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THE ASTRONOMICAL JOURNAL, 120 : 741õ751, 2000 August
2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
(
FACE­ON GALAXIES NGC 524 AND NGC 6340 : CHEMICALLY DECOUPLED NUCLEI AND INCLINED
CIRCUMNUCLEAR DISKS
O. K. SILîCHENKO1
Sternberg Astronomical Institute, Moscow, 119899 Russia and Isaac Newton Institute of Chile, Moscow Branch ; olga=sai.msu.su
Received 1999 September 1 ; accepted 2000 May 2
ABSTRACT
The central regions of the early­type disk galaxies NGC 524 and NGC 6340 have been investigated
with the Multipupil Field Spectrograph at the 6 m telescope of the Special Astrophysical Observatory of
the Russian Academy of Sciences. We conïrm the existence of chemically distinct stellar nuclei in these
galaxies, which had been claimed earlier. The metallicity di+erences that we have found between the
nuclei and the bulges, 0.5õ1.0 kpc from the centers, reach 0.5õ0.6 dex. Both nuclei are magnesium over­
abundant, but the bulges have di+erent magnesium­to­iron ratios : it is solar in NGC 6340 and the same
as the nuclear one in NGC 524. Kinematical and morphological analyses reveal the existence of inclined
central disks in these galaxies. In NGC 524 the central disk consists of stars, dust, and ionized gas ; its
extension may be as large as up to RB 3 kpc, and it is inclined by some 20¡ to the global galactic plane.
In NGC 6340 only a gaseous polar disk is detected, the extension of which does not exceed RB 500 pc.
Key words : galaxies : evolution õ galaxies : individual (NGC 524, NGC 6340) õ galaxies : nuclei õ
galaxies : structure
1. INTRODUCTION
Early­type disk galaxies di+er from late­type disk (spiral)
galaxies by the smooth appearance of their disks. In
general, they lack a massive gaseous component and notice­
able star formation regions. However, often some gas is
present in the circumnuclear regions ; it is widely accepted
that this gas has an external origin. Bertola and coworkers
have reported a large body of statistics of decoupled dis­
tributions and kinematics of gaseous and stellar com­
ponents in nearby lenticulars. Particularly, Bertola et al.
(1992a) have reported three cases of gas counterrotation in
a sample of 15 objects, and they concluded, drawing on data
in the literature, that at least 40% of gas­possessing S0 have
to accrete this gas during their late evolution. Kuijken et al.
(1996) also noted that a signiïcant fraction of gas disks in
lenticulars (24% ^ 8% in their sample) demonstrate
counterrotation with respect to stars. Among the known
decoupled distributions of gas and stars, a class of polar
rings is the most spectacular one. Besides the extreme case
of NGC 2685, where the radius of the gaseous polar ring is
comparable to the radius of the global stellar disk, there
exist more ```` minor­axis dust­lane îî galaxies, such as NGC
1947 (Bertola et al. 1992b) or NGC 7280 (Carollo et al.
1997b ; Afanasiev & Silîchenko 2000) where compact cir­
cumnuclear gaseous disks are shown to rotate orthogonally
with respect to the global stellar rotation.
In 1992 we published our ïrst paper (Silîchenko et al.
1992) on chemically distinct nuclei in early­type disk gal­
axies. In three lenticulars and in three SaõSbîs we have
found a sharp drop in magnesium absorption­line strength
when passing from the nuclei to the surrounding bulges.
The di+erence in Mg absorption strength indicates an order
of magnitude change in metallicity on scales of a few arcse­
conds, whereas smooth metallicity gradients in galactic
spheroids are usually only 0.3 dex per decade in radius in
ellipticals (Carollo et al. 1993) or somewhat larger, with a
mean of [0.5 and an extreme of [1.0 per dex, in bulges of
õõõõõõõõõõõõõõõ
1 Guest Investigator of the RGO Astronomy Data Centre.
early­type disk galaxies (Balcells & Peletier 1994 ; Fisher et
al. 1996). The origin of chemically decoupled nuclei in disk
galaxies may be related to a gas accretion event and a sub­
sequent star formation burst in the nucleus where the acc­
reted gas is accumulated. With this hypothesis we try to ïnd
a connection between the chemical distinctness of the
nuclear stellar population and the presence of gas sub­
systems of obviously external origin. We (Silîchenko et al.
1992 ; Silîchenko 1999a) have already noted chemically dis­
tinct nuclei in the lenticulars NGC 1023, where an inclined
extended H I disk is present, and NGC 7332 where circum­
nuclear gas counterrotates with respect to the stars. The
lenticular galaxy with the chemically decoupled nucleus
NGC 7280 represents a remarkable example. The circum­
nuclear gas coupling with tiny dust lanes is extended, and it
rotates orthogonally to the circumnuclear stellar disk,
which is, on its own, inclined with respect to the main galac­
tic disk (Afanasiev & Silîchenko 2000).
In this paper we present the results of a complex study of
the central parts of two nearby face­on, early­type disk gal­
axies, NGC 524 and NGC 6340. Some time ago we found
chemically decoupled nuclei in these galaxies. NGC 524
(Silîchenko et al. 1992) and NGC 6340 (Silîchenko 1995)
were observed at the 6 m telescope with the Multipupil
Field Spectrograph (MPFS) equipped by IPCS, and the
drops of the magnesium absorption­line equivalent width
along the radius were detected. Recently we reobserved the
galaxies because the MPFS is now equipped with a CCD,
making the data become more accurate. Besides the investi­
gation of the radial variations of absorption­line indices, we
also present results of kinematical and morphological
analyses for the central parts of NGC 524 and NGC 6340.
