Structure of the galaxies of the NGC 80 group: Two-tiered disks

Two-color photometric data obtained on 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 studied galaxies are classified as lenticular galaxies. The stellar populations in the galaxies are very diverse, from old stars with ages of T > 10 billion years (IC 1541) to relatively young stars with ages of T ∼ 1–3 billion years (IC 1548, NGC 85); in one case, star formation is ongoing (UCM 0018+2216). In most of the studied galaxies, more precisely in all of them brighter than M B ∼ −18, two-tiered stellar disks are detected, whose radial surface-brightness profiles can be described by two exponential segments with different characteristic scales—shorter near the center and longer at the periphery. All of the dwarf S0 galaxies with single-tiered disks are close companions to larger galaxies. Except for this fact, no dependence of the properties of S0 galaxies on distance from the center of the group is found. Morphological signs of a “minor merger” are found in the lenticular galaxy NGC 85. Based on these last two results, it is concluded that the most probable mechanism for their transformation of spiral into lenticular galaxies in groups is gravitational (minor mergers and tidal interactions).     

Do the galaxies NGC 936 and NGC 3198 possess massive spheroidal subsystems?

The condition for gravitational stability of the stellar disks of the galaxies NGC 936 and NGC 3198 makes maximum disk models unacceptable. We present mass estimates for these objects' spheroidal components. The mass of the dark halo of NGC 3198, within four disk radial scale lengths, exceeds its disk mass by a factor of 1.6 to 2. The masses of the disk and spheroidal subsystem (halo + bulge), within four radial scale lengths, are approximately the same for NGC 936.     

Multicolor CCD photometry of six lenticular and spiral galaxies. Stellar population of the galaxies

The results of multicolor surface photometry of the S0 galaxies NGC 524, NGC 1138, and NGC 7280 and the spiral galaxies NGC 532, NGC 783, and NGC 1589 are analyzed. UBVRI observations were acquired with the 1.5-m telescope of the Maidanak Observatory (Uzbekistan), while JHK data were taken from the 2MASS catalog. The brightness and color distributions in the galaxies are analyzed. Extinction in dust lanes in three spiral galaxies is estimated. The contributions of the radiation of the spherical and disk components in different photometric bands are estimated. Two-color diagrams are used to estimate the composition of the stellar populations in various galaxy components. The variations of the color characteristics in the S0 galaxies is due mostly to radial metallicity gradients.     

Estimating the masses of the spherical and disk components of galaxies via numerical simulations

A series of numerical N-body simulations is performed in order to dynamically model the properties of four galaxies (NGC 5603, NGC 3198, NGC 891, and NGC 1566) with known rotation curves, radial disk scales L, and velocity dispersions of old disk stars at various galactocentric distances r. Each model includes a three-dimensional collisionless disk and rigid spherical components, whose relative mass μ was treated as a free parameter that differed from simulation to simulation. The observed disk stellar velocity dispersions were assumed to be equal to or (in the general case) greater than the corresponding line-of-sight projections of the simulated values for the adopted μ after the initially unstable disk is heated and arrives at a steady state. A comparison of the simulated and observed rotational velocities and velocity dispersions provides evidence for “light” disks with μ≥2 in the disk (r<4L).     

The volume density of gas in disk galaxies with low HI surface densities

Published data on rotation curves and the radial distribution of the surface density of neutral hydrogen (HI) in galaxies with a low gas content are used to calculate radial profiles of the volume density of HI in the planes of the galactic disks. A self-consistent model for the disks is used, taking into account the self-gravitation of the gas and the presence of a pseudo-isothermal, massive halo. Eleven low-surface-brightness (LSB) galaxies and three S0 galaxies in which HI is detected are considered. The gaseous and stellar disks are taken to be in equilibrium and axially symmetric, and the velocity dispersion in the stellar disk to be equal to the marginal value for gravitational perturbations; in general, this gives an upper limit for the gas density. It is shown that, on average, the gas volume densities are two orders of magnitude lower in LSB galaxies than in galaxies with normal brightnesses at the same R values, while the three S0 galaxies occupy an intermediate position. The volume density of gas observed at the galaxy peripheries are less than 10⁻²⁷ g/cm³, even in the plane of the disk. The role of the UV background in ionizing outer regions is discussed. The obtained gas densities can be used to estimate the star-forming efficiency in regions of low density.     

