arXiv subjects

A. V. Zasov

Publications and source records attributed to A. V. Zasov.

At least 37 records · Page 2Linked to original sources

arXiv2008

The Pressure of an Equilibrium Interstellar Medium in Galactic Disks

Based on an axisymmetric galactic disk model, we estimate the equilibrium gas pressure P/k in the disk plane as a function of the galactocentric distance R for several galaxies (MW, M33, M51, M81, M100, M101, M106, and the SMC). For this purpose, we solve a self-consistent system of equations by taking into account the gas self-gravity and the presence of a dark pseudo-isothermal halo. We assume that the turbulent velocity dispersions of the atomic and molecular gases are fixed and that the velocity dispersion of the old stellar disk corresponds to its marginal stability (except for the Galaxy and the SMC). We also consider a model with a constant disk thickness. Of the listed galaxies, the SMC and M51 have the highest pressure at a given relative radius R/R_25, while M81 has the lowest pressure. The pressure dependence of the relative molecular gas fraction confirms the existence of a positive correlation between these quantities, but it is not so distinct as that obtained previously when the pressure was estimated very roughly. This dependence breaks down for the inner regions of M81 and M106, probably because the gas pressure has been underestimated in the bulge region. We discuss the possible effects of factors other than the pressure affecting the relative content of molecular gas in the galaxies under consideration.

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arXiv2008

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 AstrophysicalObservatory 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.

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arXiv2007

Midplane Gas Density and the Schmidt Law

The thickness of isothermal gaseous layers and their midplane volume densities ρ_{gas}(R) were calculated for several spiral and LSB galaxies by solving the self-consistent equilibrium equations for gaseous discs embedded into a stellar one. The self-gravity of the gas and influence of dark halo on the disk thickness were taken into account. The resulting midplane volume densities of spiral galaxies were compared with the azimuthally averaged star formation rate SFR to verify the feasibility and universality of the Schmidt law SFR ~ ρ_{gas}^n.

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arXiv2007

Gas Density and the Volume Schmidt Law for Spiral Galaxies

The thickness of the equilibrium isothermal gaseous layers and their volume densities ρ_{gas}(R) in the disc midplane are calculated for 7 spiral galaxies (including our Galaxy) in the frame of self-consistent axisymmetric model. Local velocity dispersions of stellar discs were assumed to be close to marginal values necessary for the discs to be in a stable equilibrium state. Under this condition the stellar discs of at least 5 of 7 galaxies reveal a flaring. Their volume densities decrease with R faster than ρ_{gas}, and, as a result, the gas dominates by the density at the disc periphery. Comparison of the azimuthally averaged star formation rate SFR with the gas density shows that there is no universal Schmidt law SFR ρ_{gas}^n, common to all galaxies. Nevertheless, SFR in different galaxies reveals better correlation with the volume gas density than with the column one. Parameter n in the Schmidt law SFR ρ_{gas}^n, formally calculated by the least square method, lies within 0.8-2.4 range and it's mean value is close to 1.5. Values of n calculated for molecular gas only are characterized by large dispersion, but their mean value is close to 1. Hence the smaller ρ_{gas} the less is a fraction of gas actively taking part in the process of star formation.

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arXiv2006

The Star-Formation Efficiency and Density of the Disks of Spiral Galaxies

For four well studied spiral galaxies (M33, M81, M100 and M101) we consider the dependencies of star formation rate (SFR) and star formation efficiency (SFE = SFR/M_{gas}) both on the radial distance R and on some kinematic parameters of galactic discs. To estimate SFR(R) we used a combined UV+FIR method based on the UV profiles corrected for the interstellar extinction presented by Boissier et al, 2004. It is demonstrated that the most tight correlation, similar for all four galaxies we considered, exists between the local SFE and the surface brightness (density) of discs at a given R (beyond their central regions). To account for the observed surface densities of discs in the frame of a simple conservative model of evolution of gas content (a toy model) it is necessary for the local parameter N in the Schmidt law for a disc (SFR \sim σ_{gas}^N) not to exceed unit. Only in this case it is possible to reconcile the observed dependencies σ_{gas}(R) and SFE(R) assuming a gas accretion, more intense in the inner regions of galaxies.

