Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://sn.sai.msu.ru/~sil/preprints/2300028.ps.gz Äàòà èçìåíåíèÿ: Thu Apr 20 16:09:31 2000 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 19:44:03 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ï ð ï ð ï ð ï ð ï ð ï ð ï

Astron. Astrophys. 354, 28--34 (2000) ASTRONOMY
AND
ASTROPHYSICS
NGC 759: A giant elliptical with a just­forming decoupled nucleus #
V.V. Vlasyuk 1 and O.K. Sil'chenko 2,3
1 Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Russia
2 Sternberg Astronomical Institute, University av. 13, Moscow 119899, Russia
3 RGO Astronomy Data Centre, Guest Investigator
Received 25 August 1999 / Accepted 12 November 1999
Abstract. By undertaking a spectral and photometric investi­
gation of the central part of the bright elliptical galaxy NGC 759,
we have found an optical counterpart for the circumnuclear
molecular gas ring with the radius of # 3 ## (# 1 kpc) which
was earlier reported by Wiklind et al. (1997). This counter­
part consists of the ionized gas excited by young massive stars
and of the dust. The gaseous ring exhibits fast rotation: its pro­
jected rotation velocity at R # 3 ## is # 220 km/s. The large
projected rotation velocity value together with the asymmetric
appearance of the dust ring on the (V - I) colour map con­
firm the hypothesis of Wiklind et al. (1997) about a rather high
inclination of the gas rotation plane: our data favours i # 40 #
proposed by them for the molecular gas ring. Meantime the 2D
decomposition of the galaxy images both in the V and I bands
has revealed an existence of the brightness excess with respect
to the de Vaucouleurs' spheroid: this extracomponent is seen in
the radius range of 3 ## --16 ## , its boundaries looking nearly round,
and has a radial brightness distribution well­fitted by two expo­
nential laws with different characteristic scales. We argue that
two stellar disks are embedded into the bright elliptical galaxy:
the outer one is seen nearly face­on and so its origin is prob­
ably related to that of the main galactic body, the inner one is
inclined by i # 40 # and so its origin is probably related to the
circumnuclear gaseous ring. Within the radius range of their
appearance the disks contributes about 10% into the integrated
surface brightness.
Key words: galaxies: elliptical and lenticular, cD -- galaxies:
kinematics and dynamics -- galaxies: photometry -- galaxies:
structure -- galaxies: nuclei -- galaxies: individual: NGC 759
1. Introduction
According to a common opinion, elliptical galaxies have
the simplest visible structure among the galaxies. They con­
Send offprint requests to: O.K. Sil'chenko (olga@sai.msu.su)
# Based on observations collected with the 6m telescope of the Spe­
cial Astrophysical Observatory (SAO) of the Russian Academy of Sci­
ences (RAS) which is operated under the financial support of Science
Ministry of Russia (registration number 01­43) and on observations
collected with the 1m and 0.6m telescopes of the Special Astrophysi­
cal Observatory (SAO) of the Russian Academy of Sciences (RAS).
tain only one structural unit, a spheroid, whose stellar pop­
ulation looks relatively homogeneous, old and metal­rich,
and usually lacks interstellar medium. During the last years
this widely­held opinion has been strongly opposed. Firstly,
close inspection has revealed the presence of dust and ion­
ized gas in more than the half of all elliptical galaxies (e.g.