The thickness of stellar disks in early-type galaxies

We suggest and justify a new photometric method enabling the derivation of the relative thickness of a galactic disk from the 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 piece-wise-exponential (with a flatter outer profile) surface-brightness distributions. The data were taken from the open SDSS archive. The statistics of the estimated relative thicknesses of the stellar disks of early-type galaxies show the following features. The disks of lenticular and spiral early-type galaxies have similar thickness. The presence of a bar results in only a slight increase of the thickness. However, there is a substantial difference between the thicknesses of disks with a single exponential brightness profile and exponential disks that represent the inner segments of Type III profiles (after Erwin); i.e., they have an outer exponential disk with a larger characteristic scale. The disks are significantly thicker in the former than in the latter case. This may provide evidence that a single exponential scale in a disk surface-brightness distribution forms due to viscosity effects acting over the entire period of star-formation evolution in the disk.     

Disk boundaries in spiral galaxies

We explore the hypothesis that the outer boundaries (“cutoffs”) of the stellar disks observed in many galaxies are determined by the condition of local gravitational (Jeans) stability for the gaseous protodisks at large galactocentric distances. The ratio of the surface density of the disk Σ_disk to the critical value for Jeans instability Σ_crit is computed for a number of galaxies, assuming that the gas velocity dispersion in the forming disk corresponded to its current thickness and that the disk itself is in a quasi-equilibrium state. The mean estimated stellar velocity dispersion in the vicinity of the cutoff (12 km/s) is close to the typical velocity dispersions of gaseous clouds in disk galaxies. At greater distances, such velocity dispersions should ensure gravitational stability of the disk both at the present epoch and in the past. The cutoff radius of the disk R _cut is correlated with other disk parameters, and the ratio Σ_disk/Σ_crit at R _cut is close to unity in most cases. We conclude that the available observational data agree well with the hypothesis that stellar disk cutoffs are due to a rapid decrease in the star-formation rate beyond R _cut, where the gaseous disk has always been stable.     

The structure of the stellar disks of southern S0 galaxies in sparse environments

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 meter-class telescopes. Structural parameters of the global stellar disks of the galaxies are calculated—the exponential scale and relative thickness. The presence of substructure 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 substructure of the disks of lenticular galaxies: they can be classified as blue (ongoing star formation) or red (concentration of dust). The rings do not always lie in the main plane of the disk; there are cases of clearly inclined, or even polar, compact rings.     

Supermassive black holes and central star clusters: Connection with the host galaxy kinematics and color

The relationship between the masses of the central, supermassive black holes (M _bh) and of the nuclear star clusters (M _nc) of disk galaxies with various parameters galaxies are considered: the rotational velocity at R = 2 kpc V ₍₂₎, the maximum rotational velocity V _max, the indicative dynamical mass M ₂₅, the integrated mass of the stellar populationM _*, and the integrated color index B-V. The rotational velocities andmasses of the central objects were taken from the literature. ThemassM _nc correlatesmore closely with the kinematic parameters and the disk mass than M _bh, including with the velocity V _max, which is closely related to the virial mass of the dark halo. On average, lenticular galaxies are characterized by higher massesM _bh compared to other types of galaxies with similar characteristics. The dependence of the blackhole mass on the color index is bimodal: galaxies of the red group (red-sequence) with B-V >0.6–0.7 which are mostly early-type galaxies with weak star formation, differ appreciably from blue galaxies, which have higher values of M _nc and M _bh. At the dependences we consider between the masses of the central objects and the parameters of the host galaxies (except for the dependence of M _bh on the central velocity dispersion), the red-group galaxies have systematically higher M _bh values, even when the host-galaxy parameters are similar. In contrast, in the case of nuclear star clusters, the blue and red galaxies form unified sequences. The results agree with scenarios in which most red-group galaxies form as a result of the partial or complete loss of interstellar gas in a stage of high nuclear activity in galaxies whose central black-hole masses exceed 10⁶?10⁷ M _⊙ (depending on the mass of the galaxy itself). The bulk of disk galaxies with M _bh > 10⁷ M _⊙ are lenticular galaxies (types S0, E/S0) whose disks are practically devoid of gas.     

