The gas density and “volume” Schmidt law for spiral galaxies

The equilibrium thickness of the isothermal layers of interstellar gas and volume gas densities ρ _gas in the plane of the disk as a function of galactocentric distance R are computed for seven spiral galaxies (including the Milky Way) using an axisymmetrical model. In this model, the thickness of the stellar disk varies with R and remains approximately equal to the minimum thickness of a stable equilibrium disk. We found the disk thickness to increase toward the periphery in at least five of the seven galaxies. The density of the stellar disk decreases with R faster than ρ _gas , so that gas dominates at the disk peripheries in terms of density. A comparison of the azimuthally averaged star formation rate SFR and the gas density shows the absence of a universal Schmidt law SFR ～ρ _gas ⁿ for galaxies. However, the SFRs in various galaxies are better correlated with the volume than the gas surface density. The parameter n in the Schmidt law formally calculated using the least-squares method lies in the interval 0.8–2.4, being, on average, close to 1.5. The values of n calculated separately for the molecular gas display substantial scatter, but are, on average, close to unity. The value of n appears to increase with decreasing ρ _gas , so that the fraction of gas that actively participates in star formation decreases with n.     

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.     

Properties of galactic disks at optical and near-IR wavelengths

We have analyzed the radial scales, central surface brightnesses, and colors of 400 disks of various types of galaxies. For nine galaxies, the brightness decrease and the central disk brightness were obtained via a two-dimensional decomposition of the U BV RI J H K photometric images into bulge and disk components. We used published disk parameters for 392 of the galaxies. The central surface brightness μ _0,i ⁰ and linear (disk) scale length h vary smoothly along the Hubble sequence of galaxies within a rather narrow interval. The disks of relatively early-type galaxies display higher central K surface brightnesses, higher central surface densities, higher central mass-to-luminosity ratios M/L(B), smaller sizes (relative to the diameter of the galaxy D ₂₅), redder integrated colors, and redder central colors. The color gradient normalized to the radius of the galaxy and the “blue” central surface brightness of the disk, μ _0,i/0(B), are both independent of the galaxy type. The radial disk scales in different photometric bands differ less in early-type than in late-type galaxies. A correlation between the central disk surface brightness and the total luminosity of the galaxy is observed. We also consider the influence of dust on the photometric parameters of the disks.     

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.     

Galaxies with anomalously high abundances of molecular hydrogen

A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with anomalously high molecular-to-atomic hydrogen mass ratios (M _mol/M _HI > 2) is analyzed. The sample galaxies do not differ systematically from the other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the integrated masses of gas (for galaxies with the same linear sizes and disk angular momenta). This suggests that the overabundances of H₂ are due to the molecularization of HI. Galaxies with bars and active nuclei are found more frequently among galaxies with M _mol estimates in CISM. In a small fraction of cases, high M _mol/M _HI ratios are due to overestimation of M _mol due to overstimating of the conversion factor for the translation of CO-line intensities into the number of H₂ molecules along the line of sight. It is argued that the molecularization of the bulk of the gas mass could be due to the concentration of gas in the inner regions of the galactic disks and the resulting high gas pressures and relative low star-formation efficiencies, as is indeed observed in galaxies with high M _mol/M _HI ratios.     

Evolution of the star-formation rate and extinction in disk galaxies at high <Emphasis Type="Italic">z</Emphasis>

A model for the evolution of disk galaxies is used to investigate evolutionary variations in the star-formation rate (SFR) and extinction in disk galaxies beginning from their formation. The results obtained are compared with observational estimates of the SFRe and extinction at cosmological redshifts z ≤ 7. The formof themass-radius relation for disk galaxies is discussed. It is proposed that the mass-radius relation is a consequence of a mass-angular momentum relation. Analysis of the influence of the form of the mass-radius relation on the evolutionary variations in the SFR and extinction testify that this influence is very small. The SFRs and extinctions at various redshifts z obtained in the models are in agreement with observations.     

Age and interstellar absorption in young star-formation regions in the galaxies NGC 1068, NGC 4449, NGC 4490, NGC 4631, and NGC 4656/57 derived from multicolor photometry

We have compared the results of multicolor U BV R and Hα photometry for 169 young star-formation complexes in five galaxies using a grid of evolutionary models for young star clusters. The ages and interstellar absorptions are estimated for 102 star-formation complexes with the standard uncertainties σ _t = 0.30 dex and $ \sigma _{A_V } = 0.45^m We have compared the results of multicolor U BV R and Hα photometry for 169 young star-formation complexes in five galaxies using a grid of evolutionary models for young star clusters. The ages and interstellar absorptions are estimated for 102 star-formation complexes with the standard uncertainties σ _t = 0.30 dex and . The accuracies of these parameters were verified using numerical simulations. Original Russian Text ? A.S. Gusev, V.I. Myakutin, A.E. Piskunov, F.K. Sakhibov, M.S. Khramtsova, 2008, published in Astronomicheskiĭ Zhurnal, 2008, Vol. 85, No. 9, pp. 794–809.     