Roberts et al. 1991, Goudfrooij et al. 1994). Secondly, decou­
pled stellar cores were discovered in some elliptical galax­
ies: Jedrzejewski and Schechter (1988) and Bender (1988)
have found a half­dozen galaxies where the central regions
rotate faster and often around different rotation axes than the
main bodies. Later, Bender and Surma (1992) have proved
that the stellar populations in four kinematically distinct cores
are prominently more metal­rich than those in the rest of the
galaxies. Several more kinematically and chemically decou­
pled cores in elliptical galaxies have been added to the sample
of Bender and Surma (1992) -- see e.g. Carollo and Danziger
(1994), Mehlert et al. (1998). Though not one­to­one, the de­
coupled cores are usually related to compact circumnuclear
stellar disks embedded into spheroidal bodies of some ellip­
tical galaxies; in almost all cases these disks are found to
be younger than the common estimate of the age of ellipti­
cal galaxies, namely, are 5--8 Gyr old (Surma & Bender 1995,
Sil'chenko 1997, de Jong & Davies 1997). So a natural ex­
planation of the presence of decoupled cores in the centers
of elliptical galaxies may be a hypothesis of a secondary
star formation burst provoked by merging a gas­rich galaxy
(``dissipative merger''). According to our statistical estimate
(Sil'chenko 1994), up to 30% of all nearby ellipticals may have
chemically distinct cores. Secondary star formation bursts in el­
liptical galaxies had to last typically for 10 8
-10 9 years; the low
limit is caused by the need to provide a substantial magnesium
enrichment in the nuclear integrated stellar population and the
upper limit to preserve a high magnesium­to­iron ratio that is
usually observed. So, about 0.3%--3% of all nearby elliptical
galaxies may be caught just in the moment of the secondary star
formation burst in their centers. Here we present the first exam­
ple of an elliptical galaxy probably experiencing its decoupled
core formation -- NGC 759. The main global parameters of the
galaxy are given in Table 1.
The unusual structure of the central region of NGC 759 has
been revealed by Wiklind et al. (1997) as a result of interfer­

V.V. Vlasyuk & O.K. Sil'chenko: NGC 759: A giant elliptical with a just­forming decoupled nucleus 29
Table 1. Global parameters of NGC 759
Hubble type E0
R25 15.2 kpc
B 0
T 13.33
MB --20.7
(B - V ) 0
T 1.00
(U -B) 0
T 0.52
Vr (CO) 4665 km · s -1 (Wiklind et al. 1997)
Distance 66 Mpc (H0=75 km · s -1 · Mpc -1 )
Inclination 3.3 o
ometric observations in the CO (1­0) emission line. Though
the spatial resolution of these observations, 3. ## 1 â 2. ## 3, is not
quite sufficient to insist on details, a large mass of molecular gas,
2.4·10 9 M# , has certainly been found within a radius of 3 ## . The
most probable shape of the molecular gas distribution is a flat
ring; it demonstrates fast rotaton, v proj
rot # 200 km/s, consistent
with the visible mass concentration to the center. The conclu­
sion has been made that NGC 759 may represent a late stage of
a merger between two gas­rich disk galaxies. If this is so, there
must also be other consequences of such a catastrophic event:
present star formation in the gaseous circumnuclear ring and
perhaps intermediate­age stellar population and some structure
peculiarities in the outer regions of the galaxy. This has stim­
ulated us to undertake a spectral and photometric investigation
of NGC 759 in the optical range. The results are presented in
this paper.
2. Observations and data reduction
The spectral observations of NGC 759 were undertaken in
January 1998, at the 6­meter telescope of the Special Astro­
physical Observatory of the Russian Academy of Sciences.
We have used the Multi­Pupil Field Spectrograph (MPFS,
Afanasiev et al. 1990) in the prime focus and the long­slit spec­
trophotometric complex in the Nasmyth focus. The detailed pa­
rameters of the spectral observations are given in Table 2.
The Multi­Pupil Field Spectrograph allows us to obtain a set
of about one hundred spectra from a rectangular area of an ex­
tended astronomical object (e.g. of a galaxy). We have obtained
two such sets for the central region of NGC 759: one in the green
spectral range, 4800--5400 š
A, which we have used to derive a
two­dimensional velocity field of stars by cross­correlating in­
dividual spectra with spectra of bright K giant stars, and another
in the red, 6200--6900 š
A, -- it contains emission lines H# and
[N ii]#6583 which have been used to calculate a two­dimen­
sional velocity field of the ionized gas. These observations have
been performed with the reciprocal dispersion of 1.6 š
A per pixel
(the spectral resolution of 4--6 š
A slightly varying over the frame).
To calibrate the wavelength scale, we exposed separately a spec­
trum of the hollow cathod lamp filled with helium, neon, and
argon, but the accuracy of the calibration was also checked by
measuring positions of night­sky emission lines. We found no
systematic velocity shifts. The typical random error of an indi­
vidual measurement of a weak emission line is 40 km/s.