A correlation between dark matter and neutral hydrogen in spiral galaxies

We test the hypothesis put forward by Bosma (1981) that the surface density of dark matter is proportional to the surface density of HI, using decompositions of the rotation curves of a number of galaxies according to the THINGS, along with data for the galaxy NGC 6822. The rotation curves of these galaxies can be explained by assuming the existence of a massive gaseous disk in the absence of a dark halo, although the proportionality factor ??_dark/??_HI between the surface densities of dark matter and HI is different for different galaxies. However, there emerges the problem of the gravitational stability of galaxies whose stellar-velocity dispersions have been estimated, if the thickness of the dark-matter disk is similar to or less than the thickness of the stellar disk. The proportionality between ?? _dark and ??_HI is probably due to the fact that the radial profiles of ??_HI for galaxies with flat rotational curves are close to the critical density of a gravitationally stable gaseous layer (??_HI ?? R ^?1), and ??_dark(R) for a pseudo-isothermal halo obeys the same law.     

Stellar subsystems of the galaxy NGC 2366

Hubble Space Telescope archive data are used to perform photometry of stars in seven fields at the center and periphery of the galaxy NGC 2366. The variation of the number density of stars of various ages with galactocentric radius and along the minor axis of the galaxy are determined. The boundaries of the thin and thick disks of the galaxy are found. The inferred sizes of the subsystems of NGC 2366 (Z _thin = 4 kpc and Z _thick = 8 kpc for the thin and thick disks, respectively) are more typical for spiral galaxies. Evidence for a stellar halo is found at the periphery of NGC 2366 beyond the thick disk of the galaxy.     

Minimum velocity dispersion in stable stellar disks. Numerical simulations

N-body dynamical simulations are used to analyze the conditions for the gravitational stability of a three-dimensional stellar disk in the gravitational field of two rigid spherical components—a bulge and halo whose central concentrations and relative masses vary over wide ranges. The number of point masses N in the simulations varies from 40 to 500 000 and the evolution of the simulated systems is followed over 10–20 rotation periods of the outer edge of the disk. The initially unstable disks are heated and, as a rule, reach a quasi-stationary equilibrium with a steady-state radial-velocity dispersion c_r over five to eight turns. The radial behavior of the Toomre stability parameter Q_T(r) for the final state of the disk is estimated. Simple models are used to analyze the dependence of the gravitational stability of the disk on the relative masses of the spherical components, disk thickness, degree of differential rotation, and initial state of the disk. Formal application of existing, analytical, local criteria for marginal stability of the disk can lead to errors in c_r of more than a factor of 1.5. It is suggested that the approximate constancy of Q_T?1.2–1.5 for r?(1–2)×L (where L is the radial scale of disk surface density), valid for a wide range of models, can be used to estimate upper limits for the mass and density of a disk based on the observed distributions of the rotational velocity of the gaseous component and of the stellar velocity dispersion.     

Kinematics of disk galaxies with known masses of their supermassive black holes. Observations

This is the first paper in a project aimed at analyzing relations between the masses of supermassive black holes or nuclear clusters in galaxies and the kinematic features of the host galaxies. We present long-slit spectroscopic observations of galaxies obtained on the 6-m telescope of the Special Astrophysical Observatory using the SCORPIO focal reducer. Radial profiles of the line-of-sight velocities and velocity dispersions of the stellar populations were obtained for seven galaxies with known masses of their supermassive black holes (Mkn 79, Mkn 279, NGC 2787, NGC 3245, NGC 3516, NGC 7457, and NGC 7469), and also for one galaxy with a nuclear cluster (NGC 428). Velocity profiles of the emitting gas were obtained for some of these galaxies as well. We present preliminary galactic rotation curves derived from these data.     

Multicolor CCD photometry of six lenticular and spiral galaxies. Structure of the galaxies

The results of multicolor surface photometry of the S0 galaxies NGC 524, NGC 1138, and NGC 7280 and the spiral galaxies NGC 532, NGC 783, and NGC 1589 are reported. U BV RI observations were acquired with the 1.5-m telescope of the Maidanak Observatory (Uzbekistan), while JHK data were taken from the 2MASS catalog. The overall structure of the galaxies is analyzed and the galaxy images decomposed into bulge and disk components. The parameters of the galaxy components—rings, bars, spiral arms, and dust lanes—are determined. The bulge/disk decompositions based on averaged one-dimensional photometric profiles yield incorrect parameters for the bulges of the S0-Sa galaxies with bars and/or rings, whose inner regions are dominated by the radiation of the bulge.     