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arXiv2006

Galaxies with unusually 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 considered. The sample galaxies do not differ systematically from other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the total mass of gas in comparison with galaxies of similar linear sizes and disk angular momentum. This suggests that the overabundance of $H_2$ is due to transition of HI to H_2. Galaxies with bars and active nuclei are found more frequently among galaxies which have M_{mol} estimates in CISM. In a small fraction of galaxies, high M_{mol}/M_{HI} ratios are caused by the overestimation of M_{mol} due to a low conversion factor for the translation of CO-line intensities into the number of H_2 molecules along the line of sight. It is argued that the "molecularization" of the bulk of the gas mass could be due 1) to the concentration of gas in the inner regions of the galactic disks, resulting to a high gas pressure and 2) to relatively low star-formation rate per unit mass of molecular gas which indeed takes place in galaxies with high M_{mol}/M_{HI} ratios.

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arXiv2005

The Gas Content in Galactic Disks: Correlation with Kinematics

We consider the relationship between the total HI mass in late-type galaxies and the kinematic properties of their disks. The mass $M_HI$ for galaxies with a wide variety of properties, from dwarf dIrr galaxies with active star formation to giant low-brightness galaxies, is shown to correlate with the product $V_c R_0$ ($V_c$ is the rotational velocity, and $R_0$ is the radial photometric disks scale length), which characterizes the specific angular momentum of the disk. This relationship, along with the anticorrelation between the relative mass of HI in a galaxy and $V_c$, can be explained in terms of the previously made assumption that the gas density in the disks of most galaxies is maintained at a level close to the threshold (marginal) stability of a gaseous layer to local gravitational perturbations. In this case, the regulation mechanism of the star formation rate associated with the growth of local gravitational instability in the gaseous layer must play a crucial role in the evolution of the gas content in the galactic disk.

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arXiv2005

Super-Massive Black Holes and Dark Halos

The relations between masses of Super-Massive Black Holes, Mbh, in galactic nuclei, maximal rotational velocities, Vm, and indicative masses, Mi, of galaxies are studied for galaxies with the available rotation curves. Mbh correlates with both Vm and Mi, although much weaker than with the central velocity dispersion S0. Masses Mbh for early-type galaxies (S0-Sab), which usually possess more luminous bulges, are, on average, larger than those for late-type galaxies with similar velocities of rotation. It enables to conclude that masses of black holes are determined mainly by the bulge properties, and - much weaker - by dark haloes of galaxies.

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arXiv2004

The orientation parameters and rotation curves of 15 spiral galaxies

We analyzed ionized gas motion and disk orientation parameters for 15 spiral galaxies. Their velocity fields were measured with the H-alpha emission line by using the Fabry-Perot interferometer at the 6m telescope of SAO RAS. Special attention is paid to the problem of estimating the position angle of the major axis (PA_0) and the inclination (i) of a disk, which strongly affect the derived circular rotation velocity. We discuss and compare different methods of obtaining these parameters from kinematic and photometric observations, taking into account the presence of regular velocity (brightness) perturbations caused by spiral density waves. It is shown that the commonly used method of tilted rings may lead to systematic errors in the estimation of orientation parameters (and hence of circular velocity) being applied to galaxies with an ordered spiral structure. Instead we recommend using an assumption of constancy of i and PA_0 along a radius, to estimate these parameters. For each galaxy of our sample we present monochromatic H-alpha- and continuum maps, velocity fields of ionized gas, and the mean rotation curves in the frame of a model of pure circular gas motion. Significant deviations from circular motion with amplitudes of several tens of km/s (or higher) are found in almost all galaxies. The character and possible nature of the non-circular motion are briefly discussed.