Due to the face­on orientation of their main planes, any
inclined disks, both stellar and gaseous, can be easily
detected if systematic visible velocity gradients are observed
that can be attributed to rotation velocity projection onto
the line of sight. The paper is organized as follows. Section 2
describes the observations and their reduction, as well as
the data from open archives which we use. In ° 3 we reconsi­
der radial variations of metal absorption­line indices and
741

742 SILîCHENKO Vol. 120
conïrm that the stellar nuclei of NGC 524 and NGC 6340
are chemically decoupled. In ° 4 we study the morphology
of distributions of gas and stars in the galaxies, and in ° 5 we
present two­dimensional velocity ïelds both for ionized gas
and stars in the central regions. Section 6 contains the main
conclusions. In Table 1 we give the basic parameters for the
galaxies under consideration.
2. OBSERVATIONS AND DATA REDUCTION
The observations of NGC 524 and NGC 6340 presented
in this paper were performed in 1996õ1997 with the MPFS
of the 6 m telescope at the Special Astrophysical Observa­
tory (SAO), Nizhnij Arkhyz, Russia (Afanasiev et al. 1990,
1996). The detailed log of the observations is given in Table
2. The detector used was 520 ] 580 CCD ISD015A
(Electron, St. Petersburg), with a pixel size of 18 km] 24
km and read­out noise of 13 e~1. The gain was 3.4 e~1 in
1996 and 7 e~1 in 1997.
The MPFS, which resembles the French integral­ïeld
spectrograph TIGER (Bacon et al. 1995) in the principles of
its design, makes it possible to obtain simultaneously a set
of spectra from an extended area (8 ] 12 square elements in
the present work) ; each spatial element is , so the
1A. 3 ] 1A. 3
full ïeld of view which we center on the nuclei of the gal­
axies is 10A ] 16A. Two spectral ranges were exposed, blue
and red, with a (reciprocal) dispersion of 1.6 pixel~1 (with
A#
the spectral resolution of 4õ6 slightly varying over the
A#
ïeld of view). The blue spectral range under consideration
contains several strong absorption lines, such as the Mgb
feature. We have used it to derive stellar velocity ïelds by
cross­correlating spectra of the galaxies with the spectra of
K giant stars observed during the same nights with the
same spectrograph. Also we have calculated absorption­line
indices Hb, Mgb, and Fe5270 in the popular Lick system
(Worthey et al. 1994) to study radial variations of the mean
metallicity of the stellar populations. The sky background
in the blue was exposed separately after each galaxy expo­
sure. Properly normalized and smoothed, it was then sub­
tracted from the spectra of the galaxies. The exposure times
for the galaxies were long enough to achieve a signal­to­
noise ratio (S/N) not less than 50õ70 in the nucleiõthis
A# ~1
level of S/N provides an accuracy of the absorption­line
indices of better than 0.2 (Cardiel et al. 1998). However,
A#
the accuracy drops to 0.5õ0.6 for the outermost elements
A#
observed, and, to keep a constant level of signal­to­noise
ratio up to RB 8@@, we added the element spectra in concen­
tric circular rings with a width of and the center in the
1A. 3
nuclei. After azimuthal averaging, the accuracy of all indices
measured along the radius is about 0.1 To check consis­
A# .
tency of our index system with the standard Lick one, we
have observed nine G8õK3 stars, both giants and dwarfs,
from the list of Worthey et al. (1994) and calculated their
indices in the same manner as those for the galaxies. The
agreement between stellar indices measured by us and those
tabulated in Worthey et al. (1994) is excellent within the
errors cited by them ; the mean deviations of our measure­
ments from those of Worthey et al. (1994) for all the indices
are less than 0.05 so we need not degenerate our spectral
A# ,
resolution to come into agreement with the standard Lick
index system. The indices Mgb and Fe5270 measured in the
galaxies have been corrected for the stellar velocity disper­
sion broadening ; the correction values are determined by
artiïcial Gauss broadening the stellar spectra with varying
ps. For NGC 6340 this correction is small (only 0.1 A# )
because the stellar velocity dispersion in the center of this
galaxy is 100õ140 km s~1 (Bottema 1989). For NGC 524
this correction is 0.5 in the nucleus and 0.4 beyond it,
A# A#
because the nuclear stellar velocity dispersion is 246 km s~1
(LEDA) or 236 ^ 19 km s~1 (Schechter 1983), and it
decreases only slightly in the nearest vicinity of the nucleus
(our impression from the visual inspection of the spectra).
