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The Sc galaxy NGC 6181 was observed at the 6m telescope of SAO RAS with the scanning Perot-Fabry interferometer in the H-alpha emission line and at the 1m telescope of SAO RAS in BVRI broadband filters with CCD. Subtraction of the mean circular rotation curve from the two-dimensional velocity field has revealed a ring-like zone with a diameter about of 2 kpc where strong radial gas motions are present. The form of the ring is almost perfectly circular in the plane of the galaxy. It is located closer to the center than the beginning of the well-defined spiral structure, but outside of the central bulge-dominated region. The detected radial velocity reduced to the plane of the galaxy is about 100 km/s and probably is azimuthally dependent. The very inner region of the galaxy, with radius less than 3 arcsec or 0.5 kpc, shows a turn of the dynamical major axis by about 30 degrees. Central continuum isophotes are also twisted which suggests the presence of a small nuclear bar.
The Sb galaxy NGC 2841 was observed at the 6 m telescope of SAO RAS with the Multi-Pupil Field Spectrograph and at the 1 m telescope of SAO RAS with the long-slit spectrograph. An unresolved nucleus of NGC 2841 is shown to be chemically decoupled both in magnesium and in iron with abundance break estimates of 0.36 dex for Mg and 0.6 dex for Fe; an abundance gradient in the bulge is seen only in the magnesium index and is typical for early-type disk galaxies. The rotation axis of the nuclear ionized gas in NGC 2841 is orthogonal to that of the central stellar population; an existence of a bulge stellar component with decoupled rotation momentum in the radius range 5" - 12" is suspected. A possible scenario for the origin of the unusual central structure in NGC 2841 is proposed.
Results of two-dimensional spectroscopy for central parts of polar-ring galaxies NGC 2685 and IC 1689 are presented. An orthogonality of star and gas rotations is detected. A strong break of absorption-line index Mgb between the nuclei and the inner bulges ("chemically decoupled nuclei") and perhaps an intermediate age of the stellar population in the nucleus of IC 1689 imply secondary star formation bursts in the centers of the polar-ring galaxies which are possibly related to accretion events having produced polar rings themselves.
The results of a spectroscopic and photometric investigation of the central region of M~31 are presented. An analysis of absorption-index radial profiles involving magnesium, calcium, and iron lines has shown that the unresolved nucleus of M~31 is distinct by its increased metallicity; unexpectedly, among two nuclei of M~31, it is the faintest one located exactly in the dynamical center of the galaxy (and dynamically decoupled) which is chemically distinct. The Balmer absorption line $H\beta$ has been included into the analysis to disentangle metallicity and age effects; an age difference by a factor 3 is detected between stellar populations of the nucleus and of the bulge, the nucleus being younger. The morphological analysis of CCD images has revealed the presence of a nuclear stellar-gaseous disk with a radius of some 100 pc, the gas component of which looks non-stationary, well inside the bulge of M~31.
By using bidimensional spectral data obtained at the 6m telescope
for the Virgo spirals NGC 4216 and NGC 4501, we have found chemically
distinct metal-rich nuclei in these galaxies. Under the assumption
of equal ages for the nuclear and bulge stellar populations, the
metallicity difference between the nuclei and their environments in
the galaxies is estimated as a factor of 2. But we have also found
an age difference between the nucleus and the bulge in NGC 4216:
age-metallicity disentangling on the diagrams (H-beta, Mgb),
(H-beta, [MgFe]), and (H-beta, $
Investigation of gaseous and stellar kinematics and of broad-band VRI and narrow-band H-alpha and [NII]6583 images is performed for the central part (R < 4 kpc) of the regular spiral galaxy NGC 2841. We have found emission-line splitting at R < 20" and three-component LOSVD for the stars in the radius range 6" - 100". Morphological analysis reveals strong narrow shock fronts close to the major axis in the radius range of 30" - 50", a turn of the isophote major axis by 5 degrees and strongly negative Fourier coefficient a4 (boxy isophotes) in the radius range of 15" - 33". In principle, all these features may be explained in the frame of a triaxial bulge hypothesis.
As a result of bidimensional spectroscopy of the central parts of two nearby lenticular galaxies, NGC 1023 and NGC 7332, undertaken with the Multi-Pupil Field Spectrograph of the 6m telescope of the Special Astrophysical Observatory, their chemically decoupled stellar nuclei are found to be substantially younger than the surrounding bulges: the mean age of the nuclear stellar populations is 7 billion years in NGC 1023 and 2.5 +/- 0.5 billion years in NGC 7332. The morphological analysis undertaken by Seifert and Scorza (1996) for NGC 7332 and by us for NGC 1023 has revealed an existence of separate circumnuclear stellar disks with the radius of 80 pc in NGC 1023 and of 400 pc in NGC 7332; probably, the intermediate-age stellar populations are related to these structures.