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.     

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.     

Evolution of galaxies and the Tully-Fisher relation

We study the evolution of the [O/Fe]-[Fe/H] relation and the dependence of the iron abundance on distance from the galactic plane z in a one-zone model for a disk galaxy, starting from the beginning of star formation. We obtain good agreement with the observational data, including, for the first time, agreement for the [Fe/H]-z relation out to heights of 16 kpc. We also study the influence of the presence of dark matter in the galaxies on the star-formation rate. Comparison of the observed luminosity of the Galaxy with the model prediction places constraints on the fractional mass of dark matter, which cannot be much larger than the fractional mass of visible matter, at least within the assumed radius of the Galaxy, ～20 kpc. We studied the evolution of disk galaxies with various masses, which should obey the Tully-Fisher relation, M ? R². The Tully-Fisher relation can be explained as a combination of a selection effect related to the observed surface brightnesses of galaxies with large radii and the conditions for the formation for elliptical galaxies.     

Stellar structure of irregular galaxies: Edge-on galaxies

Stellar photometry obtained using the Hubble Space Telescope is used to study the distributions of the number densities of stars of various ages in 12 irregular and dwarf spiral galaxies viewed edge-on. Two subsystems can be distinguished in all the galaxies: a thin disk comprised of young stars and a thick disk containing a large fraction of old stars (primarily red giants) in the system. Variations of the stellar number density in the thin and thick disks in the Z direction perpendicular to the plane of the galaxy follow an exponential law. The size of the thin disk corresponds to the visible size of the galaxy at the μ = 25 mag/arcsec² isophote, while the thick disk is a factor of two to three larger. In addition to a thick disk, the massive irregular galaxy M82 also has a more extended stellar halo that is flattened at the galactic poles. The results of our previous study of 12 face-on galaxies are used together with the new results presented here to construct an empirical model for the stellar structure of irregular galaxies. Original Russian Text ? N.A. Tikhonov, 2006, published in Astronomicheskiĭ Zhurnal, 2006, Vol. 83, No. 7, pp. 579–588.     

The role of external factors in the evolution of galaxies

We consider the evolution of galaxies in dense galactic clusters. Observations and theoretical estimates indicate that this evolution may be specified to a large extent by collisions between galaxies, as well as interactions between the gaseous components of disk galaxies and intergalactic gas. We analyze collisions between disk galaxies with gaseous components using a simple model based on a comparison of the duration of a collision and the characteristic cooling time for the gas heated by the collision, and also of the relative masses of stars and gas in the colliding disk galaxies. This model is used to analyze scenarios for collisions between disk galaxies with various masses as a function of their relative velocities. Our analysis indicates that galaxies can merge, lose one or both of their gaseous components, or totally disintegrate as a result of a collision; ultimately, a new galaxy may form from the gas lost by the colliding galaxies. Disk galaxies with mass M _G and velocities exceeding ～300 (M _G/10¹⁰ M _⊙)^1/2 km/s in intergalactic gas in clusters with densities ～10^?27 g/cm³ can lose their gas due to the pressure of inflowing intergalactic gas, thereby developing into E(SO) galaxies.     

The Individual Thicknesses of Stellar Disks of Galaxies Viewed at Arbitrary Angles

A method for determining the thickness of the stellar disk of a galaxy from a photometric image of the galaxy in the plane of the sky is proposed and justified. The method can be applied to determine the thickness of plane-parallel exponential disks with an arbitrary, radius-independent, luminosity distribution perpendicular to the plane of the disk J(r, z) = exp(-r/h)f(z). A special feature and advantage of the method is that it enables determination of the thicknesses of disks viewed at arbitrary angles to the plane of the sky (but not strictly edge-on or face-on). The key idea of the method is finding the true inclinations of galaxies viewed at arbitrary angles not from their isophotes, but instead from the azimuthal distribution of the exponential parameter h. The difference between the inclination determined in the traditional way using the isophotes and the true inclination enables estimation of the thickness of the disk. The effectiveness of the method for determining the inclinations of plane-parallel disks is confirmed using a sample of model isothermal galactic disks: I(r, z) = I₀ exp(-r/h)sech²(z/z₀). The inclinations of the planes of the model galaxies to the line of sight and the relative thicknesses in the model sample vary arbitrarily, making it possible to determine the limits of applicability of the method: z₀/h < 0.7 and 10° <i < 75°.A sample of 44 piecewise-exponential disks of galaxies of the southern sky clusters is used to illustrate the application of the technique to observational data. Comparing the distribution of inclinations calculated using the new method and the traditional isophote method shows that the new method yields a more uniform distribution of inclinations to the plane of the sky for the sample galaxies. The derived average disk thicknesses and the disk-thickness distributions are consistent with statistical estimates and observational data from the literature for samples of galaxies viewed edge-on

     

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.     