The red spectral range under consideration contains the
TABLE 1
GLOBAL PARAMETERS OF THE GALAXIES
Parmeter NGC 524 NGC 6340
Type (NED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SA(rs)0‘ SA (s)0/a
R 25 (kpc) (RC3]LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 9.3
B T 0 (RC3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.17 11.67
M B (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [21.39 [19.94
(B[V ) T 0 (RC3) . . . . . . . . . . . . . . . . . . . . . . . . . . 1.00 0.79
(U[B) T 0 (RC3) . . . . . . . . . . . . . . . . . . . . . . . . . . 0.58
V r (RC3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (opt) 2421 km s~1 (radio) 1198 km s~1
Distance (Mpc) (LEDA, H 0 \ 75 km s~1 Mpc~1) . . . . . . 33 19.8
Inclination (deg) (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 25.6
P.A. 0 (deg) (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
p 0 (km s~1) (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 146
v m (km s~1) (LEDA) . . . . . . . . . . . . . . . . . . . . 245
TABLE 2
TWO­DIMENSIONAL SPECTROSCOPY OF NGC 524 AND NGC 6340
Exposure Spectral Range P.A.(top) Seeing
Date Galaxy (min) (A# ) (deg) (arcsec)
1996 Aug 14õ15 . . . . . . . . . . NGC 6340 90 4800õ5400 209 1.7
1996 Aug 14õ15 . . . . . . . . . . NGC 6340 90 6250õ6900 180 1.7
1996 Oct 14õ15 . . . . . . . . . . NGC 524 60 6250õ6900 67 1.6
1997 Oct 31õNov 1 . . . . . . NGC 6340 81 4800õ5400 164 2.2
1997 Oct 31õNov 1 . . . . . . NGC 524 94 4800õ5400 149 2.2

No. 2, 2000 FACE­ON GALAXIES NGC 524 AND NGC 6340 743
emission line [N II] j6583, which is strong in the centers of
NGC 524 and NGC 6340. It is used to obtain the two­
dimensional velocity ïelds of the ionized gas. The wave­
length calibration for both spectral ranges was made by
using separate exposures of the hollow­cathode lamp ïlled
with helium, neon, and argon. The accuracy and absence of
systematic shifts of the velocity scale was checked by mea­
suring the night­sky emission lines. The accuracy of the
individual velocity measurements both for stars and ionized
gas is about 25 km s~1. The primary data reductionõbias
subtraction, ÿat ïelding, cosmic­ray removal, extraction of
one­dimensional spectra, wavelength calibration, construc­
tion of surface brightness mapsõhave been performed with
software developed in the Special Astrophysical Observa­
tory (Vlasyuk 1993). The absorption­line indices were calcu­
lated with our own FORTRAN programs.
We have also retrieved a long­slit spectrum of NGC 524
from the La Palma Archive. The galaxy was observed on
1995 December 24 at the ISIS William Herschel Telescope
(WHT) with a dispersion of 0.4 pixel~1 in the spectral
A#
range 5000õ5400 (blue arm of the spectrograph) ; the slit
A#
(width equal to was aligned almost along the minor
0A. 62)
axis of the galaxy, P.A. \ 135¡. We have used this spectrum
to calculate Mgb and Fe5270 indices along the slit with the
same techniques as the indices from our MPFS data (the
sky for subtracting was taken from the edges of the slit),
though we cannot reduce these indices into the standard
Lick system because of the lack of standard Lick star obser­
vations.
The photometric data involved in our analysis are taken
from the La Palma and Hubble Space Telescope (HST )
Archives. The details of the observations are given in Table
3. The programs in the frame of which the central parts of
the galaxies have been observed by the HST are Core
Properties of Bulges of Spiral Galaxies (PI : M. Stiavelli,
program ID 6359) and Nuclear Structure of S0 Galaxies
(PI : A. Phillips, program ID 5999). We derived morphologi­
cal characteristics of the surface brightness distribution in
the galaxies by analyzing these images ; the FITELL
program of V. V. Vlasyuk has been used for this purpose.
3. CHEMICALLY DECOUPLED NUCLEI IN NGC 524 AND
NGC 6340
As noted in the introduction, the galaxies under consider­
ation exhibit stellar nuclei with enhanced magnesium
absorption lines (for NGC 524, see Silîchenko et al. 1992 ;
for NGC 6340, see Silîchenko 1995). Now we present more
precise radial proïles of Mgb together with those of Fe5270
and Hb. The azimuthally averaged index measurements are
presented in Table 4. The accuracy of the azimuthally aver­
aged measurements is improved by a factor of 3 with respect
to our earlier results. However, the qualitative conclusions
remain the same.
NGC 524 (Fig. 1) shows radial proïles of the magnesium
and iron indices of very similar shapes. The maximum
equivalent widths of the metal lines are observed in the
nucleus and decrease radially up to RB 4@@ (600 pc).
Between R\ 4@@ and R\ 8@@ the MPFS proïles ÿatten o+.
Their extensions by the long­slit data conïrm the ÿat
behavior of the metal­index proïles in the radius range
5Aõ20A (the bulge­dominated region), though the long­slit
measurements are not reduced into the Lick system. By
calculating index gradients in the bulge in a traditional
manner (versus d log R in the radial range 3Aõ8A), we have
obtained dMgb/d log R\[0.85^ 0.18 dex~1 and
A#
dFe5270/d log R\[0.59^ 0.02 dex~1, both gradients
A#
corresponding to d[m/H]/log R\[0.4õa value quite
typical for early­type galaxies (Balcells & Peletier 1994 ;
Fisher et al. 1996). The zero points of the linear regressions
corresponding to the extrapolated bulge indices at R\ 1@@,
TABLE 3
PHOTOMETRIC OBSERVATIONS OF NGC 524 AND 6340
Exposure Seeing Scale
Date Galaxy Telescope Filter (seconds) (arcsec) (arcsec)
1988 May 9õ10 . . . . . . . NGC 6340 JKT V 600 1.6 0.30
1988 May 9õ10 . . . . . . . NGC 6340 JKT R 600 1.7 0.30
1988 May 9õ10 . . . . . . . NGC 6340 JKT I 720 1.6 0.30
1998 May 17õ18 . . . . . . NGC 6340 JKT R 900 1.1 0.33
1996 Nov 6õ7 . . . . . . . . . NGC 524 JKT I 360 2.6 0.31
1995 Sep 24 . . . . . . . . . . . NGC 524 HST ]WFPC2 F555W 160 0.13 0.045
1995 Sep 24 . . . . . . . . . . . NGC 524 HST ]WFPC2 F814W 320 0.13 0.045
1996 Jun 2 . . . . . . . . . . . . NGC 6340 HST ]WFPC2 F606W 400 0.15 0.045
TABLE 4
AZIMUTHALLY AVERAGED LICK INDICES IN NGC 524 AND NGC 6340
NGC 524 NGC 6340 (1996 Aug) NGC 6340 (1997 Oct)
R
(arcsec) Hb Mgb Fe5270 Hb Mgb Fe5270 Hb Mgb Fe5270
0 . . . . . . . . 1.33 4.87 3.06 1.05 4.65 2.68 1.56 4.49 2.93
1.3 . . . . . . 1.42 4.65 2.97 1.24 4.20 2.88 1.35 4.38 2.83
2.6 . . . . . . 1.23 4.20 2.75 1.30 3.60 2.43 1.00 3.83 2.48
3.9 . . . . . . 1.16 3.88 2.34 0.91 2.97 2.34 1.23 3.18 2.34
5.2 . . . . . . 0.94 3.70 2.27 0.80 3.16 2.14 1.24 3.17 2.30
6.5 . . . . . . 1.22 3.68 2.21 0.92 2.26 1.64 1.39 3.10 2.00
7.8 . . . . . . 0.98 3.61 . . . 0.78 2.24 2.03 1.59 2.49 0.99

744 SILîCHENKO Vol. 120
FIG. 1.õNGC 524 : Radial proïles of the absorption­line indices Hb,
Mgb, and Fe5270. Open circles present our MPFS data ; the bell with the
error bar is the data of Trager et al. (1998). Asterisks show the long­slit
data from the ISIS WHT (La Palma Archive) not corrected for stellar
velocity dispersion and not reduced to the standard Lick system.
