arXiv subjects
Publications and source records attributed to Olga K. Sil'chenko.
By studying the stellar population properties along the radius in 15 nearby S0 galaxies, I have found that the outer stellar disks are mostly old, with the SSP-equivalent ages of 8-15 Gyr, being often older than the bulges. This fact puts into doubt a currently accepted paradigm that S0 galaxies have formed at z=0.4 by quenching star formation in spiral galaxies.
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 lenticular barred 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 angle between the disks to be 34+/-9 or 77+/-9 deg. The most probable origin of the inclined disk is the 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. In the circumnuclear region (R< 200 pc) some evidences of the AGN jet interaction with an ambient interstellar medium were found.
In this paper we propose a new method for reconstructing the surface density of matter in flat disks of spiral galaxies. The surface density is expressed through observational rotation velocity curves of visible matter in the disks of spiral galaxies. The new method is not based on quadrature of special functions. The found solution is used for processing and analysis of observational data from several spiral galaxies. The new method can be used to more accurately estimate the amount of dark matter in spiral galaxies.
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.
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. The scenarios for the origin of both decoupled nuclei and lenticular galaxies as a whole are discussed.
The results of photometric decomposition of surface brightness distributions in 85 early-type unbarred galaxies are presented. The SDSS r-images are analysed. Double-tiered exponential disks are found in all galaxies which are studied; the statistics of the disk parameters is derived.
I present some results of 3D spectroscopy for a small sample of dwarf elliptical galaxies, mostly members of small groups. The galaxies under consideration have a typical absolute magnitude of -18 (B-band), and at the Kormendy's relation they settle within a transition zone between the main cloud of giant ellipticals and the sequence of diffuse ellipticals. By measuring Lick indices and investigating radial profiles of the SSP-equivalent ages and metallicities of the stellar populations in their central parts, I have found evolutionary distinct cores in all of them. Typically, the ages of these cores are 2-4 Gyr, and the metallicities are higher than the solar one. Outside the cores, the stellar populations are always old, T>12 Gyr, and the metallicities are subsolar. This finding implies that the well-known correlation between the stellar age and the total mass (luminosity) of field ellipticals (Trager et al. 2000, Caldwell et al. 2003, Howell 2005) may be in fact a direct consequence of a larger contribution of nuclear starbursts into the integrated stellar population in dwarfs with respect to giants, and does not relate to `downsizing'.
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 pseudo-bulge 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.
BV-bands photometric data obtained at the 6-m telescope of the Special Astrophysical Observatory 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, with the 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 (`antitruncated') 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.
The presence of exponential bulges and anti-truncated 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 anti-truncated disk, and also what particular mechanisms may be responsible for the formation of anti-truncated disks. Some bright examples of lenticular galaxies with the multi-tiers exponential stellar structures are presented, among them -- two central group giant S0s seen face-on and perfectly axisymmetric.
Panoramic spectroscopic data of the sample of 80 nearby lenticular galaxies obtained with the Multi-Pupil Fiber Spectrograph of the 6-m telescope are presented. The SSP-equivalent ages, [Z/H], and [Mg/Fe] are determined through the Lick indices H-beta, Mgb, and separately for the nuclei and for the bulges. About a half of the sample contain chemically distinct nuclei, more metal-rich and younger than the bulges. The correlations of the stellar population properties for the nearby S0s are discussed.
The kinematics, structure, and stellar population properties in the centers 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 centers of these galaxies, with radii of 6" and 5" respectively, are found also to be chemically distinct. These stellar structures are characterized by enhanced magnesium-line strength in the integrated spectra. However, we have not found any mean stellar age differences between the decoupled cores and their outskirts. Analysis of the two-dimensional line-of-sight velocity fields reveals systematic turns of the kinematical major axes near the nuclei of both galaxies; in NGC 3608 the ionized gas rotates in the orthogonal plane with respect to the stellar component rotation. By taking into account some morphological features, we conclude that both NGC 3607 and NGC 3608 have large triaxial stellar spheroids. We argue that the magnesium-enhanced cores are not circumnuclear disks; instead they resemble rather compact triaxial structures which may be a cause of formation of polar disks around them - a gaseous one in NGC 3608 and a stellar-gaseous one in NGC 3607. In the latter galaxy the star formation is perhaps still proceeding over the polar disk.
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 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 the sparse environments are younger than those in the 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 on average less massive than those in dense environments). However 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 kinematics, structure, and stellar population properties in the centers of two early-type spiral galaxies of the Leo Triplet, NGC 3623 and NGC 3627, 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 LGG379, 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 250-350 pc which has been formed not later than 5 Gyr ago.
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 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 is triaxial.
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 a 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.
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/sec and probably is azimuthally dependent. The very inner region of the galaxy, r < 3 arcsec or 0.5 kpc, shows a turn of the dynamical major axis by about 30 deg. Central continuum isophotes are also twisted which suggests the presence of small nuclear bar.