Early-type disk galaxies: Structure and kinematics

Spectroscopic observations of three lenticular (S0) galaxies (NGC 1167, NGC 4150, and NGC 6340) and one SBa galaxy (NGC 2273) have been taken with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences aimed to study the structure and kinematic properties of early-type disk galaxies. The radial profiles of the stellar radial velocities and the velocity dispersion are measured. N-body simulations are used to construct dynamical models of galaxies containing a stellar disk, bulge, and halo. The masses of individual components are estimated formaximum-mass disk models. A comparison of models with estimated rotational velocities and the stellar velocity dispersion suggests that the stellar disks in lenticular galaxies are “overheated”; i.e., there is a significant excess velocity dispersion over the minimum level required to maintain the stability of the disk. This supports the hypothesis that the stellar disks of S0 galaxies were subject to strong gravitational perturbations. The relative thickness of the stellar disks in the S0 galaxies considered substantially exceed the typical disk thickness of spiral galaxies.     

Star-forming regions in the ring galaxies NGC 5585 and IC 1525

We use UBVRI CCD photometry to study star-forming regions (SFRs) in the galaxies NGC 5585 and IC 1525. The observations were acquired with the 1.5-m telescope of the Mt. Maidanak Observatory of the Astronomical Institute of the Uzbek Academy of Sciences (Uzbekistan), with seeing of 0.8″–1.8″. We identified 47 SFRs in NGC 5585 and 16 SFRs in IC 1525. We estimated the ages and internal extinctions of the SFRs using the PEGASE2 evolution models. The sizes of the SFRs were also determined. We discuss in detail the techniques applied to evaluate the SFR parameters from photometric analysis. The age range for the studied SFRs is (2–40) × 10⁶ yrs, and the internal extinctions are A(V ) ≤ 1.5m. The age distributions of the SFRs in both galaxies are typical of stellar systems with intense, extended star formation. The internal extinction in the SFRs decreases with distance from the galactic centers: A(V ) ∝ −r. For both galaxies, the scale length for the decrease of the dust surface density, estimated from the A(V )−r relation for SFRs, is close to the scale length for the disk brightness decrease in the V and R bands. Relatively larger and older SFRs are observed in the galaxies’ rings, while such SFRs are not found in the spiral arms. We detected different SFR parameters for different spiral arms of NGC 5585.     

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.     

Optical variability of the NLS1 galaxy Ark 564 in 1987–2004

We present the results of our U BV R _c I _c photoelectric and CCD photometry of the NLS1 galaxy Ark 564 in 1987–2004. Between late 1997 and 2002, a slight increase in brightness was observed followed by a small but systematic decline until 2004. Ark 564 has a low variability amplitude, within 0.2^m in the U BV R _c I _c bands. The galaxy’s spectral energy distribution at 3600–9000 Å is unlike those for Seyfert 1 (Syl) galaxies. A delay of the Ic flux relative to the B flux of about three days was found, which is in agreement with the standard disk model. This delay probably reflects the geometric size of the region that emits the observed continuum.     

The role of close passages of galaxies and the asymmetry of their dark haloes in the formation of their spiral patterns

The influence of close passages of galaxies on the shapes of disk galaxies and the distribution of stars in them is studied for several types of interactions in the framework of the restricted N-body problem. Depending on the conditions adopted, either two spiral density waves or ring structures are formed in the stellar disk of the galaxy. These structures can generate star formation fronts with the corresponding shape, as are observed in disk galaxies. Our calculations can also be applied to study the influence of the passage of a nearby star on a protoplanetary disk. The formation of ring structures there could specify the type of planet formation in the outer regions of the planetary system and the distribution of semimajor axes for the planetary orbits. We use the same model to study the generation and evolution of spiral density waves in the stellar disks of galaxies as a result of the recently found asymmetry of the gravitational potential in the massive dark haloes in disk galaxies. The dipole component of the gravitational field of the halo can continuously permanently generate the spiral structure in disk galaxies.     

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.