To refine a kinematical analysis, we have also obtained a
long­slit spectrum with the Nasmyth­focus spectrograph of the
6m telescope in the red spectral range under a slightly higher
spectral resolution, of about 3 š
A. Unfortunately, in this spectro­
graph there is no rotation­field mechanism; but the short expo­
sure time and a large distance from the culmination point during
the observations have allowed us to fix the position angle of the
slit at the value of 90 # ± 2 # . The 3 š
A--resolution spectral ob­
servation has revealed a complex structure of the profiles of the
emission lines H#, [N ii]#6583, and [S ii]#6717,6731 which
can be traced up to 5 ## from the center.
The photometric observations of NGC 759 were performed
with the small telescopes of the Special Astrophysical Obser­
vatory -- at the 1­meter and 0.6­meter Zeiss reflectors (Z1000
and Z600, respectively). The 1040 â 1160 CCD detector was
used. The observations at the 1­meter telescope were made with
binning 2 â 2, through BV RI filters and in the photometric
night, but without guiding -- so only short exposures were pos­
sible. As the photometric standards, we exposed a field of the
star cluster NGC 7790 which has been previously studied in
V RI by Christian et al. (1985) and in BV R by Odewahn et
al. (1992); it was exposed with the same exposure times and
at the same zenith distance as NGC 759. Though the short ex­
posure times have prevented surface photometry deep enough
and only the measurements of the galaxy within the radius of
# 20 ## were relevant, this observation has allowed us to de­
termine V RI magnitudes for a dozen stars around NGC 759
which were used later to calibrate our photometry with the 0.6­
meter telescope into the standard Johnson­Cousins photometric
system. The observations at the 0.6­meter telescope have been
made without binning in V RI filters, with guiding -- so the ex­
posures were long enough. Unfortunately, the guiding of the
telescope is imperfect, so despite guiding, the stellar images
are slightly elongated, with artificial ellipticity of # 0.2. No
photometric standards were observed with the 0.6­meter tele­
scope. These observations appear to be deep enough: we trace
surface brightness profiles almost up to the optical border of the
galaxy, R # 50 ## . The detailed parameters of our photometric
observations are given in the Table 3.
Besides our own data, we include also photometric data from
the La Palma Archive: NGC 759 has been observed through the
RI filters at the Jacobus Kapteyn Telescope (JKT) with the EEV
1280 â 1180 CCD on different dates. The standards of Landolt
(1992) were used to calibrate the measurements of the galaxy
and its surrounding stars into the Cousins system.
All the data, both spectroscopic and photometric, have been
reduced by using the software developed in the Special Astro­
physical Observatory (Vlasyuk 1993).
3. Photometric structure of NGC 759
NGC 759 is thought to be a ``bona­fide'' elliptical galaxy (see
e.g. RC3), however the detailed CCD photometric study of this
galaxy has never been published. In this section we analyse
mostly the data obtained at the 0.6­meter telescope, because
they are the deepest ones.

30 V.V. Vlasyuk & O.K. Sil'chenko: NGC 759: A giant elliptical with a just­forming decoupled nucleus
Table 2. Spectral observations of NGC 759
Date Configuration Exposure Field Scale Spectral range PA of long side Seeing
21/22.01.98 MPFS+CCD 520 â 580 90 min 10. ## 4 â 15. ## 6 1. ## 30 per lens 4700--5450 š
A 96 # 2. ## 4
22/23.01.98 LS+CCD 1024 â 1024 30 min 2 ## â 34 ## 0. ## 4 per pixel 6000--7350 š
A 90 # 2 ##
23/24.01.98 MPFS+CCD 1040 â 1160 120 min 11 ## â 22 ## 1. ## 36 per lens 6300--7300 š
A 285 # 2. ## 2
Fig. 1. Radial variations of the isophote morphological characteristics
in NGC 759 according to our data and to those from the JKT La Palma
First of all we have undertaken a morphological study of
the isophotes. Fig. 1 shows radial variations of the major­axis
position angle and of the ellipticity. The measurements of the
0.6­meter telescope images look to be the most accurate ones
and give the most extended dependencies, but in the central
part they are distorted by guiding errors; the La Palma R­band
data of 1994 are not so deep, but they were obtained under
the best seeing, of 1. ## 15, and with excellent guiding so they
are considered as a reference set for the central 5 ## . One can
see that all the measurements agree rather well in the radius
range of 4 ## --30 ## . Though the galaxy is classified as E0, it is not
perfectly round: the minimum ellipticity measured is 0.05, and
the average one is 0.07--0.08. It is enough to measure reliably the
orientation of the isophote major axis: in the full radius range
the major­axis position angle looks approximately constant and
is confined to the interval PA 0 = 20 # ± 10 # .