Mgb\ 4.34 ^ 0.14 and Fe5270 \ 2.68 ^ 0.01 are
A# A# ,
noticeably smaller than the nuclear indices, Mgb(nuc) \
4.87 ^ 0.16 and Fe5270(nuc) \ 3.16 ^ 0.19 We have
A# A# .
taken the mean bulge characteristics by averaging the
MPFS data in the radial range 4Aõ8A, resulting in
SMgb(bul)T \ 3.75 ^ 0.06 and SFe5270(bul)T \
A#
2.27 ^ 0.04 If we now apply the models of Worthey
A# .
(1994) for old stellar populations to the di+erences *Mgb
and *Fe5270 between the nucleus and the mean bulge,
under the assumption of equal mean stellar population
ages, we obtain *[Fe/H] \ 0.50 ^ 0.10 dex from *Mgb
and *[Fe/H] \ 0.56 ^ 0.15 dex from *Fe5270, the nucleus
being more metal­rich. Unfortunately, we cannot determine
the mean age of the stellar population in the center of NGC
524 by using the absorption­line index Hb, because it is
notably a+ected by the emission line. However, chemically
decoupled nuclei in lenticular galaxies usually appear to be
younger than the bulges (Silîchenko 1999a ; Afanasiev &
Silîchenko 2000), so the derived metallicity di+erence must
be considered to be a low limit. The coincidence within the
errors of the *[Fe/H] values derived from *Mgb and
*Fe5270 gives some evidence for equal magnesium­to­iron
ratios in the nucleus and in the bulge of NGC 524.
NGC 6340 (Fig. 2) has been observed twice in the green
spectral range with the MPFS]CCD, so we can indepen­
dently estimate the accuracy of our measurements by com­
paring the two data sets. One can see in Figure 2 that the
FIG. 2.õNGC 6340 : Radial proïles of the absorption­line indices Hb,
Mgb, and Fe5270. Filled and open circles present our MPFS data ; the bell
with the error bar is the data of Trager et al. (1998).
claimed accuracy of 0.1 for the azimuthally averaged
A#
points is kept quite well up to RB 6@@. The shape of the
radial proïles of the metal­line indices demonstrates again
the presence of a chemically decoupled nucleus, though the
contrast of the Fe5270 drop is obviously smaller than that
of Mgb. As we have in this galaxy more measurements than
in NGC 524, the mean indices for the nucleus and the bulge
of NGC 6340 are estimated more precisely. For the nucleus,
Mgb(nuc) \ 4.57 ^ 0.08 and Fe5270(nuc) \ 2.80 ^ 0.13
A#
and for the bulge, R\ 4Aõ6A, Mgb(bul) \ 3.22 ^ 0.04
A# , A#
and Fe5270(bul) \ 2.32 ^ 0.07 After applying the models
A# .
of Worthey (1994) to the nucleus­bulge di+erences, *Mgb
and *Fe5270, we obtain *[Fe/H] \ 0.61 ^ 0.05 dex from
*Mgb and *[Fe/H] \ 0.30 ^ 0.13 dex from *Fe5270.
Again, these metallicity di+erences represent only low
limits, owing to age uncertainty : a narrow Hb emission line
is seen inside the broader absorption, and so the
absorption­line index, Hb, is invalid for the age diagnostics.
Unlike NGC 524, NGC 6340 demonstrates di+erent
*[Fe/H] from the magnesium and iron index di+erences ; it
may be a signature of di+erent magnesium­to­iron ratios in
the bulge and in the nucleus of this galaxy.
The question of the magnesium­to­iron ratio can be
addressed directly with a diagram (Fe5270­Mgb). The ïrst
fruitful attempt to use this diagram to diagnose the Mg/Fe
ratio was made by Worthey et al. (1992), and they found
immediately that a lot of luminous ellipticals are magne­
sium overabundant. I noted that Mgb and Fe5270 in the
centers of disk galaxies, including lenticulars, satisfy the
models with solar magnesium­to­iron ratio (Silîchenko

No. 2, 2000 FACE­ON GALAXIES NGC 524 AND NGC 6340 745
1993), though later Fisher et al. (1996) reported several
nuclei (but not the bulges !) in luminous lenticulars that
were magnesium overabundant. Figure 3 presents these dia­
grams for NGC 524 and NGC 6340 ; the model sequences
with varying metallicity and ages calculated by Worthey
(1994) for [Mg/Fe] \ 0 border the narrow locus of the solar
Mg/Fe ratio. The comparison of the observations with the
models reveals that both chemically decoupled nuclei are
magnesium overabundant ; though less accurate, the mea­
surements of Trager et al. (1998) conïrm this conclusion.
