\ 8@@ (D1 kpc) is an order of
magnitude. This is an unprecedented fall compared to the
smooth metallicity gradients seen in early­type galaxies
(Davies, Sadler, & Peletier 1993 ; Carollo, Danziger, &
Buson 1993), but we have found the same metallicity drop
by an order of magnitude in the lenticular galaxy NGC
7332 (Silîchenko 1999a), where this phenomenon is related
to the presence of the decoupled circumnuclear stellar disk.
6. CONCLUSIONS AND DISCUSSION
In the lenticular galaxy NGC 7280 we have found a
rather young stellar nucleus, with a mean stellar population
FIG. 10.õAge diagnostics diagrams for NGC 7280. Top : Hb vs. Mg b,
the models of Worthey (1994). Middle Hb vs. [MgFe], the models of
Worthey (1994). Bottom : Hb vs. SFeT for [Mg/Fe] \ 0, the models of
Tantalo et al. (1998). The measurements are azimuthally averaged and
taken along the radius with the step of The ages of the models are
1A. 0.
given in Gyr ; the metallicities for Wortheyîs models are ]0.50, ]0.25,
0.00, [0.22, [0.50, [1.00, [1.50, and [2.00, if one takes the signs from
right to left, and for the models of Tantalo et al. they are ]0.4, 0.0, and
[0.7.
age of 1.5 ^ 0.5 Gyr, which is more metal­rich by an order
of magnitude than the bulge at RB 1 kpc. The statistical
error of the nuclear age estimate is obtained by projecting
error bars of the metal and Balmer line indices onto the line
of equal metallicity, [m/H] \]0.5. In general, there may
be a systematic age error related to a Balmer emission con­
fusion of the absorption­line index Hb. Some early­type
galaxies demonstrate a narrow emission line Hb at the

134 AFANASIEV & SILîCHENKO Vol. 119
bottom of the broader absorption line, and this e+ect may
result in age overestimation through the artiïcial diminu­
tion of the absorption­line index Hb. However, we have
already mentioned that in NGC 7280 the measurable
Balmer emissions are absent. We have tried to estimate an
upper limit on the possible systematic error due to Hb emis­
sion. The equivalent width of the noiselike emission feature
at the position of redshifted Ha (which is always the strong­
est among the Balmer emission lines) is less than 0.3 at
A#
R¹ 5@@ and less than 0.6 at R\ 6@@õ7@@. Since the contin­
A#
uum is almost ÿat between 4800 and 6600 this means
A# ,
that the equivalent width of the Hb emission, if it exists, is
less than 0.1 at R¹ 5@@ and less than 0.2 at R\ 6@@õ7@@,
A# A#
and this is a possible correction for the emission that should
be applied to the absorption­line index Hb. For the nucleus
of NGC 7280, it means that the possible age systematic
error due to Hb emission is less than 0.1 Gyr.
The age of the stellar populations that we determine here
is not, of course, an absolute age value ; our estimate must
be considered in the frame of the stellar isochrone sets that
are used in model calculations. Particularly, Worthey (1994)
used the stellar isochrones of VandenBerg and coworkers
and revised Yale isochrones that were calculated in the mid­
1980s. Since then some improvements have been made in
this ïeld ; particularly, the necessity of taking into account
mild overshooting has been proved. This e+ect is important
for stars more massive than 1.6 or for stellar systems
M _
younger than 4õ5 Gyr (Maeder & Meynet 1991), and there­
fore is important for the nucleus of NGC 7280. The possible
systematic error of the age estimate due to neglecting the
overshooting e+ect can be evaluated by the analogy with
open star clusters of similar ages. Carraro et al. (1993) have
measured the ages of two intermediate­age (1 \ T \ 2 Gyr)
open clusters, NGC 752 and NGC 3680, by comparing
observed C­M diagrams with the theoretical isochrones cal­
culated with and without overshooting ; they have found
that including overshooting increases the age estimates by
0.4 Gyr. Therefore, the age estimate for the nucleus of NGC
7280 must perhaps be increased up to 2 Gyr if one refers to
the state­of­art stellar evolution theory.