We have constructed azimuthally averaged surface bright­
ness profiles in the V ­ and I­bands by taking data along ellip­
tical rings with the axis ratio 0.93 and orientation PA 0 = 17 #
corresponding to the results of our morphological analysis (the
I­band profile is shown in Fig. 2). We have applied the program
of de Vaucouleurs­law fitting to these profiles to analyse the
Fig. 2. Azimuthally averaged profile of the surface brightness in the
I­band according to our Z600 data fitted by a single de Vaucouleurs's
law. The accuracy of observational points is comparable to the size of
signs
structure of the brightness distribution. The best­fitted I­band
model profile obtained is also plotted in Fig. 2. The impression
is that these profiles cannot be perfectly fitted only by a de Vau­
couleurs' law in the full range of radii: the r. m. s. scatters of
points around the model profiles are 0.05 mag in the V ­band and
0.07 mag in the I­band while the accuracy of azimuthally aver­
aged points is better everywhere than 0.035 mag; however the
profiles may be characterized as spheroid­dominated. For the
V ­band image we have derived the de Vaucouleurs' parameters
µ e = 22.1±0.1 V ­mag/## ## and r e = 19 ## ±1 ## (6.1 kpc); for the
I­band image the fitting of the de Vaucouleurs' spheroid implies
a scalelength r e = 18. ## 4 ± 0. ## 5 (5.9 kpc) and the corresponding
effective surface brightness µ e = 20.65 ± 0.05 I­mag/## ## . The
surface photometry of NGC 759 in the V ­band was earlier made
by Schombert (1986) through the photographic method; he fit­
ted the brightness profile at R > 3 kpc by a de Vaucouleurs'
law and had found µ e = 21.6 V ­mag/## ## and r e = 6.9 kpc --
not too different from our results.
After obtaining the best de Vaucouleurs' model for the
NGC 759 surface brightness distribution, we have subtracted
the model brightness maps from the observed images and have
obtained smooth and round brightness residual structures with
the radius of # 16 ## (Fig. 3); the mean contribution of this resid­
ual surface brightness to the total one is about 10% in both filters.
Well inside this round brightness excess something more elon­
gated, with the PA 0 # 15 # , the major semiaxis of # 8 ## , and
b/a # 0.7, can be detected. The azimuthally averaged I­band
profile of the residual surface brightness is presented in Fig. 4,
in the top part. In the radius range of 3 ## --16 ## one can see two ex­
ponential segments meeting at R # 8 ## . Obviously, the residual

V.V. Vlasyuk & O.K. Sil'chenko: NGC 759: A giant elliptical with a just­forming decoupled nucleus 31
Table 3. Photometric observations of NGC 759
Date Telescope Filter Exposure Seeing Scale
7/8.09.94 JKT R 300 s 1. ## 15 0. ## 31
5/6.11.96 JKT I 360 s 2. ## 2 0. ## 31
9/10.08.98 Z1000 B 180 s 2. ## 7 0. ## 51
9/10.08.98 Z1000 V 120 s 2. ## 5 0. ## 51
9/10.08.98 Z1000 R 60 s 2. ## 6 0. ## 51
9/10.08.98 Z1000 I 120 s 2. ## 2 0. ## 51
19/20.10.98 Z600 V 1800 s 1. ## 55 0. ## 45
19/20.10.98 Z600 R 1800 s 1. ## 5 0. ## 45
19/20.10.98 Z600 I 1800 s 1. ## 6 0. ## 45
Fig. 3. The residual brightness map in the I­band after subtraction of
the best­fit de Vaucouleurs' model. The full dimensions of the field
shown is 115 ## â 115 ## .