We present the results of the spectral investigation of the regular Sb galaxy NGC 7331 with the Multi-Pupil Field Spectrograph of the 6m telescope. The absorption-line indices H-beta, Mgb, and Fe are mapped to analyse the properties of the stellar populations in the circumnuclear region of the galaxy. The central part of the disk inside ~3" (200 pc) -- or a separate circumnuclear stellar-gaseous disk as it is distinguished by decoupled fast rotation of the ionized gas -- is very metal-rich, rather young, ~ 2 billion years old, and its solar magnesium-to-iron ratio evidences for a very long duration of the last episode of star formation there. However the gas excitation mechanism now in this disk is shock-like. The star-like nucleus had probably experienced a secondary star formation burst too: its age is 5 billion years, much younger than the age of the circumnuclear bulge. But [Mg/Fe]=+0.3 and only solar global metallicity imply that the nuclear star formation burst has been much shorter than that in the circumnuclear disk. The surrounding bulge is rather old, 9-14 billion years old, and moderately metal-poor. The rotation of the stars and gas within the circumnuclear disk is axisymmetric though its rotation plane may be slightly inclined to the global plane of the galaxy. Outside the circumnuclear disk the gas may experience non-circular motions, and we argue that the low-contrast extended bulge of NGC 7331 may be triaxial.
We have undertaken bidimensional spectroscopy of the central part of the nearby lenticular galaxy NGC 7280 with the Multi-Pupil Fiber Spectrograph of the 6m telescope of the Special Astrophysical Observatory. A rather young stellar nucleus, with a mean population age of 1.5 +/- 0.5 Gyr, is found which is more metal-rich than the bulge at R~ 1 kpc by an order of magnitude. The chemically and age-decoupled nucleus seems to be spatially resolved: the circumnuclear absorption index isolines represent ellipses elongated in PA~ 100 degr -- 110 degr. The same orientation, PA=103 degr, is found for the elongated circumnuclear stellar structure, revealed from the morphological analysis of the HST WFPC2 and NICMOS images of NGC 7280 and seen best of all at R=1". Taking into account the stellar kinematics inside R< 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 2" - 8" imply an existence of the intermediate-scale bar elongated in PA~ 60 degr. The circumnuclear ionized gas is distributed and rotates in the plane orthogonal to the plane of the circumnuclear stellar disk.
By undertaking a spectral and photometric investigation of the central part of the bright elliptical galaxy NGC759, 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 counterpart consists of the ionized gas excited by young massive stars and of the dust. The gaseous ring exhibits fast rotation: its projected 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 confirm the hypothesis of Wiklind et al. (1997) about a rather high inclination of the gas rotation plane: our data favours i= 40 degr 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 well-fitted by two exponential laws with different characteristic scales. We argue that two stellar disks are embedded into the bright elliptical galaxy: the outer one is seen nearly face-on and so its origin is probably related to that of the main galactic body, the inner one is inclined by i~ 40 degr 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.
The central regions of the early-type disk galaxies NGC 524 and NGC 6340 have been investigated with the Multi-Pupil Field Spectrograph at the 6m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We confirm the existence of chemically distinct stellar nuclei in these galaxies which have been claimed earlier. By myself, the metallicity differences which are found between the nuclei and the bulges, 0.5--1.0 kpc from the centers, reach 0.5--0.6 dex. Both nuclei are magnesium overabundant, but the bulges have different magnesium-to-iron ratios: it is solar in NGC 6340 and the same as the nuclear one in NGC 524. The 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 R=3 kpc, and it is inclined by some 20 degrees to the global galactic plane. In NGC 6340 only a gaseous polar disk is detected which extension does not exceed R~ 500 pc.
The regular isolated Sab galaxy NGC 7217 has been studied
with the Multi-Pupil Fiber Spectrograph of the 6m telescope
of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz,
Russia) in two spectral ranges, the blue one including
the strong absorption lines MgI and FeI and the red one including
the emission lines H-alpha and [NII]6583.
We confirm the existence of a circumnuclear gaseous polar
disk with a radius of 3" which we reported earlier.
