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 global gaseous disk seems also to be inclined to the symmetry plane of the stellar body, being probably of external origin. Although the rather young mean stellar age and spatial orientations of the circumnuclear disks in NGC 3379, NGC 3384, and NGC 3368 could imply their recent formation from material of the intergalactic HI cloud, the time scale of these secondary formation events, of order 3 Gyr, does not support the collision scenario of Rood and Williams (1985), but is rather in line with the ideas of Schneider (1985, 1989) regarding tidal interactions of the galaxies with the HI cloud on timescales of the intergroup orbital motions.
We have investigated a sample of S0 galaxies, mostly with circumnuclear dust lanes orthogonal to their major axes chosen from HST/WFPC2 images. Two-dimensional spectroscopy undertaken with the MPFS of the 6m telescope of SAO RAS has revealed that indeed the ionized gas in the centers of these eight lenticulars rotate in the planes nearly orthogonal to the rotation (and symmetry) planes of their central stellar components. Though almost all the galaxies are located in dense environments, an external origin of this rotation plane tilt is not obvious because all the galaxies but one are known to have extended HI disks, and in two cases where the angular resolution of HI observations allows we find orthogonality of the external HI- and inner ionized-gas disks. We discuss a possible relation of the inner gas polar rings to a triaxiality of galactic potential. The stellar populations in the nuclei of all but two galaxies are very old that excludes recent star formation bursts and proves that the polar orbits of the circumnuclear gas are rather stable. In the nuclei of NGC~2655 and NGC~4111 we have found signatures of star formation bursts some 1.5--2~Gyr ago. This finding can be related to very central gas in NGC~2655 which is coplanar to the circumnuclear stellar disk and to radial gas inflow in NGC~4111; just these gas reservoirs and not the polar rings may be responsible for fueling nuclear star formation.
By means of 2D spectroscopy we have found a lot of compact inner stellar and gaseous disks in morphologically normal galaxies. The evolution and spatial orientation of these inner structures are often decoupled from the evolution and spatial orientation of the large-scale galactic disks and bulges. We argue that this finding requires substantial change of common approaches to investigation of the central parts of galaxies, including central black holes' study.
The properties of stellar populations in the centers of 54 nearby lenticular galaxies are investigated by means of 2D spectroscopy, with the Multi-Pupil Spectrograph of the 6m telescope of SAO RAS. All the galaxies are divided into four groups depending on the environment type; every subsample contains more than 10 galaxies. Clear distinctions between the mean stellar ages and abundance ratios both of the nuclei and of the bulges are found for the S0s in the different environments.
For five lenticular galaxies where earlier the chemically distinct nuclei have been detected from the observations with the Multi-Pupil Spectrograph of the 6m telescope, we analyse the data from the integral-field spectrograph SAURON of the 4.2m William Herschel Telescope. We confirm the presence of the peaks of equivalent widths of the magnesium absorption line MgIb in the nuclei of all five galaxies over the larger field of view than previously it was done. However, the structure of the chemically distinct regions within our modest sample has appeared to be very different: from compact unresolved condensations to disks extending over several hundred parsecs, and even a triaxial compact body like "a minibar" in one particular case. It is confirmed in NGC 7280 and revealed in NGC 7332 that inner gas polar rings exist. In their outer parts the planes of these polar rings demonstrate a warp toward the planes of stellar rotation, but in the sense that the outer gas counterrotates the stars. Such gas distribution and kinematics have been predicted for a tumbling triaxial potential by several independent dynamical simulations.
The kinematics, structure, and stellar population properties in the centers of two early-type spiral galaxies of the Leo Triplet, NGC 3627 and NGC 3623, are studied by means of integral-field spectroscopy. Unlike our previous targets, NGC 3384/NGC 3368 in the Leo I Group and NGC 5574/NGC 5576 in LGG 379, NGC 3623 and NGC 3627 do not appear to experience a synchronous evolution. The mean ages of their circumnuclear stellar populations are quite different, and the magnesium overabundance of the nucleus in NGC 3627 is evidence for a very brief last star formation event 1 Gyr ago whereas the evolution of the central part of NGC 3623 looks more quiescent. In the center of NGC 3627 we observe noticeable gas radial motions, and the stars and the ionized gas in the center of NGC 3623 demonstrate more or less stable rotation. However, NGC 3623 has a chemically distinct core -- a relic of a past star formation burst -- which is shaped as a compact, dynamically cold stellar disk with a radius of about 300 pc which has been formed not later than 5 Gyr ago.
We have studied the unbarred Sb galaxy with a nuclear star-forming ring, NGC 7742, by means of 2D spectroscopy, long-slit spectroscopy, and imaging, and have compared the results with the properties of another galaxy of this type, NGC 7217, which is studied by us earlier. Both galaxies have many peculiar features in common: each has two global exponential stellar disks with different scalelengths, each possesses a circumnuclear inclined gaseous disk with a radius of 300 pc, and each has a global counterrotating subsystem, gaseous one in NGC 7742 and stellar one in NGC 7217. We suggest that past minor merger is the probable cause of all these peculiarities, including appearance of the nuclear star-forming rings without global bars; the rings might be produced as resonance features by tidally induced oval distortions of the global stellar disks.
We have obtained 2D spectral data for a sample of 58 nearby S0 galaxies with the Multi-Pupil Spectrograph of the 6m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The Lick indices H-beta, Mgb, and combined Fe-index are calculated separately for the nuclei and for the bulges taken as the rings between R=4" and 7"; and the luminosity-weighted ages, metallicities, and Mg/Fe ratios of the stellar populations are estimated by confronting the data to SSP models. Four types of galaxy environments are considered: clusters, centers of groups, other places in groups, and field. The nuclei are found to be on average slightly younger than the bulges in any types of environments, and the bulges of S0s in sparse environments are younger than those in dense environments. The effect can be partly attributed to the well-known age correlation with the stellar velocity dispersion in early-type galaxies (in our sample the galaxies in sparse environements are in average less massive than those in dense environments), but for the most massive S0s, with the stellar velocity dispersion of 170-220 km/s, the age dependence on the environment is still significant at the confidence level of 1.5 sigma.