structure represents two stellar disks whose appearance in the
V ­filter inside R = 5 ## is strongly affected by dust. The param­
eters of fitting by two exponential laws applied to the I­band
residual brightness profile are:
-- in the radius range of 8 ## --14 ## µ 0 = 20.8 I­mag/## ## and
r 0 = 5. ## 0 (this disk is seen under i = 21.5 # to provide the
ellipticity of 0.07),
-- and in the radius range of 3 ## --8 ## µ 0 = 18.8 I­mag/## ## and
r 0 = 1. ## 9 (this disk is seen under i = 40 # to provide the
ellipticity of 0.3).
If we assume the distance to NGC 759 to be 66 Mpc (Wiklind
et al. 1997), we obtain the stellar disks with the radii of 2.6
and 5 kpc and the exponential scalelengths of 0.6 and 1.6 kpc,
respectively. The final fitting result in the frame of the composite
I­band model is plotted in the bottom part of Fig. 4.
Fig. 5 presents azimuthally averaged profiles of the (V -R)
and (V - I) colour distributions. Besides the smooth colour
gradients in the main body of the galaxy, one can see a prominent
feature -- a sharp bump at R = 3. ## 5. The colour image detail
Fig. 4. The azimuthally averaged profile of the surface brightness resid­
uals after subtracting the pure de Vaucouleurs' spheroid which we have
fitted by two exponential laws (top) and the azimuthally averaged pro­
file of the total surface brightness in the I­band according to our Z600
data fitted by a composite model bulge+two disks (bottom).
Fig. 5. Azimuthally averaged colour profiles
corresponding to this peak (Fig. 6) looks like a red semi­ellipse
with the major axis aligned in PA # 20 # . The asymmetry of
the feature obviously related to the line of nodes implies that
the dust ring is inclined with respect to the symmetry plane of
the galaxy. The ellipticity of this structure, 1 - b/a # 0.2, is
consistent with the flat ring inclination of 40 # ; curiously, just
the same inclination of the circumnuclear molecular gas ring
has been suggested by Wiklind et al. (1997) from dynamical
arguments, and the inner stellar disk is seen under a similar
inclination. We think that the red semi­ellipse on the (V - I)

32 V.V. Vlasyuk & O.K. Sil'chenko: NGC 759: A giant elliptical with a just­forming decoupled nucleus
Fig. 6. The gray­scaled (V - I) map of the central part of NGC 759:
dark regions correspond to the redder ones. North is up, east is to the
left, the total map dimensions are 57. ## 6 â 57. ## 6. The artificially white
square marks the nucleus of the galaxy.
map results from a flat ring­like dust concentration related to
the molecular gas ring with the inner radius of 1 ## and the outer
radius of 3 ## reported by Wiklind et al. (1997). This suggestion
is also confirmed by the central (R < 5 ## ) depression of the V
surface brightness profile of the disk component.
4. Kinematics of stars and ionized gas
in the center of NGC 759
Fig. 7 presents isovelocity maps for the stars and ionized gas
in the center of NGC 759 which have been obtained with the
Multi­Pupil Field Spectrograph.
First of all, one must note that the stellar rotation in the very
center of NGC 759 is much faster than one could expect for
the E0­type galaxy. It may be that the stellar core of NGC 759
is dynamically decoupled. The direction of the maximum cen­
tral line­of­sight velocity gradient (``the dynamical major axis'')
is PA 0 = 22 # ; it coincides with the photometric major axis
(Fig. 1), and this coincidence signifies that we see clear evidence
of regular axisymmetric rotation. The low limit of the rotation
angular velocity defined by projection effects is 66 km/s/kpc.
Such a high central velocity gradient is typical rather for an
early­type disk galaxy.