The same area, with a radius of 3" -4", elongated
orthogonally to the line of nodes, is distinguished by high
values of the Lick index
We present the results of a comprehensive spectrophotometric study of the central region in the regular lenticular galaxy NGC 4036 with two spectrographs of the 6-m telescope. The unresolved nucleus of NGC 4036 is shown to be chemically decoupled: [Mg/Fe] =+0.3 at the very center, whereas in the immediate vicinity of its nucleus, this ratio abruptly drops to +0.1 and does not change further along the radius. A study of isophotal morphology in combination with a kinematic analysis has proven that the rotation of stars at the NGC 4036 center is axisymmetric. However, the major-axis turn within R < 5" should be considered real. We interpret this turn as evidence for the existence of a tilted circumnucelar stellar disk with a radius of 250 kpc in NGC 4036. The NGC 4036 bulge may be triaxial, and the ionized gas at the galactic center is then concentrated toward the principal plane of the ellipsoidal potential.
The results of the complex photometric and spectral investigation of the regular Sb galaxy NGC 615 are presented. The observations are made with the 6m, 1m, and 0.6m telescopes of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) and 1.5m ESO telescope (La Silla, Chile). The analysis of the radial brightness profiles reveals an existence of at least two decoupled exponential components; the parameters of the outer one are typical for global disks of Sb galaxies, and the inner component seen in the radius range 9" - 30" (1.0--3.5 kpc) is distinguished by compactness and high surface brightness. A combination of 2D velocity fields for stars and ionized gas in the central part of the galaxy and of the long-slit velocity profiles along the major axis up to the optical border of NGC 615 provides some interesting findings. In the very center, R<3" (0.3 kpc), stars and gas rotates together, and the dynamical and photometric major axes are turned with respect to the global line of nodes; we conclude that an inclined circumnuclear disk is detected. In the bulge, R=3" - 6" (0.35-0.75 kpc), the stellar velocity dispersion is more than 100 km/s, the stars rotate twice slower than the ionized gas, and a counterrotating gaseous component is observed. In the inner exponential component the stars rotate together with the ionized gas, and the stellar velocity dispersion drops to about 50 km/s. As the photometric major axis of this component is turned with respect to the line of nodes, we would called it `an oval inner disk'. The mean age of the stellar populations in the nucleus and in the inner disk beyond its HII regions is 5 Gyr, while the bulge is much older. Both the chemically decoupled nucleus of NGC 615 and the inner disk probably formed a bulk of their more luminous stars in a secondary star formation burst provoked perhaps by a close passage of another galaxy some billion years ago.
A chemically distinct compact stellar nucleus with [Fe/H]=+0.6, [Mg/Fe]=+0.1 - +0.2, and a mean age of the stellar population of 5 Gyr is found in the nearby luminous Sb galaxy with a large-scale bar NGC 4548 as a result of the investigation of its central part with the Multi-Pupil Spectrograph of the 6m telescope. This nucleus representing probably a circumnuclear disk co-planar to the main disk of the galaxy is embedded into a bulge possessing stellar population though more metal-poor by a factor of 2.5 than that of the nucleus, but also rather young, about 4 Gyr old. The bulge of NGC 4548 is triaxial and has a de Vaucouleurs' surface brightness profile; the unusual characteristics of its stellar population force us to suggest a formation or transformation of the bulge by a secular evolution provoked by a triaxial potential effect of the large-scale bar. The ionized gas within 3" from the nucleus of NGC 4548 rotates in a plane inclined to the main symmetry plane of the galaxy, perhaps even in the polar plane; this may be also a consequence of the large-scale bar influence.
The central regions of the regular lenticular galaxies NGC 4429 and NGC 7013 have been studied with the Multi-Pupil Field (Fiber) Spectrograph (MPFS) of the 6m telescope of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) in two spectral ranges, the blue one including the strong absorption lines MgI and FeI and the red one including the emission lines H-alpha and [NII]6583. We confirm the presence of the chemically distinct nucleus in NGC 7013 reported earlier. Besides this unresolved (in the magnesium index) structure, we have found a ring of younger stellar populations than those in the nucleus and in the bulge; the radius of this ring is about 6" (400 pc). A similar ring, distinguished by high magnesium- and iron-index values and bordered by H-alpha emission at its inner edge, with a radius of 6" (500 pc), is found in NGC 4429. We try to relate the ringed structure of the chemically decoupled cores in these galaxies with past, now dissolved, large-scale bars whose remnants are now seen in NGC 4429 and NGC 7013 as lenses between the bulges and global disks. An analysis of the gas and star kinematics in the centers of the galaxies has revealed a presence of an inclined circumnuclear disk in NGC 7013 and the existence of minibar in NGC 4429.