The central regions of the galaxies in the group of NGC 3169/NGC 3166/NGC 3156 are investigated with the Multi-Pupil Spectrograph of the 6m telescope; the former galaxy which is the central galaxy of the group is a spiral one (Sa), and the last two galaxies are lenticular ones. The group is known to possess an extended HI cloud with the size of more than 100 kpc which is related with the central galaxy NGC 3169 by its position, orientation and rotation. We have found that the mean age of the stellar populations in the centers of all three galaxies is the same, about 1 Gyr. Since the galaxies have been classified as early-type ones and since NGC 3166 and NGC 3156 lack global star formation, we conclude that we deal with the consequences of the synchronous nuclear star formation bursts in all three galaxies which have been evidently provoked by the galactic interaction with the intragroup HI cloud.
We report the discovery of young embedded structures in three diffuse elliptical galaxies (dE) in the Virgo cluster: IC 783, IC 3468, and IC 3509. We performed 3D spectroscopic observations of these galaxies with the MPFS spectrograph at the 6-m Special Astrophysical Observatory telescope and obtained spatially resolved distributions of kinematic and stellar population parameters by fitting high-resolution PEGASE.HR synthetic single stellar populations (SSP) in pixel space. In all three galaxies, the luminosity-weighted age of the nuclei (< 4 Gyr) is considerably younger than that of the population in the outer regions of the galaxies. We discuss two possibilities for the formation of such structures — a dissipative merger event and a different ram pressure stripping efficiency during two consecutive crossings of the Virgo cluster centre
We present the first 3D observations of a diffuse elliptical galaxy (dE). The good quality data (S/N up to 40) reveal the kinematical signature of an embedded stellar disc, reminiscent of what is commonly observed in elliptical galaxies, though similarity of their origins is questionable. Colour map built from Hubble Space Telescope Advanced Camera for Surveys (ACS) images confirms the presence of this disc. Its characteristic scale (about 3 arcsec =250 pc) is about a half of galaxy’s effective radius, and its metallicity is 0.1–0.2 dex larger than the underlying population. Fitting the spectra with synthetic single stellar populations (SSP), we found an SSP-equivalent age of 5 Gyr and nearly solar metallicity [Fe/H]=-0.06 dex. We checked that these determinations are consistent with those based on Lick indices, but have smaller error bars. The kinematical discovery of a stellar disc in dE gives additional support to an evolutionary link from dwarf irregular galaxies due to stripping of the gas against the intracluster medium.
We probe the stellar population age and metallicity distributions in nearby elliptical galaxies over the largest extension to date. Long-slit spectroscopy is made by using the spectrograph SCORPIO of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciencies. The Lick indices H$\beta$, Mgb, Fe5270, and Fe5335 are calculated along the slit up to radii of 1.3 to 3 $r_e$ in 4 galaxies and up to 0.5 $r_e$ in the fifth one. The comparison with evolutionary synthesis models of simple stellar populations allows us to disentangle age and metallicity and to measure both. We have found that the mean stellar age is constant along the radius only in one galaxy out of 5. The other 4 galaxies demonstrate quite different behaviour of the mean stellar age: the outer parts are older than the centres in 3 cases and younger -- in one case. The metallicity gradients cannot be approximated by a single power law over the full radial extension in 4 galaxies of 5. The inner metallicity gradients within 0.5$r_e$ are all rather steep, steeper than -0.4 metallicity dex per radius dex, and are inconsistent with the origin of the elliptical galaxies by a major merger.
The kinematics, structure, and stellar population properties in the centres of two brightest early-type galaxies of the Leo II group, NGC 3607 and NGC 3608, are studied by means of integral-field spectroscopy. The kinematically distinct areas in the centres of these galaxies, with radii of 6" and 5", respectively, are found also to be chemically distinct: they are characterized by enhanced magnesium-line strength. However, no stellar age differences have been found between the decoupled cores and their outskirts. An analysis of two-dimensional line-of-sight velocity fields reveals systematic turns of kinematical major axes near the nuclei of both galaxies; in NGC 3608 the ionized gas rotates perpendicular to the stellar component rotation. When taking into account some morphological features, it is concluded that both NGC 3607 and NGC 3608 have large triaxial stellar spheroids. It is argued that the magnesium-enhanced cores are not circumnuclear disks; instead they resemble rather compact triaxial structures that force the formation of polar disks around them – a gaseous one in NGC 3608 and a stellar-gaseous one in NGC 3607; in the latter, star formation is perhaps still proceeding.
The review of structural and kinematical features observed in disk galaxies which are thought to be consequences of secular evolution will be presented. Those include: `pseudobulges', namely, bulges with exponential surface brightness profiles which are common in late-type spirals but are also found in some lenticulars; `nested disks', or stellar disks constituted by two exponential segments with the inner ones being more compact (short-scaled); circumnuclear rings of young stars; circumnuclear gaseous rings. The phenomenon of circumnuclear polar gas rings/disks will be discussed with its possible connection to some intrinsic mechanisms of galactic secular evolution.