The rotation of the ionized gas in the center of NGC 759
is even more striking. Its projected angular rotation velocity
reaches 235 km/s/kpc; the whole velocity field (Fig. 7b) looks
like a solid­body rotating disk with a radius of # 3 ## while
v rot sin i is 220 km/s at the edge of this disk. This estimate
agrees very well with the projected rotation speed of the molec­
ular gas torus having the same radius of # 3 ## which has been
reported by Wiklind et al. (1997). However as we have some­
what higher spatial resolution than Wiklind et al. (1997) had,
we estimate the orientation of the dynamical major axis of gas
rotation more precisely: our value PA 0 = 18 # agrees with the
stellar dynamical and photometric major axis orientations.
Additional evidence for the fast rotation of the circumnu­
clear ionized gas is given by our long­slit spectrum in PA =
90 # . This cross­section is taken by 70 # to the dynamical major
axis. The emission lines in the vicinity of the nucleus consist
of two components separated by some 300 km/s. The appear­
ance of two discrete peaks of emission­line profiles argues an
existence of the hole in the gaseous disk emission surface bright­
ness in the very center of the galaxy -- here a clear analogy to the
double­horned CO­line profile obtained during the single­dish
CO observations of NGC 759 (Wiklind et al. 1997) is seen.
5. Star formation in the circumnuclear gaseous disk
When Wiklind et al. (1997) obtained a ring­like distribution
of the molecular gas in the center of NGC 759, they had cal­
culated a radial dependence of the gravitational stability pa­
rameter, # gas /# crit , demonstrating a maximum, close to 1, at
R # 2 ## , and had concluded that there may be a ring of star­
forming H ii regions with the same radius in this galaxy. The
significant present star formation in NGC 759 had also been
implied by its prominent FIR luminosity: it has L fir /LB = 0.3
(Wiklind et al. 1995) which is higher by an order of magnitude
than a typical L fir /LB ratio for elliptical galaxies and exceeds
by a factor of two the maximum ratios found by Goudfrooij
& de Jong (1995) for their complete RSA sample of nearby
elliptical galaxies. Now we can confirm the supposition of Wik­
lind et al. (1997) by a direct two­dimensional observation of the
emission­line ratio distribution.
Fig. 8 gives a comparison of the spectra in the vicinity of H#
between the nucleus and the ring with the radius of 2. ## 7 and the
width of 1. ## 4. One can see that while in the nucleus the intensi­
ties of the emission lines H# and [N ii]#6583 are roughly equal
which is a signature of substantial contribution from the LINER­
like nucleus, in the three­arcsecond ring the H# is twice as strong
as [N ii] -- it is typical for a star­forming site where massive stars
ionize the gas (Veilleux & Osterbrock 1987). We can estimate
roughly the star formation rate by using the integrated H# flux.
First of all, we have summed the individual element spectra
from our MPFS frame over the area of 7 ## â 8 ## centered onto
the nucleus which contains noticeable H# emission. We then
calculated an equivalent width of the H# emission line in the in­
tegrated spectrum, showing that it is equal to 4.1 š
A. As we must
also take into account the presence of H# absorption we add
some 2 š
A to this value (because it is a typical H# absorption­line
equivalent width for old stellar populations; see for example our
model calculations, Balinskaya & Sil'chenko, 1993) and obtain
6 š
A. Next we calculate a V magnitude in the same aperture by
using our photometric data, showing it to be 14.7 mag. Through
transforming this estimate into energetic units by a standard way
(Allen 1973), we obtain LH# (7 ## â8 ## ) = 1.65 · 10 40 erg/s. This
H# luminosity corresponds to the total star formation rate of
0.13 M
# per year if we use the calibration of Kennicutt et al.
(1994). The uncertainty of this estimate may be a factor of 2--3;
however it is quite inconsistent with the estimate of 7 M
# per

V.V. Vlasyuk & O.K. Sil'chenko: NGC 759: A giant elliptical with a just­forming decoupled nucleus 33
Fig. 7. Two­dimensional line­
of­sight velocity fields for the
stars (a) and for the ionized gas
(b) in the center of NGC 759
Fig. 8. Emission­line spectra of the nucleus and of the 3 ## --ring of
NGC 759
year made by Wiklind et al. (1997) from the IRAS luminosity
of NGC 759. The discrepancy of these two estimates proba­
bly results from the ambiguous nature of IRAS flux in ellipti­
cal galaxies: e.g. dust mass in elliptical galaxies estimated from
L fir is larger by an order of magnitude than dust mass estimated
from optical observations (Goudfrooij et al. 1994). There were
suggestions that dust which produces FIR radiation in ellipti­
cal galaxies may be mostly the dust in the envelopes of highly
evolved stars (red giants), and not the dust heated by young
stars. As NGC 759 only has upper limits for S 12 and S 25 , this
may be the case.