Analysis of the structure of disk galaxies in the NGC 2300 group

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 disksmay 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.     

The stellar population and evolution of galaxies of the NGC 80 group

Seven early-type galaxies that are members of the massive X-ray group containing NGC 80 have been studied using two-dimensional spectroscopy with the 6-m telescope of the Special Astrophysical Observatory. We searched for evidence for the synchronous secular evolution of the galaxies in the group. The bulges of five of the seven galaxies appear to be old, with the average age of the bulge stars being 10–15 billion years. Signs of a relatively recent star-formation burst are observed in the small S0 galaxy IC 1548, whose average bulge age is 3 billion years and average core age is 1.5 billion years. A circumnuclear polar gas ring was also detected in this galaxy; in its outer regions, it makes a smooth transition to a gas disk that counter-rotates relative to the stars. IC 1548 probably underwent a close interaction, which resulted in its transformation from a spiral to a lenticular galaxy; the same interaction may also have induced the central burst of star formation. In the giant E0 galaxy NGC 83, a compact massive stellar-gas disk with a radius of about 2 kpc and very rapid rotation is observed, with ongoing star formation; the so-called “minor merger” is likely to have occurred there. We conclude that the NGC 80 group is in a state of formation, with the small NGC 83 subgroup “falling into” the large, old NGC 80 subgroup.     

Gas dynamics of a central collision of two galaxies: Merger, disruption, passage, and the formation of a new galaxy

Results of numerical simulations of a collision of the gaseous components of two identical disk galaxies during a head-on collision of the galaxies in the polar direction are presented. When the relative velocity of the galaxy collision is small, their gaseous components merge. At high relative velocities (100–500 km/s), the massive stellar components of the galaxies (M _g = 10⁹ M _⊙) pass through each other nearly freely, leaving behind the gaseous components, which are decelerated and heated by the collision. If the overall gaseous component of the colliding galaxies is able to cool to the virial temperature during the collision, a new galaxy forms. At velocities V ≥ 500 km/s, the gaseous component does not have time to cool, and the gas is scattered into intergalactic space, supplying it with heavy elements produced in supernovae in the colliding galaxies. High-velocity (V ≥ 100 km/s) collisions of identical low-mass galaxies (M _g ≤ 10⁹ M _⊙) whose mass is dominated by the mass of gas lead to the disruption of their stellar components. The overall gaseous component forms a new galaxy when V ≤ 500 km/s, and is scattered into intergalactic space if the velocity becomes higher than this. A galaxy collision increases the star-formation rates in the disk galaxies by nearly a factor of 100. Rotation of the colliding galaxies in the same direction increases the changes of the disruption of both the stellar and gaseous components of the galaxies. The merger of galaxies during their collision can explain the presence of gaseous disks rotating opposite to the rotation of the stellar component in some ordinary elliptical galaxies. Moreover, galaxy mergers can help explain the origin of a comparatively young stellar population in some elliptical galaxies.     

Kinematic properties of the Galactic thick disk from data from the RAVE DR4 catalog

The analysis of the kinematic properties of the Galactic thick disk based on data from modern catalogs of stellar radial velocities and proper motions is presented. A new aspect of new determination of the kinematic characteristics of the thick disk is that the selected objects define this disk’s properties near the plane of symmetry. The velocity dispersion of stars in the Galactic thick disk in the radial direction and the direction of the Galactic rotation have been determined. The stellar-velocity distribution in the direction of the rotation is asymmetric. The parameters of this asymmetry have been determined, and the lag of the rotational velocity of the thick disk relative to objects in the thin disk estimated. The value of this “asymmetric drift,” about 20 km/s, suggests larger spatial scales for the kinematic characteristics in the radial direction for the Galactic thick disk than for the thin disk.     

Structural analysis of disk galaxies of the NGC 524 group

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.