We have studied the central parts of seven early-type galaxies -- the members of the X-ray-bright galaxy group NGC 80 -- by means of integral-field spectroscopy at the Russian 6m telescope. We searched for signatures of synchronous evolution of the group galaxies. The following results have been obtained. Five galaxies have revealed old stellar populations in the bulges, with the SSP-equivalent ages from 10 to 15 Gyr. A moderate-luminous S0 galaxy IC 1548 demonstrates consequences of recent star formation burst: the SSP-equivalent age of the bulge is 3 Gyr, that of the nucleus -- 1.5 Gyr. It is also in this galaxy that we have found a circumnuclear polar gaseous disk which changes smoothly to counterrotating one at radii larger than 3" (1 kpc). Probably, IC 1548 had suffered an interaction followed by external gas accretion which might also provoke the central star formation burst. In the giant E0 galaxy NGC 83 which is projected close to the group center but has a line-of-sight velocity redshifted by 600 km/s with respect to the group center, we have observed a compact massive stellar-gaseous disk with the radius of some 2 kpc demonstrating current star formation. Consequently, NGC 83, just as IC 1548, has the young stellar population in the center. We speculate that a small subgroup leaded by NGC 83 is in process of infalling into the old massive group around NGC 80.
Panoramic spectroscopy of the sample of 80 nearby lenticular galaxies
is presented. The SSP-equivalent ages, [Z/H], and [Mg/Fe] are determined
through the Lick indices H-beta, Mgb, and
We have studied stellar and gaseous kinematics as well as stellar population properties in the center of the early-type barred galaxy NGC 4245 by means of integral-field spectroscopy. We have found a chemically distinct compact core, more metal-rich by a factor of 2.5 than the bulge, and a ring of young stars with the radius of 300 pc. Current star formation proceeds in this ring; its location corresponds to the inner Lindblad resonance of the large-scale bar. The mean age of stars in the chemically distinct core is significantly younger than the estimate by Sarzi et al. (2005) for the very center, within R=0.25", made with the HST spectroscopy data. We conclude that the `chemically distinct core' is in fact ancient ultra-compact starforming ring with the radius less than 100 pc which marks perhaps the past position of the inner Lindblad resonance. In general, the pattern of star formation history in the center of this early-type gas-poor galaxy confirms the predictions of dynamical models for the secular evolution of a stellar-gaseous disk under bar influence.
The presence of exponential bulges and antitruncated disks has been noticed in many lenticular galaxies. In fact, it could be expected because the very formation of S0 galaxies includes various processes of secular evolution. We discuss how to distinguish between a pseudobulge and an antitruncated disk, and also what particular mechanisms may be responsible for the formation of antitruncated disks. Some bright examples of lenticular galaxies with the multi-tiers exponential stellar structures are presented, among them -- two central group giant S0s (NGC 80 and NGC 524) seen face-on and perfectly axisymmetric.
We report a discovery of extended counterrotating gaseous disks in early-type disk galaxies NGC 2551 and NGC 5631. To find them, we have undertaken complex spectral observations including integral-field spectroscopy of the central parts of the galaxies and long-slit deep spectroscopy to probe the external parts. The line-of-sight velocity fields have been constructed and compared to the photometric structure of the galaxies. As a result, we have revealed full-size counterrotating gaseous disks, the one coplanar to the stellar disk in NGC 2551 and the other inclined to the main stellar disk in NGC 5631. We suggest that we observe the early stages of minor-merger events which may be two different stages of the process of lenticular galaxy formation in rather sparse environments.
BV photometric data obtained at the 6-m telescope of the Special Astrophysical Observatory of Russian Academy of Sciences are used to analyze the structure of 13 large disk galaxies in the NGC 80 group. Nine of the 13 galaxies under consideration are classified by us as lenticular galaxies. The stellar populations in the galaxies are very different, from old ones with ages of T>10 Gyrs (IC 1541) to relatively young ones with ages of T<2-3 Gyr (IC 1548, NGC 85). In one case, current star formation is known (UCM 0018+2216). In most of the galaxies, more precisely in all of them more luminous than M(B)~-18, two-tiered stellar disks are detected, whose radial surface-brightness profiles can be fitted by two exponential segments with different scalelengths -- shorter near the center and longer at the periphery. All dwarf S0 galaxies with single-scalelength exponential disks are close companions to giant galaxies. Except for this fact, no dependence of the properties of S0 galaxies on distance from the center of the group is found. Morphological traces of minor merger are found in the lenticular galaxy NGC 85. Basing on the last two points, we conclude that the most probable mechanisms for the transformation of spirals into lenticular galaxies in groups are gravitational ones, namely, minor mergers and tidal interactions.
Using the method of integral-field (3D) spectroscopy, we have investigated the kinematics and distribution of the gas and stars at the center of the early-type spiral galaxy with a medium scale bar NGC 7177 as well as the change in the mean age of the stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius R = 1.5"–2", where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate; basically, it has already turned into a pseudobulge as a result of secular dynamical evolution. The mean stellar age inside the star formation ring, in the galactic nucleus, is old, ~10 Gyr. Outside, at a distance R = 6"-8" from the nucleus, the mean age of the stellar population is ~2 Gyr. If we agree that the bar in NGC 7177 is old, then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions of some dynamical models.
Spheroidal stellar systems on various scales include elliptical galaxies, dwarf spheroidal galaxies, and globular stellar clusters. Elliptical galaxies are thought to be formed by major mergers of disk galaxies: it is the easiest way to create dynamicallly hot stellar systems without rotation, whose shape is supported only by anisotropic chaotic motions (by stellar velocity dispersion). However some recent observational findings have put into doubt this commonly accepted scenario. Some features of elliptical galaxies structure can only be explained if minor merging has mostly shaped these spheroidal stellar systems. The dwarf spheroidal galaxies represent quite certainly former disk galaxies shaped and transformed by tidal interactions with their large host galaxies. Globular clusters differ from the (dwarf) spheroidal galaxies by an absence of their own dark matter component. So they cannot be either downscaled version of galaxies nor the direct precursors of elliptical galaxies during the hierarchical gravitational clustering of baryons. However they are the oldest stellar systems in the Universe -- it is an observational fact. The formation mechanisms of the oldest globular clusters represent a puzzle yet.