Kinematics and stellar population properties in the center of nearby Sbc galaxy NGC 5055 are studied with the Multi-Pupil Spectrograph of the 6m telescope of the Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS). We confirm rotation and stellar velocity dispersion asymmetries along the major axis reported earlier by other authors. We have found a resolved chemically distinct core in NGC 5055, with the magnesium-enhanced region shifted by 2.5" (100 pc) to the south-west from a photometric center, toward a kinematically identified circumnuclear stellar disk. Mean ages of stellar populations in the true nucleus, defined as the photometric center, and in the magnesium-enhanced substructure are coincident and equal to 3-4 Gyr being younger by several Gyr with respect to the bulge stellar population. A possible origin of the asymmetries in the center of NGC 5055 is discussed.
Two lenticular galaxies with counterrotating stellar components in their disks have been studied with the Multi-Pupil Fiber Spectrograph of the 6m telescope (Special Astrophysical Observatory). In NGC 4138 the nucleus is certainly chemically distinct, with the central concentration of magnesium enhancement marginally exceeding that of iron. The ionized gas within R=4" of the nucleus rotates circularly in an inclined ring, perhaps a kind of a circumnuclear polar ring surrounding a compact stellar minibar. NGC 4550 exhibits complex extended structures in its center that can be identified by enhanced levels of magnesium and iron indices; the stellar population of the unresolved nucleus is younger than that of the circumnuclear region. We conclude that although NGC 4550 does not possess a chemically distinct nucleus in the ordinary sense of the term, there are nevertheless clear signatures of a secondary star formation burst confined perhaps to the counterrotating stellar-gaseous disk. We argue that the inclination of the counterrotating disk differs from that of the main stellar disk, implying that the two disks are not co-planar. Both galaxies seem to have possessed bars, either actually present or already dissolved.
The stellar population properties in the centers of NGC 5574 and NGC 7457 -- two lenticular galaxies previously known by radially homogeneous blue optical-band colors -- are studied by the means of integral-field spectroscopy. The compact nuclei of the galaxies have appeared to be chemically distinct: in NGC 5574 the nucleus is distinguished by a higher than solar iron-to-magnesium ratio, and in NGC 7457 a drop of mean stellar metallicity by a factor of 2 exists between the nucleus and the bulge. Both galaxies demonstrate a rather young mean luminosity-weighted age of the stellar populations in the bulges, not older than 5 - 7 Gyr; but the chemically distinct nuclei are still younger, 2 - 2.5 Gyr old. The kinematics of the stars in the center of NGC5574 is probably affected by an influence of the global bar. The compact core of NGC 7457, with a radius of about 1.5", shows a visible counterrotation; a combined analysis of the photometric and kinematical 2D maps allows to conclude that the core rotates axisymmetrically but its rotation plane is inclined to the main symmetry plane of the galaxy. The stellar velocity dispersion in the centers of both galaxies is anomalously low, of 60 - 80 km/s.
We have found an evolutionary decoupled central region in the giant lenticular galaxy NGC 80 belonging to a rich galaxy group: the mean stellar age in the nucleus and in the circumnuclear ring with a radius of 5"-7" is only 7 Gyr whereas in the bulge the stellar population is older than 10 Gyr. Besides, the nucleus of NGC 80 is chemically distinct: it is more metal-rich than its nearest outskirts by a factor of 2 -- 2.5 and is distinguished by a magnesium overabundance, [Mg/Fe]=+0.3. The global stellar disk of NGC 80 appears to be two-tiers: its outer part has an exponential scalelength of 11 kpc and a normal surface brightness, whereas the inner disk, also exponential and axisymmetric, is more compact and abnormally bright. Though it is evident that the two-tiers structure of the global disk and the chemically distinct nucleus of NGC 80 have a common origin and are produced by a catastrophique radial gas re-distribution in the protogalactic disk, the event having provoked this re-distribution remains a puzzle because the galaxy lacks any disturbance or triaxiality signatures.
The central regions of the three brightest members of the LeoI galaxy group -- NGC 3368, NGC 3379, and NGC 3384 -- are investigated by means of 2D spectroscopy. In all three galaxies we have found separate circumnuclear stellar and gaseous subsystems -- more probably, disks -- whose spatial orientations and spins are connected to the spatial orientation of the supergiant intergalactic HI ring reported previously by Schneider et al. (1983) and Schneider (1985, 1989). In NGC 3368 the