The magnesium overabundance of the chemically decou­
pled nuclei in lenticular galaxies is not uniqueõwe have
found it, for example, in NGC 1023 (Silîchenko 1999a). But
it is not a ruleõNGC 7332 and NGC 7280, being of the
same total luminosity as NGC 6340, demonstrate the solar
Mg/Fe in their chemically decoupled nuclei as well as in
their bulges (Silîchenko 1999a ; Afanasiev & Silîchenko
2000). NGC 6340 shows a solar magnesium­to­iron ratio in
its bulge ; the [Mg/Fe] di+erence between the nucleus and
the bulge is about 0.3 dex. NGC 524 demonstrates a quite
di+erent Mg/Fe behavior. The data taken along the radius
lie parallel to the model sequence but shifted to the right.
This is interpreted as [Mg/Fe] B]0.3, constant up to
RB 8@@ (1 kpc) in this galaxy. Such behavior of Mg/Fe
resembles that in ellipticals (Worthey et al. 1992) and in
some bulges of supergiant spirals, such as M31 (Silîchenko
et al. 1998) or NGC 488 (Silîchenko 1999b).
4. MORPHOLOGY OF STELLAR AND GAS DISTRIBUTIONS
IN THE CENTER OF NGC 524 AND NGC 6340
To understand the structure of the central regions in
NGC 524 and NGC 6340, we have analyzed digital images
obtained from the La Palma and HST archives. The radial
dependencies of the major­axis position angle and ellipticity
of the isophotes approximated by ellipses are presented in
Figure 4 for NGC 524 and in Figure 5 for NGC 6340.
The galaxies have been studied photometrically earlier,
especially NGC 524. For NGC 6340 only one­dimensional
brightness proïles can be found in the literature (Boroson
1981 ; Whitmore & Kirshner 1982) ; they give evidence that
the global disk begins to dominate at Rº 15@@. For NGC
524 the information is rich and somewhat controversial.
According to Kent (1985) and Bothun & Gregg (1990), who
have performed the brightness proïle decomposition in the
r band and B band, respectively, the bulge dominates over
FIG. 4.õRadial variations of the isophote morphological character­
istics in the center of NGC 524 according to HST data and La Palma data.
the disk at all radii ; so in some respects NGC 524 may be
reclassiïed as an elliptical with embedded disk. However,
the proïles presented by Hodge & Steidl (1976) and Mag­
relli et al. (1992) reveal an extended exponential disk domin­
ating over the bulge at Rº 30@@. The ellipticity estimates for
the outer isophotes range from 0.03 ^ 0.01 (Hodge & Steidl
1976) to 0.06 (Kent 1984) ; our estimate from the La Palma
data is 0.06 ; in any case, the galaxy inclination cannot be
larger than 20¡. The line­of­nodes orientation lies some­
where between (Hodge & Steidl 1976) and
P.A.
0 \ 5¡ ^ 23¡
41¡ (Kent 1984 ; Magrelli et al. 1992). More exact determi­
nation is prevented by the low ellipticity of the isophotes ;
LEDA and RC3, in particular, omit the estimate of the
major­axis position angle (see our Table 1).
The combination of the photometric data of di+erent
spatial resolution allows us to study the structure of the
inner regions of NGC 524 and NGC 6340 in more detail.
Figure 4 shows the radial variations of the major­axis
position angle and ellipticity in the center of NGC 524. At
R[ 5@@ they look rather constant, with at the level of
P.A. 0
FIG. 3.õDiagrams (Fe5270­Mgb). (left), NGC 524 ; (right) NGC 6340. The measurements are azimuthally averaged and taken along the radius with the
step of The nuclear measurements from Trager et al. (1998) are plotted for comparison with their error bars. The ages of Wortheyîs (1994) models for
1A.3.
[Mg/Fe] \ 0 are given in billion years.

746 SILîCHENKO Vol. 120
FIG. 5.õRadial variations of the isophote morphological character­
istics in the center of NGC 6340 according to HST data and La Palma
data.
about 40¡ and 1 [ b/a at the level of 0.03. This behavior
does not contradict the data for the outer regions according
to, e.g., Magrelli et al. (1992). However, at R¹ 4@@ the major
axis seems to be turned by some 20¡ and, more importantly,
the ellipticity seems to increase up to about 0.10õa value
that is not reached even in the outermost part of the galaxy.
Though the accuracy of the morphological parameter esti­
mates is low for such roundish isophotes and though some
dust is present inside RB 1@@ (Byun et al. 1996), which
causes a discrepancy of two HST measurements of the ellip­
ticity through the di+erent ïlters, it seems quite probable
that a visibly elongated stellar structure is present in the
center of NGC 524.
The variations of the morphological parameters in the
center of NGC 6340 are even more prominent than in the
center of NGC 524. In the disk­dominated region, at
R\ 30Aõ35A, the position angle of the isophote major axis,
that is, the orientation of the line of nodes, is P.A.
0 \ 131¡.
The asymptotic ellipticity is 0.08, so the inclination of the
galactic plane may be as large as 23¡. However, at R\ 5@@
(Fig. 5) the major axis is surely turned and its P.A. B 85¡
di+ers from the orientation of the line of nodes at least by
45¡. The ellipticity demonstrates a local maximum at
RB 5@@, the most prominent in the high­resolution HST
data. Though some discrepancy of the data inside RB 3@@
(caused by the di+erent spatial resolutions of the
observations) can be seen, the elongated stellar structure in
the center of NGC 6340 is also rather probable.