We have studied unbarred S0 galaxies, NGC 3599 and NGC 3626, the members of the X-ray bright group Leo II, by means of 3D spectroscopy, long-slit spectroscopy, and imaging, with the aim to identify epoch and mechanisms of their transformation from spirals. Both galaxies have appeared to bear a complex of features resulting obviously from minor merging: decoupled gas kinematics, nuclear starforming rings, and multi-tiered oval large-scale stellar disks. The weak emission-line nucleus of NGC 3599 bears all signs of the Seyfert activity, according to the line-ratio diagnostics of the gas excitation mechanism. After all, we conclude that the transformation of these lenticular galaxies has had place about 1-2 Gyr ago, through the gravitational mechanisms not related to hot intragroup medium of Leo II.
We have investigated the stellar population properties in the central regions of a sample of lenticular galaxies with bars and single-exponential outer stellar disks using the data from the SAURON integral-field spectrograph retrieved from the open Isaac Newton Group Archive. We have detected chemically decoupled compact stellar nuclei with a metallicity twice that of the stellar population in the bulges in seven of the eight galaxies. A starburst is currently going on at the center of the eighth galaxy, and we have failed to determine the stellar population properties from its spectrum. The mean stellar ages in the chemically decoupled nuclei found range from 1 to 11 Gyr. Scenaria for the origin of both decoupled nuclei and lenticular galaxies as a whole are discussed.
By using the public UV imaging data obtained by the GALEX (Galaxy Ultraviolet Explorer) for nearby galaxies, we have compiled a list of lenticular galaxies possessing ultraviolet rings -- starforming regions tightly confined to particular radial distances from galactic centers. We have studied large-scale structure of these galaxies in the optical bands by using the data of the SDSS (Sloan Digital Sky Survey): we have decomposed the galactic images into large-scale disks and bulges, have measured the ring optical colours from the residual images after subtracting model disks and bulges, and have compared the sizes of the rings in the optical light and in the UV-band. The probable origin of the outer starforming ring appearances in unbarred galaxies demonstrating otherwise the regular structure and homogeneously old stellar population beyond the rings is discussed.
NGC 7217 is an unbarred early-type spiral galaxy having multi-segment exponential brightness profile and a system of starforming rings of unknown origin. We analysed new long-slit spectroscopic data for NGC 7217 and derived radial distributions of its stellar population parameters and stellar and gaseous kinematics up to the radius of 100" (~ 8 kpc). We performed the dynamical analysis of the galaxy by recovering its velocity ellipsoid at different radii, and estimated the scaleheights of its two exponential discs. The inner exponential stellar disc of NGC 7217 appears to be thin and harbours intermediate age stars (T~5 Gyr). The outer stellar disc seen between the radii of 4 and 7 kpc is very thick, with the scaleheight of 1 to 3 kpc, metal-poor, [Fe/H] <-0.4 dex, and has predominantly young stars, t(SSP)=2 Gyr. The remnants of minor mergers of gas-rich satellites with an early-type giant disc galaxy available in the GalMer database well resemble different structural components of NGC 7217, suggesting two minor merger events in the past responsible for the formation of the inner polar gaseous disc and large outer starforming ring. Another possibility to form the outer ring is the re-accretion of the tidal streams created by the first minor merger.
We used deep long-slit spectra and integral-field spectral data to study the stars and ionized gas kinematics and stellar population properties in the barred lenticular galaxy NGC 7743. We have shown that ionized gas at the distances larger than 1.5 kpc from the nucleus settles in the disk which is significantly inclined to the stellar disk of the galaxy. Making different assumptions about the geometry of the disks and involving different sets of emission lines into the fitting, under the assumption of thin flat disk circular rotation, we obtain the full possible range of angles between the disks to be 34 or 77 degree. The most probable origin of the inclined disk is external gas accretion from a satellite, orbiting the host galaxy with a corresponding angular momentum direction. The published data on the HI distribution around NGC 7743 suggest that the galaxy has a gas-rich environment. The emission-line ratio diagrams imply the domination of shock waves in the ionization state of the gaseous disk, whereas the contribution of photoionization by recent star formation seems to be negligible. In some parts of the disk a difference between the velocities of the gas emitting in the forbidden lines and in the Balmer lines is detected. It may be caused by the fact that the inclined disk is mainly shock-excited, whereas some fraction of the Balmer-line emission is produced by a small amount of gas excited by young stars in the main stellar disk of NGC 7743.
Members of the NGC 524 group of galaxies are studied using data obtained on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences, with the SCORPIO reducer in an imaging mode. Surface photometry has been carried out and parameters of the large-scale structural components -- disks and bulges -- have been determined for the six largest galaxies of the group. A lowered percentage of bars and enhanced percentage of ring structures were found. The integrated B–V colors of a hundred of dwarf galaxies in the vicinity (within 30 kpc) of the six largest galaxies of the group have been measured. A considerable number of blue irregular galaxies with ongoing star formation is present among nearby dwarf satellites of lenticular galaxies of the group. The luminosity function for dwarf members of the group suggests that the total mass of the group is not very high, and that the X-ray emitting gas observed in the direction of NGC 524 does not belong to the group halo.