The chemically and age­decoupled nucleus seems to be
spatially resolved ; the circumnuclear absorption index iso­
lines represent ellipses elongated in P.A. B 100¡õ110¡. The
same orientation, P.A. \ 103¡, is found for the elongated
circumnuclear stellar structure seen best at R\ 1@@, the exis­
tence of which has been revealed by morphological analysis
of the HST WFPC2 and NICMOS images of NGC 7280.
Taking into account the stellar kinematics inside RB 2@@, we
conclude that this structure is a circumnuclear stellar disk
inclined with respect to the global plane of the galaxy. In the
meantime, both photometric and kinematical data in the
radius range 2Aõ8A imply the existence of an intermediate­
scale bar elongated in P.A. B 60¡. Interestingly, the circum­
nuclear ionized gas is distributed and rotates in the plane
orthogonal to the plane of the circumnuclear stellar disk.
The main problem that has attracted our attention for
some years is the origin of decoupled nuclei. Is it possible to
form a decoupled nucleus without capturing ```` external îî
matter, or does its presence always signify merging in the
past? When we found a circumnuclear ```` polar îî gaseous
disk similar to what we report now in NGC 7280 in the
regular Sb galaxy with the chemically decoupled nucleus
NGC 2841 (Silîchenko, Vlasyuk, & Burenkov 1997), we
concluded that it was a result of past gas accretion. But
later, having made an analysis of the global structure and
kinematics of this galaxy, we ascertained that it has a tri­
axial bulge that can explain all the spectacular phenomena
observed in NGC 2841, including the counterrotating
stellar component and the circumnuclear gaseous polar ring
(Afanasiev & Silîchenko 1999). Moreover, the majority of
circumnuclear stellar disks accompanying the presence of
the decoupled nuclei lie in the main galactic planes (M31,
Silîchenko, Burenkov, & Vlasyuk 1998 ; NGC 4216 and
4501, Silîchenko et al. 1999 ; NGC 1023, Silîchenko 1999a),
so there is no reason to think that they are products of
accreted material consumption. However, the case of NGC
7280 strongly complicates the picture. We observe several
populated planes in the center of this galaxy ; neither the
plane of the circumnuclear stellar disk nor the plane of the
circumnuclear gaseous disk coincides with the main galactic
plane. There is a global bar in NGC 7280, but we cannot
identify any populated circumnuclear planes with the prin­
cipal planes of this triaxial structure. Perhaps we have
encountered for the ïrst time a transient situation in the
center of an early­type galaxy related to some cyclic inter­
action with the gas­rich satellite galaxy. Both bar and
decoupled circumnuclear disk may be produced by this
interaction, but by di+erent episodes of this interaction. A
tidally induced bar forms during several rotation periods
(Miwa &Noguchi 1998), which are only 107 yr in the center
of NGC 7280 ; in the mean while, the decoupled nuclear
disk is approximately 109 years old. In such a case they are
not obliged to be related geometrically. We would also like
to stress that the decoupled nucleus in NGC 7280 is the
youngest of all the ones found by us, so this galaxy may be
quite unique in the evolution of its central region structure.
We thank the astronomers of the Special Astrophysical
Observatory S. N. Dodonov and A. V. Moiseev for support­
ing the observations at the 6 m telescope. The 6 m telescope
is operated under the ïnancial support of Science Ministry
of Russia (registration number 01­43). 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 Propulsion Laboratory, California Institute of Tech­
nology, under contract to NASA. This research has made
use of the La Palma Archive. The telescope JKT is operated
on the island of La Palma by the Royal Greenwich Obser­
vatory at the Spanish Observatorio del Roque de los
Muchachos of the Instituto de Astroïsica de Canarias. The
work is partly based on observations made with the NASA/
ESA Hubble Space Telescope, obtained from the data
archive at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5­26555. We
have also used the software ADHOC, developed at the
Marseille Observatory, France. The work was supported by
a grant of the Russian Foundation for Basic Researches
98­02­16196, by a grant of the President of Russian Federa­
tion for young Russian doctors of sciences 98­15­96029, and
by the Russian State Scientiïc­Technical Program
```` Astronomy : Basic Space Researches îî (Astronomy
section).

No. 1, 2000 YOUNG NUCLEI IN LENTICULAR GALAXIES. II. 135
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