Our spectral observations have detected the emission line
[N II] j6583, and thus the presence of ionized gas, in the
centers of both galaxies. We wonder whether the distribu­
tions of di+use matter, gas, and dust resemble those of the
stellar components. First, we can examine the dust. Figure 6
presents direct WFPC2 (namely, PC) images of both gal­
axies. One can see a rather extended roundish dust disk in
the center of NGC 524 consisting of tightly wrapped thin
dark spirals. Lower resolution observations made by
Veron­Cetty & Veron (1988) also revealed a red nucleus
and an ```` almost face­on dust ring îî 16A ] 12A in size. Let us
note, however, that such an axis ratio, 0.75, favors a dust
disk inclination rather close to 40¡, not exactly face­on. In
the center of NGC 6340 (left panel of Fig. 6) Carollo et al.
(1997b) noticed ```` a tiny dust lane. îî We can add that the
very compact (with a radius of less than dust disk is
0A. 5)
seen almost edge­on and therefore we deal with
(i dust º 70¡),
a circumnuclear polar ring. Its line of nodes is close to
P.A. 0 B 90¡.
FIG. 6.õDirect views of the images of NGC 524 (left) and NGC 6340 (right), obtained by the HST WFPC2 in the visual passbands. The sizes of the maps
shown are 36A ] 36A and , respectively. The orientations are P.A.(top) \[68¡ and P.A.(top) \[129¡.
4A.5 ] 4A.5

No. 2, 2000 FACE­ON GALAXIES NGC 524 AND NGC 6340 747
Our MPFS observations allow us to refer directly to
emission­line brightness maps. Figure 7 presents surface
distributions of the [N II] emission intensity in arbitrary
units. In NGC 6340 the nitrogen emission­line isophotes are
elongated in P.A. B 130¡, so they imply the existence of a
rather extended gaseous disk, which in the line of nodes, but
not the inclination, coincides with that of the global disk. In
NGC 524 the emission distribution is strongly asymmetric.
It is conïned to the western part of the circumnuclear
region, and its geometry is not quite evident. However, if we
recall the work of Macchetto et al. (1996), who observed
lenticular galaxies through the narrowband II]
H a ][N
ïlter, we ïnd that the emission distribution in the center of
NGC 524 matches perfectly that of the dust : there is the
same roundish patchy disk with a radius of some 20@@. A
bright emission knot in the western vicinity of the nucleus is
also seen in the map presented by Macchetto et al. (1996), so
our left panel of Figure 7 probably represents a high­level
slice of the overall emission distribution.
5. KINEMATICS OF GAS AND STARS IN THE CENTRAL
REGIONS OF NGC 524 AND NGC 6340
When we look at a rotating disk from its pole, it is pro­
jected ```` face­on îî (on the sky plane), and the projection of its
rotation velocity onto the line of sight is zero. Generally,
face­on galaxies should lack any line­of­sight velocity gra­
dients (over their images). It is just what we would expect
from NGC 524 and NGC 6340. However, the line­of­sight
velocity ïelds obtained by us for these galaxies look quite
di+erent.
Figure 8 presents velocity ïelds for the ionized gas in the
centers of the galaxies. Both maps demonstrate obvious
signs of regular rotation : the measured line­of­sight veloci­
ties gradually changes from one map corner to another. In
the case of axisymmetrical (circular, cylindric) rotation, the
direction of the highest velocity gradient (which we call the
```` dynamical major axis îî) should coincide with the line of
nodes. In NGC 524 (left panel of Fig. 8) this direction is
in NGC 6340 (right panel of Fig. 8),
P.A.
0 B 23¡ ; P.A.
0 B
Both directions are indeed close to the major axes of
115¡.
the inner isophotes as reported in the previous section,
though it is not quite true for NGC 6340. P.A. 0 (phot)
changes along the radius in the center of this galaxy, and it
is not quite clear what orientation must be chosen as a
reference one. But as the ïrst approximation, one can con­
clude that we see circumnuclear rotating thin (cold) gaseous
disks ; and these disks, unlike the main stellar disks, are
obviously not face­on : their visible rotation is very fast.
As for stellar rotation, we have not found clear signs of it
in the center of NGC 6340. The systematic velocity varia­
tion over the full line­of­sight velocity ïeld of this galaxy,
the spatial base of which is some 10A, if it exists, does not
exceed the random error of one velocity measurement,
namely, 25 km s~1. On the contrary, stars in the center of
NGC 524 demonstrate quite noticeable rotation (Fig. 9).
This is somewhat unexpected. LEDA (see Table 1) gives for
NGC 524 and for NGC 6340 so the latter
i \ 8¡. 7 i \ 25¡.6,
galaxy would have a projection factor 3 times less than the
former. Besides, the stellar velocity dispersion in the center
of NGC 524 is rather high. The galaxy is thought by many
photometrists to be bulge dominated, so this galaxy is
closer to ellipticals than NGC 6340, and, as a luminous
FIG. 7.õSurface brightness distributions for the emission line [N II] j6583 (isolines) superposed on the gray­scaled red continuum map for NGC 524 (left)
and NGC 6340 (right). The maps are direct. The orientations are P.A.(top) \[23¡ for NGC 524 and P.A.(top) \ 0¡ for NGC 6340. The brightness units are
arbitrary.