By the means of panoramic spectroscopy at the SAO RAS BTA telescope, we investigated the properties of stellar populations in the central regions of five early-type galaxies - the NGC 524 group members. The evolution of the central regions of galaxies looks synchronized: the average age of stars in the bulges of all the five galaxies lies in the range of 3–6 Gyr. Four of the five galaxies revealed synchronized bursts of star formation in the nuclei 1–2 Gyr ago. The only galaxy, in which the ages of stellar population in the nucleus and in the bulge coincide (i.e. the nuclear burst of star formation did not take place) is NGC 502, the farthest from the center of the group of all the galaxies studied.
By undertaking deep long-slit spectroscopy with the focal reducer SCORPIO of the Russian 6m telescope, we studied stellar population properties and their variation with radius in 15 nearby S0 galaxies sampling a wide range of luminosities and environments. For the large-scale stellar disks of S0s, we have measured SSP-equivalent metallicities ranging from the solar one down to [Z/H]=-0.4 - -0.7, rather high magnesium-to-iron ratios, [Mg/Fe]> +0.2, and mostly old SSP-equivalent ages. Nine of 15 (60%+/-13%) galaxies have large-scale stellar disks older than 10 Gyr, and among those we find all the galaxies which reside in denser environments. The isolated galaxies may have intermediate-age stellar disks which are 7-9 Gyr old. Only two galaxies of our sample, NGC 4111 and NGC 7332, reveal SSP-equivalent ages of their disks of 2-3 Gyrs. Just these two young disks appear to be thin, while the other, older disks have scale heights typical for thick stellar disks. The stellar populations in the bulges at radii of 0.5r(eff) are on the contrary more metal-rich than the solar Z_0, with the ages homogeneously distributed between 2 and 15 Gyr, being almost always younger than the disks. We conclude that S0 galaxies could not form in groups at z~0.4 as is thought now; a new scenario of the general evolution of disk galaxies is proposed instead.
We have obtained and analyzed long-slit spectral data for the lenticular galaxy IC 719. In this gas-rich S0 galaxy, its large-scale gaseous disk counterrotates the global stellar disk. Moreover in the IC 719 disk we have detected a secondary stellar component corotating the ionized gas. By using emission-line intensity ratios, we have proved the gas excitation by young stars and so are claiming current star formation, most intense in a ring-like zone at the radius of 10'' (1.5 kpc). The oxygen abundance of the gas in the starforming ring is about half of the solar abundance and matches perfectly the metallicity of the global stellar disk. Since the stellar disk remains dynamically cool, we conclude that smooth prolonged accretion of the external gas from a neighboring galaxy provides current building of the thin large-scale stellar disk.
Due to the fast development of observational techniques, observations of high-redshift galaxies and detailed studies of nearby galaxies bring surprises every week. Many quite unexpected discoveries have been made over the last years, which have changed our views about galaxy formation and evolution substantially. Here I review some of the most important and unexpected observational facts that bring changes even to paradigms.
A sample of completely isolated S0 galaxies has been studied by means of long-slit spectroscopy at the Russian 6-m telescope. Seven of twelve galaxies have revealed a presence of extended ionized-gas discs which rotation is mostly decoupled from the stellar kinematics: 5 of 7 (71%) galaxies show a visible counterrotation of the ionized-gas component with respect to the stellar component. The emission-line diagnostics demonstrates a wide range of the gas excitation mechanisms, although a pure excitation by young stars is rare. We conclude that in all cases the extended gaseous discs in our sample S0s are of external origin, despite the visible isolation of the galaxies. Possible sources of external accretion, such as systems of dwarf gas-rich satellites or cosmological cold-gas filaments, are discussed.
Lenticular galaxies are a morphological class of disc galaxies that in general lacks current star formation and extended gaseous discs. However recent UV surveys have revealed XUV (eXtended UV) discs and rings even among these `red and dead' galaxies, and their origin is now still unclear. We have studied the nature of outer starforming, UV-detected rings in four unbarred S0 galaxies by undertaking their long-slit spectroscopy with the focal reducer SCORPIO of the Russian 6m telescope. Gaseous discs in NGC 252 and in NGC 4513 have decoupled kinematics, and the ionized gas of their rings is certainly excited by young stars. Just these two galaxies belong to galaxy groups. Two other, quite isolated S0 galaxies with the UV-rings, IC 522 and NGC 446, demonstrate shock-dominated gas excitation in the UV-detected rings, so their rings may probably have impact origin. However, in all four galaxies the ionized gas reveals oxygen abundance close to the solar one, so the hypothesis of gas accretion from cosmological filaments seems to be unfavourable. Rather another large galaxy may be a donor of outer gas in the galaxies of our sample.
In this paper we present the results of long-slit spectral observations for a sample of isolated lenticular galaxies, made with the SCORPIO and SCORPIO-2 spectrographs of the 6-meter BTA telescope of the SAO RAS. By applying direct spectra fitting using the stellar population evolutionary synthesis models, we have measured the radial profiles of the stellar line-of-sight velocity as well as the velocity dispersion, SSP-equivalent age and SSP-equivalent metallicity of stars along the radius in 12 targets. The resulting averaged ages of the stellar population in bulges and discs cover an entire range of possible values from 1.5 to 15 Gyr which indicates the absence of a certain formation epoch for the structural components in the isolated lenticular galaxies, unlike in the members of clusters and rich groups: they could have been formed at a redshift of z>2 as well as only a few billion years ago. Unlike S0 galaxies in more dense environments, the isolated galaxies typically have the same age of stars in the bulges and discs. The disc-embedded lenses and rings of increased stellar brightness, identified from the photometry in 7 of 11 galaxies, do not differ strongly from the stellar discs as concerning the properties of stellar populations and stellar velocity dispersion. We conclude that the final shaping of the morphological type of a lentiular galaxy in complete isolation is critically dependent on the possible regimes of cold-gas accretion from outside.