6. Discussion and conclusions
By undertaking a spectral and photometric investigation of the
central part of the bright elliptical galaxy NGC 759, we have
found an optical counterpart for the circumnuclear molecular
gas ring with R # 3 ## (1 kpc) which was reported by Wiklind et
al. (1997). This counterpart consists of the ionized gas excited by
young massive stars as well as of dust. The gaseous ring exhibits
fast rotation: its projected rotation velocity at R # 3 ## measured
by us is equal to about 220 km/s and agrees perfectly with the re­
sult of Wiklind et al. (1997) for the molecular gas ring. This large
projected rotation velocity value together with the asymmetric
appearance of the dust ring on the (V - I) colour map con­
firm the hypothesis of Wiklind et al. (1997) about a rather high
inclination of the gas rotation plane: our data favours i # 40 #
proposed by them for the molecular gas ring. Meantime the 2D
decomposition of the galaxy images both in the V and I bands
has revealed an existence of the brightness excess with respect
to the de Vaucouleurs' spheroid: this extracomponent is seen in
the radius range of 3 ## --16 ## and consists of two exponential disks
with different inclinations. The more outer disk is seen almost
face­on and has structural parameters typical for ordinary disk
galaxies: µ 0 = 20.8 I­mag/## ## and r 0 = 1.6 kpc. The innermost
stellar disk, if it is intrinsically round, is inclined by # 40 # , just
as the circumnuclear gaseous ring; this morphological estimate
of the disk inclination is consistent with its visible fast rotation
untypical for E0 galaxies. The visible orientation of its line of
nodes, PA 0 # 20 # , agrees also with the dynamical major axis
derived by us from the two­dimensional stellar velocity field of
the central part of NGC 759. The structural parameters of the
innermost disk, µ 0 = 18.8 I­mag/## ## and r 0 = 0.6 kpc, are
more appropriate to compact inner disks of luminous elliptical
galaxies (Iodice et al. 1999).
Having structural parameters of NGC 759, it would be in­
teresting to look what place it occupies among other elliptical
galaxies, particularly, on the ``Fundamental Plane''. We have
plotted NGC 759, with its R e # 6 kpc and µ e # 23.1 B­
mag/## ## , in Fig. 4 from the paper of Capaccioli et al. (1992)
where the Kormendy' relation for a lot of early­type galaxies is

34 V.V. Vlasyuk & O.K. Sil'chenko: NGC 759: A giant elliptical with a just­forming decoupled nucleus
presented and have assured that it follows the sequence for the
family of bright ellipticals, in accordance to its MB = -20.7.
So the galaxy is well relaxed even if it has experienced a recent
merging.
But we know now that NGC 759 possesses also stellar disks,
in addition to the de Vaucouleurs' spheroid. Iodice et al. (1999)
have reported recently results of 2D decomposition for the im­
ages of early­type galaxies in the nearby clusters Virgo and For­
nax. They have found that # 25% of elliptical galaxies demon­
strate inner exponential stellar disks; tight correlations between
their µ 0 and h, and also between R e and h are detected. The
innermost disk in NGC 759 with its µ 0 # 21 B­mag/## ## and
h = 0.6 kpc, follows the µ 0 -h relation for the disks within el­
lipticals which is presented by Iodice et al. (1999) in their Fig. 3.