748 SILîCHENKO Vol. 120
FIG. 8.õIsovelocities for the ionized gas in the centers of NGC 524 (left) and NGC 6340 (right). The maps are direct. The orientations are
P.A.(top) \[23¡ for NGC 524 and P.A.(top) \ 0¡ for NGC 6340. The positions of photometric centers are shown by the white plus signs
FIG. 9.õStellar isovelocities in the centers of NGC 524. The map is
direct. P.A.(top) \[31¡, the position of the photometric center is shown
by the plus sign.
early­type galaxy, it should rotate slowly. However, the
line­of­sight velocity gradient is quite visible in Figure 9 ; the
direction of the maximum velocity gradient, P.A. 0 B 19¡,
agrees well with the dynamical major axis of the gaseous
component. As the photometric major axis in the radius
range of 1Aõ4A is also aligned in (Fig. 4),
P.A. 0 (phot) B 20¡
we conclude that the separate stellar disk, with the radius of
4Aõ5A (0.6õ0.8 kpc) and a gaseous extension up to RB 20@@
(3 kpc), exists in the center of NGC 524. Its line of nodes is
aligned in P.A. B 20¡, whereas the global line of nodes is
close to (Fig. 4 ; see also Magrelli et al. 1992), so
P.A. 0 B 40¡
the circumnuclear disk is inclined with respect to the main
galactic plane.
To compare more thoroughly stellar and gaseous rota­
tions in the centers of NGC 524 and NGC 6340, we have
simulated one­dimensional velocity proïles along the
dynamical major axes overlapping a narrow ```` slit îî on the
two­dimensional velocity ïelds for the gas and stars. The
resulting proïles are plotted in Figure 10. In NGC 524 (left
panel of Fig. 10) there is no signiïcant discrepancy between
the stellar and gaseous rotation ; however, both velocity
variation amplitudes are too high to be attributed to the
rotation plane inclined by i ¹ 20¡, unless we accept V rot º
km s~1 at R\ 1 kpc. We would rather ascribe i B 40¡
360
implied by the morphology of the dust ring (Veron­Cetty &
Veron 1988) to this fast rotating circumnuclear disk. This is
one more sign of its decoupling. In NGC 6340 we had a
problem with determining a position angle for our cross­
section. The ionized gas shows the dynamical
P.A. 0 B 115¡,
major axis of the circumnuclear stellar component is
unknown, and the global line of nodes is close to P.A. 0 B

No. 2, 2000 FACE­ON GALAXIES NGC 524 AND NGC 6340 749
FIG. 10.õMajor­axis line­of­sight velocity proïles for the stellar and gaseous components in the centers of NGC 524 (left), P.A.(cut) \ 20¡, and NGC
6340 (right), P.A.(cut) \ 130¡.
(Fig. 5). Bottema (1989) has chosen P.A. \ 130¡ for his
140¡
long­slit observations, and, to make comparison with his
data as well, we cut the velocity ïelds along P.A. \ 130¡.
The gas and stellar velocity proïles in the center of NGC
6340 look quite di+erent (right panel of Fig. 10). The slope
of the gas velocity proïle, though it is not taken exactly
along the dynamical major axis, is at least 5 times steeper
than that for the stellar component. The central stellar
velocity dispersion is low ; galactic bulges with such p *
usually rotate rapidly (Kormendy & Illingworth 1982). An
explanation that can reconcile the visible kinematics of both
components is that the gas and stars in the center of NGC
6340 rotate in di+erent planes. While Bottema (1989)
decided that the visible stellar rotation of NGC 6340 is
quite normal for the inclination of 20¡, the rotation of the
ionized gas must be more related to the edge­on circumnu­
clear dust ring visible on the image provided by the HST
(Fig. 6b). The highly elongated isophotes of the [N II] emis­
sion surface brightness distribution (right panel of Fig. 7)
support this suggestion.
6. CONCLUSIONS AND DISCUSSION
Two early­type, face­on disk galaxies, NGC 524 and
NGC 6340, where we previously suspected the presence of
chemically decoupled nuclei, have been reinvestigated with
the Multipupil Field Spectrograph of the 6 m telescope
equipped with CCDs. We conïrm a drop of the
magnesium­line index between the nuclei and the bulges
4Aõ8A from the centers in these galaxies ; the *Mgbîs mea­
sured now imply metallicity di+erences of 0.5õ0.6 dex. In
NGC 524 the iron­line index Fe5270 also demonstrates a
drop between the nucleus and the bulge ; the magnesium
overabundance, [Mg/Fe] B]0.3, is almost constant over
the full radius range under consideration. In NGC 6340 the
nucleus is magnesium overabundant too, but the ratio
[Mg/Fe] falls to zero toward RB 4@@.
The presence of chemically decoupled nuclei in the gal­
axies is accompanied by indications of inclined disks
detected in the circumnuclear regions, though our investiga­
tion is completely conïned to the bulge­dominated regions.
In NGC 524 the central velocity ïelds of the ionized gas and
stars are similar ; the orientation of the dynamical major
axis, coincides with the photometric major axis
P.A. 0 B 20¡,
orientation in the radius range of 1Aõ5A. Though both
deviate from the line of nodes (which has P.A.
0 \ 32¡õ42¡),
the coincidence of the photometric and dynamical major
axes proves the axisymmetric character of rotation. In a
triaxial potential the photometric and dynamical major
axes turn in opposite directions with respect to the line of
nodes (Monnet et al. 1992). As the gaseous and stellar rota­
tion velocities are comparable, we are probably dealing
with a stellar and gaseous disk inclined with respect to the
main plane of the galaxy. The rather high visible velocity
variation amplitude favors larger inclination of the circum­
nuclear disk than is that of the global disk of the galaxy.
From the geometry of the dust distribution (Veron­Cetty &
Veron 1988) we would guess about i B 40¡ in the center
versus i ¹ 20¡ for the whole galaxy. The central disk may be
extended up to RB 20@@ (3 kpc), as evidenced by the dust
spirals and emission­line distribution in Macchetto et al.