We suggest and verify a new photometric method enabling derivation of relative thickness of a galactic disk from two-dimensional surface-brightness distribution of the galaxy in the plane of the sky. The method is applied to images of 45 early-type (S0-Sb) galaxies with known radial exponential or double-exponential (with a flatter outer profile) surface-brightness distributions. The digital data in the r-band have been retrieved from the SDSS archive. Statistics of the estimated relative thicknesses of the stellar disks of early-type disk galaxies show the following features. The disks of lenticular and early-type spiral galaxies have similar thicknesses. The presence of a bar results in only a slight marginal increase of the thickness. However, we have found a substantial difference between the thicknesses of the disks with a single-scaled exponential brightness profile and the disks that represent the inner segments of the Type III (antitruncated) profiles. The disks are significantly thicker in the former subsample than in the latter one. This may provide evidence for a surface-brightness distribution of a single-scaled exponential disk to be formed due to viscosity effects acting over the entire period of star formation in the disk.
We discuss a simple and fast method for estimating masses of early-type galaxies from optical data and compare the results with X-ray derived masses. The optical method relies only on the most basic observables such as the surface brightness I(R) and the line-of-sight velocity dispersion sigma(proj)(R) profiles and provides an anisotropy-independent estimate of the galaxy circular speed V(circ). The mass-anisotropy degeneracy is effectively overcome by evaluating V(circ) at a characteristic radius R(sweet) defined from local properties of observed profiles. The sweet radius is expected to lie close to R(2), where I(R) falls following an inverse square of R, and not far from the effective radius R(eff). We apply the method to a sample of five X-ray bright elliptical galaxies observed with the 6 m telescope BTA in Russia. We then compare the optical V(circ) estimates with the X-ray derived values, and discuss possible constraints on the non-thermal pressure in the hot gas and configuration of stellar orbits. We find that the average ratio of the optical V(circ) estimate to the X-ray one is equal to 0.98 with 11 per cent scatter, i.e. there is no evidence for the large non-thermal pressure contribution in the gas at R(sweet). From analysis of the Lick indices H-beta, Mgb, Fe5270 and Fe5335, we calculate the mass of the stellar component within the sweet radius. We conclude that a typical dark matter fraction inside R(sweet) in the sample galaxies is 60 per cent for the Salpeter initial mass function (IMF) and 75 per cent for the Kroupa IMF.
We review our current knowledge about a particular case of decoupled gas kinematics, inner ionized-gas polar disks. Though more difficult to be noticed, they seem to be more numerous than their large-scale counterparts; our recent estimates imply about 10% of early-type disk galaxies to be hosts of inner polar disks. Since in the most cases the kinematics of the inner polar gaseous disks is decoupled from the kinematics of the outer large-scale gaseous disks, and since they nested around very old stellar nuclei, we speculate that the inner polar disks may be relic of very early events of external gas accretion several Gyr ago. Such view is in agreement with our new paradigm of disk galaxies evolution.
By combining new long-slit spectral data obtained with the Southern African Large Telescope for nine galaxies with our previously published observations for 12 additional galaxies, we study the stellar and gaseous kinematics as well as radially resolved stellar population properties and ionized-gas metallicity and excitation for a sample of isolated lenticular galaxies. We have found that there is no particular time frame of formation for the isolated lenticular galaxies: the mean stellar ages of the bulges and disks are distributed between 1 and >13 Gyr, and the bulge and the disk in every galaxy formed synchronously demonstrate similar stellar ages and magnesium-to-iron ratios. Extended ionized-gas disks are found in the majority of the isolated lenticular galaxies, in 72%+/-11%. Half of all extended gaseous disks demonstrate a visible counterrotation with respect to their stellar counterparts. We argue that just such a fraction of projected counterrotation is expected if all the gas in isolated lenticular galaxies is accreted from outside, under the assumption of isotropically distributed external sources. The very narrow range of the gas oxygen abundances we found for the outer ionized-gas disks excited by young stars, [O/H] from 0.0 to +0.2 dex, gives evidence for satellite merging as the most probable source of this accretion. Last, we formulate a hypothesis that the morphological type of a field disk galaxy is completely determined by the outer cold-gas accretion regime.
We have studied the occurrence frequency of the current star formation in the outer stellar rings of early-type disk galaxies based on a representative sample of nearby galaxies from the ARRAKIS catalog. We show that regular rings reveal current star formation with a young stellar population age of less than 200 Myr in about half the cases, while in the pseudorings (open rings), which are only found in spiral galaxies, current star formation is present almost always.
I analyze statistics of the stellar population properties for stellar nuclei and bulges of nearby lenticular galaxies in different environments by using panoramic spectral data of the integral-field spectrograph SAURON retrieved from the open archive of the Isaac Newton Group. I also estimate the fraction of nearby lenticular galaxies having inner polar gaseous disks by exploring the volume-limited sample of early-type galaxies of the ATLAS-3D survey. By inspecting the two-dimensional velocity fields of the stellar and gaseous components with the running tilted-ring technique, I have found seven new cases of inner polar disks. Together with those, the frequency of inner polar disks in nearby S0 galaxies reaches 10%, which is much higher than the frequency of large-scale polar rings. Interestingly, the properties of the nuclear stellar populations in the inner polar ring hosts are statistically the same as those in the whole S0 sample, implying similar histories of multiple gas-accretion events from various directions.