Moreover, its scalelength satisfies the R e - h dependence for
the family of inner disks inside bright ellipticals (their Fig. 4)
though it is still one of the most extended among a dozen such
disks (perhaps it is because NGC 759 is four times farther from
us than Virgo and Fornax clusters and our estimate of h in , 1. ## 9,
is close to our spatial resolution limit). The latter dependence
is treated by Iodice et al. (1999) as evidence of coupling during
disk and spheroid formation; however the high inclination of
the innermost stellar disk, consistent with the inclination of the
circumnuclear gaseous ring, allows us to suggest that the inner­
most stellar disk is related to a recent gas accretion event and star
formation in the 3 ## ­ring around the nucleus. Besides the inner­
most inclined disk we also see a more extended face­on disk. Its
origin probably is not related either to a recent merger, nor to the
circumnuclear gaseous ring. The lack of relation is proved by
different orientations of the 1­kpc gaseous ring and of the 5­kpc
stellar disk: the former is inclined by i # 40 # and the latter is
seen face­on, hence their rotation momenta must be decoupled.
Taking into account all of the above, we would like to conclude
that the circumnuclear gaseous ring in NGC 759 is not a relic
of a merger of two large spiral galaxies; such a merger does not
seem to provide a survival of the extended face­on stellar disk.
More likely, this ring may be a consequence of a tidal encounter
between the elliptical galaxy NGC 759 and a large spiral galaxy
which was accompanied by a substantial gas accretion.
Acknowledgements. We thank the post­graduate student of the
Special Astrophysical Observatory A. V. Moiseev for supporting
the observations at the 6m telescope. The 6m telescope is operated
under the financial support of Science Ministry of Russia (regis­
tration number 01­43). During the data analysis we have used the Lyon­
Meudon Extragalactic Database (LEDA) supplied by the LEDA team
at the CRAL­Observatoire de Lyon (France) and the NASA/IPAC Ex­
tragalactic Database (NED) which is operated by the Jet Propulsion
Laboratory, California Institute of Technology, 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
Astrofisica de Canarias. The work was supported 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 Scientific­Technical
Program ``Astronomy. Basic Space Researches'' (the section ``Astron­
omy'').
References
Afanasiev V.L., Vlasyuk V.V., Dodonov S.N., Sil'chenko O.K., 1990,
Preprint SAO N54, Special Astrophys. Obs., Nizhnij Arkhyz
Allen C.W., 1973, Astrophysical Quantities. 3rd Edition, The Athlone
Press, London
Balinskaya I.S., Sil'chenko O.K., 1993, Astrofiz. issled. (Izv. SAO
RAS) 35, 43
Bender R., 1988, A&A 202, L5
Bender R., Surma P., 1992, A&A 258, 250
Capaccioli M., Caon N., D'Onofrio M., 1992, MNRAS 259, 323
Carollo C.M., Danziger I.J., 1994, MNRAS 270, 523
Christian C.A., Adams M., Barnes J.V., et al., 1985, PASP 97, 363
de Jong R.S., Davies R.L., 1997, MNRAS 285, L1
Goudfrooij P., Hansen L., Jorgensen H., Norgaard­Nielsen H., 1994,
A&AS 105, 341
Goudfrooij P., de Jong T., 1995, A&A 298, 784
Iodice E., D'Onofrio M., Capaccioli M., 1999, in press (astro­
ph/9904029)
Jedrzejewski R., Schechter P.L., 1988, ApJ 330, L87
Kennicutt R.C. Jr., Tamblyn P., Congdon C.E., 1994, ApJ 435, 22
Landolt A.U., 1992, AJ 104, 340
Mehlert D., Saglia R.P., Bender R., Wegner G., 1998, A&A 332, 33
Odewahn S.C., Bryja C., Humphreys R.M., 1992, PASP 104, 553
Roberts M.S., Hogg D.E., Bregman J.N., et al., 1991, ApJS 75, 751
Schombert J.M., 1986, ApJS 60, 603
Sil'chenko O.K., 1994, AZh 71, 706
Sil'chenko O.K., 1997, AZh 74, 643
Surma P., Bender R., 1995, A&A 298, 405
Veilleux S., Osterbrock D.E., 1987, ApJS 63, 295
Vlasyuk V.V., 1993, Astrofiz. issled. (Izv. SAO RAS) 36, 107
Wiklind T., Combes F., Henkel C., 1995, A&A 297, 643
Wiklind T., Combes F., Henkel C., Wyrowski F., 1997, A&A 323, 727