(1996) (in the bluer passbands we also trace up
P.A. 0 B 20¡
to RB 13@@, Fig. 4). But we see the fast, solid­body rotation
only up to RB 5@@, and under the assumption of i \ 40¡ the
mass contained within this radius can be estimated as
(7õ10)109 rather high value corresponding to the
M _ õa
fast circumnuclear rotation, uB 280 km s~1 kpc~1.
In the center of NGC 6340 the stellar and gaseous veloc­
ity ïelds di+er dramatically. The lack of visible stellar rota­
tion must be confronted with the isophote major­axis turn
and the local ellipticity maximum in the radius range 1Aõ5A.
Such a combination of morphological and kinematical
properties in the center of the face­on disk galaxy may be
explained by a slight bulge triaxiality, with its largest axis
aligned in P.A. B 85¡, whereas the line of nodes of the
galaxy is close to Interestingly, the tiny
P.A. 0 B 130¡õ140¡.
dust lane noticed by Carollo et al. (1997b) in their HST
observations of NGC 6340 goes through the nucleus just in
this direction. Obviously, the di+use matter is going to settle

750 SILîCHENKO Vol. 120
into one of the principal planes of the triaxial bulge ; in this
particular case we see a quasi­polar circumnuclear gaseous
disk. The alignment of the [N II] emission isophotes (right
panel of Fig. 7) and of the dynamical major axis of the gas
velocity ïeld (right panel of Fig. 8) in P.A. 0 B 115¡õ120¡
proves a planar character of the gas rotation, but not
exactly in the principal plane of the triaxial bulge. Perhaps
the circumnuclear gaseous disk is not completely stabilized
yet. However the high line­of­sight velocity gradient is evi­
dence for a rather edge­on orientation of the rotation plane.
Circumnuclear polar gaseous disks in galaxies with tri­
axial bulges or bars represent a rather new phenomenon,
which is not yet widely known. However, several examples
are found in nearby noninteracting disk galaxies. We
(Silîchenko et al. 1997) have reported on the existence of a
fast rotating gaseous polar disk with a radius of D200 pc in
the regular Sb galaxy NGC 2841 ; later we proved that its
bulge is triaxial (Afanasiev & Silîchenko 1999). The orthog­
onal planar rotations of ionized gas and stars have also
been detected by us in the center of the lenticular galaxy
NGC 7280 (Afanasiev & Silîchenko 2000) ; this galaxy also
possesses an intermediate­scale bar. Another example of the
circumnuclear polar gaseous disk is known in NGC 253
(Anantharamaiah & Goss 1996). Qualitative arguments
(Sofue & Wakamatsu 1994) suggest that the existence of
circumnuclear polar gaseous disks well inside triaxial stellar
structures may be a result of intrinsic dynamical evolution ;
an accretion of external gas is not quite necessary in such a
case. However, an extended stellar­and­gaseous disk with a
randomly inclined rotation axis in the axisymmetric galaxy
such as NGC 524 seems to require external accretion of gas
(and stars ?).
Another important question that we have tried to answer
for some time is whether a chemically distinct nucleus (core)
and a circumnuclear stellar disk are the same thing, or
whether they are di+erent substructures, though perhaps
evolutionarily related ? In elliptical galaxies the former
variant has been discussed for several years (Surma &
Bender 1995 ; Carollo et al. 1997a). But in some spiral gal­
axies we clearly resolve the circumnuclear stellar disks and
simultaneously cannot resolve their chemically distinct
nuclei. Perhaps, the brightest example is M31. The size of its
chemically distinct entity is restricted by R¹ 4@@, whereas
the circumnuclear stellar disk extends up to RB 30@@
(Silîchenko et al. 1998). Though less convincing, because of
worse spatial resolution, a similar situation appears to be
present in NGC 524. The kinematically traced inclined
stellar disk is seen at R\ 4@@, but the e+ect of the chemically
distinct nucleus is already negligible at this radius. So we
would prefer to separate the chemically distinct nucleus and
the circumnuclear stellar disk in this galaxy. Rather, we
would associate the chemically distinct nucleus in NGC 524
with its ```` core, îî which has been found from WFPC HST
observations of the central luminosity proïle (Byun et al.
1996). The radius of the photometrically distinct core in
NGC 524 is which cannot be resolved in our obser­
0A. 3,
vations ; but as it is detectable from photometry, it may
a+ect the spectral characteristics of the galactic nucleus, if
the stellar population of the core is di+erent from the stellar
population of the bulge. As for NGC 6340, we do not know
if this galaxy has a separate circumnuclear stellar diskõnot
identiïed by its kinematics, it may be only face­on. But a
photometric core detected from the HST data is also
present in NGC 6340 ; its radius is (Carollo & Stiavelli
0A. 5
1998), so we would see it as an unresolved issue.
I thank the astronomers of the Special Astrophysical
Observatory, V. L. Afanasiev, S. N. Dodonov, V. V.
Vlasyuk, and S. V. Drabek for supporting the observations
at the 6 m telescope. I am also grateful to the postgraduate
student of the Special Astrophysical Observatory A. V.
Moiseev for help in preparing the manuscript. The 6 m
telescope is operated under the ïnancial support of Science
Ministry of Russia (registration number 01­43). During the
data analysis I 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 with the National Aeronautics and
Space Administration. 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 Observatory in 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 T elescope, 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 NAS 5­26555. The work was sup­
ported by the grant of the Russian Foundation for Basic
Researches 98­02­16196, by the grant of the President of
Russian Federation for young Russian doctors of sciences
98­15­96029 and by the Russian State Scientiïc­Technical
Program Astronomy Basic Space Researches (the section
Astronomy).
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