Surface photometry data are presented for 12 southern lenticular galaxies located in regions of low density. Digital images in the gri bands were obtained on the LCOGT network of 1-meter-class telescopes. Structural parameters of the global stellar disks of the galaxies are calculated: the exponential scales and relative thicknesses. The presence of substructures in the disks is noted; in particular, more than half the studied galaxies possess ring structures, sometimes more than one. The color maps presented indicate complex evolution of the substructures in the disks of lenticular galaxies: they can be classified as blue (on-going star formation) or red (concentration of dust). The rings do not always lie in the main plane of the disks; we have found the cases of inclined, or even polar, compact rings.
Data from the 6-m telescope of the Special Astrophysical Observatory obtained using the SCORPIO instrument in imaging mode are used to study member galaxies of the NGC 2300 group. Surface photometry has been carried out for the five largest galaxies in the group, whose isophotal parameters and the parameters of their large-scale structural components (disks and bulges) have been determined. The morphological type of the central galaxy in the group has been refined, and shown to be elliptical. Studies of structural features in non-central disk galaxies have revealed an enhanced percent of bars: bars were found in all disk galaxies of this group, with all of these being compact structures. The similarity of the structural features of the disks of the group galaxies suggests that these disks may be being restructured in the process of the current merger of the two X-ray subgroups comprising NGC 2300: the group NGC 2300 itself and the group NGC 2276.
Highly noncircular outer stellar disks have been detected in two SA0 (unbarred) galaxies by comparing the spectroscopic data on the rotation of stars and the photometric data on the shape and orientation of isophotes. In NGC 502, the oval distortion of the disk is manifested in the shape of the inner and outer elliptical rings occupying wide radial zones between the bulge and the disk and at the outer disk edge; such a structure can be a consequence of the so-called 'dry minor merger', multiple cannibalization of gas-free satellites. In NGC 5485, the stellar kinematics is absolutely unrelated to the orientation of isophotes in the disk region, and for this galaxy the conclusion about its global triaxial structure is unavoidable.
A counter-rotating gaseous disk has been detected in the SA0 galaxy IC 560 which is located at the periphery of a sparse group of six late-type galaxies. The pattern of gas excitation and mid-infrared colors are indicative of ongoing star formation within 1 kpc of the center. Outside the gaseous disk with star formation the large-scale stellar disk of the galaxy has an old age and a very low metallicity, [Z/H] ? ?1. The source of external gas accretion onto IC 560 is undetected; the only option is a single infall of a companion rich in high-metallicity gas.
Although thick stellar discs are detected in nearly all edge-on disc galaxies, their formation scenaria still remain a matter of debate. Due to observational difficulties, there is a lack of information about their stellar populations. Using the Russian 6-m telescope BTA we collected deep spectra of thick discs in three edge-on S0/a disc galaxies located in different environments: NGC 4111 in a dense group, NGC 4710 in the Virgo cluster, and NGC 5422 in a sparse group.We see intermediate-age (4–5 Gyr) metal-rich ([Fe/H]~-0.2...0.0 dex) stellar populations in NGC 4111 and NGC 4710. On the other hand, NGC 5422 does not harbour young stars, its whole disc is thick and old (10 Gyr), without evidence for a second component, and its alpha-element abundance suggests a 1.5–2 Gyr long formation epoch implying its formation at high redshift. Our results suggest the diversity of thick disc formation scenaria.
The counter-rotation phenomenon in disc galaxies directly indicates a complex galaxy assembly history which is crucial for our understanding of galaxy physics. Here, we present the complex data analysis for a lenticular galaxy NGC 448, which has been recently suspected to host a counter-rotating stellar component. We collected deep long-slit spectroscopic observations using the Russian 6-m telescope and performed the photometric decomposition of Sloan Digital Sky Survey archival images. We exploited (i) a non-parametric approach in order to recover stellar line-of-sight velocity distributions and (ii) a parametric spectral decomposition technique in order to disentangle stellar population properties of both main and counter-rotating stellar discs. Our spectral decomposition stays in perfect agreement with the photometric analysis. The counter-rotating component contributes ~30 per cent to the total galaxy light. We estimated its stellar mass to be 9 billion solar mass. The radial scalelength of counter-rotating disc is three times smaller than that of the main disc. Both discs harbour old stars but the counter-rotating component reveals a detectable negative age gradient that might suggest an extended inside-out formation during 3...4 Gyr. The counter-rotating disc hosts more metal-rich stars and possesses a shallower metallicity gradient with respect to the main disc. Our findings rule out cosmological filaments as a source of external gas accretion which is considered as a potential mechanism of the counter-rotating component formation in NGC 448, and favour some satellite merger event with the consequent slow gas accretion.
Using panoramic and long-slit spectroscopy at the 6-m telescope of SAO RAS, we studied the stellar population and kinematics of five early-type disc galaxies—members of the NGC2300 group. The evolution of galaxies appears to be absolutely out of synch: while the average age of the stars in the central regions of the galaxies located close to the center of the group ranges from 2 to 7 Gyr, the peripheral spiral galaxies have old nuclei and bulges, with the ages of 10–15 Gyr. The brightest galaxy of the NGC2300 group, which up to now has been considered to be lenticular, of the SA0 type, turned out to be extremely hot dynamically: its bulge rotates slowly, v/sigma = 0.06, and the outer parts do not rotate at all.We conclude that the kinematics of the stellar component of NGC2300 indicates that it is not a disc galaxy, but a triaxial spheroid.
By obtaining imaging data in two photometric bands for 60 lenticular galaxies - members of eight southern clusters - with the Las Cumbres Observatory one-meter telescope network, we have analyzed the structure of their large-scale stellar disks. The parameters of radial surface-brightness profiles have been determined (including also disk thickness), and all the galaxies have been classified into pure exponential (Type I) disk surface-brightness profiles, truncated (Type II), and antitruncated (Type III) piecewise exponential disk surface-brightness profiles. We confirm the previous results of some other authors that the proportion of surface-brightness profile types is very different in environments of different density: in the clusters, the Type-II profiles are almost absent while according to the literature data, in the field they constitute about one-quarter of all lenticular galaxies. The Type-III profiles are equally presented in the clusters and in the field, while following similar scaling relations; but by undertaking an additional structural analysis including the disk thickness determination we note that some Type-III disks may be a combination of a rather thick exponential pseudobulge and an outer Type-I disk. Marginally, we detect a shift of the scaling relation toward higher central surface brightnesses for the outer segments of Type-III disks and smaller thickness of the Type-I disks in the clusters. Both effects may be explained by enhanced radial stellar migration during disk galaxy infall into a cluster that in particular represents an additional channel for Type-I disk shaping in dense environments.
Although S0 galaxies are usually thought to be 'red and dead', they often demonstrate star formation that is organized in ring structures. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. Two early-type galaxies with outer rings, NGC6534 and MCG11-22-015, were selected to be studied. After inspecting the gas excitation in the rings using the Baldwin–Phillips–Terlevich method, we estimated the star formation rates (SFR) in the two outer rings of our galaxies using several SFR indicators derived from narrow-band photometry in the H-alpha emission line and archival GALEX ultraviolet images of the galaxies. The ionized gas is excited by young stars in the ring of NGC6534 and partly by shocks in MCG11-22-015. The oxygen abundances in the HII regions of the rings are close to solar. The derived SFRs allow us to qualitatively restore star formation histories (SFH) in the rings: in NGC6534, the SFH has been flat during the last 100–200 Myr, and in MCG11-22-015, the star formation has started only a few Myr ago. We suggest that the rings in NGC6534 and MCG11-22-015 have different natures: the former is a resonance ring supplied with gas perhaps through tidal effects, and the latter has been produced by a satellite accretion. Recent outer gas accretion is implied in both cases.
Rings in S0s are enigmatic features that can, however, betray the evolutionary paths of particular galaxies. We have undertaken long-slit spectroscopy of five lenticular galaxies with UV-bright outer rings. The observations have been made with the Southern African Large Telescope to reveal the kinematics, chemistry, and ages of the stellar populations and the gas characteristics in the rings and surrounding disks. Four of the five rings are also bright in the H-alpha emission line, and the spectra of the gaseous rings extracted around the maxima of the H-alpha equivalent width reveal excitation by young stars betraying current star formation in the rings. The integrated level of this star formation is 0.1–0.2 solar mass per year, with the outstanding value of 1 solar mass per year in NGC 7808. The difference of chemical composition between the ionized gas of the rings, which demonstrate nearly solar metallicity, and the underlying stellar disks, that are metal-poor, implies recent accretion of the gas and star formation ignition; the star formation history estimated by using different star formation indicators implies that the star formation rate decreases with an e-folding time of less than 1 Gyr. In NGC 809, where the UV ring is well visible but the H-alpha emission line excited by massive stars is absent, the star formation has already ceased.
We present results of long-slit and panoramic spectroscopy of extended gaseous disks in 18 nearby S0 galaxies, mostly in groups. The gas in our S0s is found to be often accreted from outside that is implied by its decoupled kinematics: at least 5 galaxies demonstrate strongly inclined large-scale ionized-gas disks smoothly coupled with their outer HI disks, 7 galaxies reveal circumnuclear polar ionized-gas disks, and in NGC 2551 the ionized gas though confined to the main galactic plane however counterrotates the stellar component. The ionized-gas excitation analysis reveals the gas ionization by young stars in 12 of 18 S0 galaxies studied here; the current star formation in these galaxies is confined to the ring-like zones coinciding with the UV-rings. The gas oxygen abundance estimates in the rings are closely concentrated around the value of [Z/H]=-0.15 dex and do not correlate either with the ring radius nor with the metallicity of the underlying stellar population. By applying the tilted-ring analysis to the 2D velocity fields of the ionized gas, we have traced the orientation of the gas rotation-plane lines of nodes along the radius. We have found that current star formation proceeds usually just where the gas lies strictly in the stellar disk planes and rotates there circularly; the sense of the gas rotation does not matter (the counterrotating gas in NGC 2551 forms stars currently). In the galaxies without signs of current star formation the extended gaseous disks are either in steady-state quasi-polar orientation (NGC 2655, NGC 2787, NGC 3414, UGC 9519), or are acquired recently through the highly inclined external filaments provoking probably shock-like excitation (NGC 4026, NGC 7280). Our data imply crucial difference of the external-gas accretion regime in S0s with respect to spiral galaxies: the geometry of the gas accretion in S0s is typically off-plane.
Galactic stellar discs, such as that of the Milky Way, have usually a complex structure consisting of a thin and a thick component. The study of galactic disc substructures and their differences can shed light on the galaxy assembling processes and their evolution. However, due to observational difficulties there is a lack of information about the stellar populationS of the thick disc componentS in external galaxies. Here we investigate three edge-on early-type disc galaxies in the Fornax cluster (IC 335, NGC 1380A, NGC 1381) by using publicly available photometrical data and our new deep long-slit spectroscopy along galactic mid-planes obtained with the 10-m SALT telescope. We report that significant changes of the stellar population properties beyond the radius where photometrical profiles demonstrate a knee are caused by an increasing thick disc contribution. Stellar population properties in the the outermost thick-disc dominated regions demonstrate remarkably old ages and a low metallicity. We interpret these findings as a consequence of star formation quenching in the outermost regions of the discs due to ram pressure gas stripping from the disc periphery at the beginning of the cluster assembly while subsequent star formation occurring in the inner discs being gradually extinguished by the starvation.