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arXiv2004

Disk-to-halo mass ratio evaluations based on the numerical models of collisionless disks

We propose that the lower bound of the stellar radial velocity dispersion c_r of an equilibrium stellar disk is determined by the gravitational stability condition. We compared the estimates of stellar velocity dispersion at radii r>(1.5 - 2)R_0 (where R_0 is the photometric radial scalelength of a disk), found in the literature, with the minimal values of c_r necessary for the disk to be in a stable state, using the results of numerical simulations of 3D collisional disks. This approach enables to estimate an upper limit of the local surface density and (if R_0 is known) a total masses of a disk and a dark halo. We argue that the old stellar disks of spiral galaxies with active star formation usually have the velocity dispersions which are close to the expected marginal values. A rough values of disk-to-total mass ratios (within the fixed radius) are found for about twenty spiral galaxies. Unlike spirals, the disks of the ``red'' Sa--S0 galaxies are evidently ``overheated'': their radial dispersion of velocities at r ~ 2 R_0 exceeds significantly the marginal values for gravitational stability.

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arXiv2003

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 a 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 thousands and the evolution of the simulated models 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 periods of rotation. The radial behavior of the Toomre stability parameter $Q_T (r)$ for the final state of the disk is estimated. Numerical 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 radial velocity dispersion $c_r$ of more than a factor of 1.5. It is suggested that the approximate constancy of $Q_T \simeq 1.2 -- 1.5$ for $r\simeq (1÷2)\times L$ (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.

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arXiv2002

A Thickness of Stellar Disks of Edge-on Galaxies and Position of Their Truncation Radii

The relationship between the geometrical properties of stellar disks (a flatness and truncation radius) and the disk kinematics are considered for edge-on galaxies. It is shown that the observed thickness of the disks and the approximate constancy of their thickness along the radius well agrees with the condition of their marginal local gravitational stability. As a consequence, those galaxies whose disks are thinner should harbor more massive dark haloes. The correlation between the de-projeced central brightness of the disks and their flatness is found (the low surface brightness disks tend to be the thinniest ones). We also show that positions of observed photometrically determined truncation radii $R_{cut}$ for the stellar disks support the idea of marginal local gravitational stability of gaseous protodisks at $R =R_{cut}$, and hence the steepening of photometric profiles may be a result of too inefficient star formation beyond $R_{cut}$.

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arXiv2002

Relation Between the Thickness of Stellar Disks and the Relative Mass of Dark Halo in Galaxies

We consider a thickness of stellar disks of late-type galaxies by analyzing the R and K_s band photometric profiles for two independent samples of edge-on galaxies. The main goal is to verify a hypotesis that a thickness of old stellar disks is related to the relative masses of the spherical and disk components of galaxies. We confirm that the radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with the increasing of total mass-to-light ratio of the galaxies, which characterize the contribution of dark halo to the total mass, and with the decreasing of central deprojected disk brightness (surface density). Our results are in good agreement with numerical models of collisionless disks evolved from subcritical velocity dispersion state to a marginally stable equilibrium state. This suggests that in most galaxies the vertical stellar velocity dispersion, which determine the equilibrium disk thickness, is close to the minimum value, that ensures disk stability. The thinnest edge-on disks appear to be low brightness galaxies (after deprojection) in which a dark halo mass far exceeds a mass of the stellar disk.

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arXiv2001

Restoring the full velocity field in the gaseous disk ofthe spiral galaxy NGC 157

We analyse the line-of-sight velocity field of ionized gas in the spiral galaxy NGC 157 which has been obtained in the Hαemission at the 6m telescope of SAO RAS. The existence of systematic deviations of the observed gas velocities from pure circular motion is shown. A detailed investigation of these deviations is undertaken by applying a Fourier analysis of the azimuthal distributions of the line-of-sight velocities at different distances from the galactic center. As a result of the analysis, all the main parameters of the wave spiral pattern are determined: the corotation radius, the amplitudes and phases of the gas velocity perturbations at different radii, and the velocity of circular rotation of the disk corrected for the velocity perturbations due to spiral arms. At a high confidence level, the presence of the two giant anticyclones in the reference frame rotating with the spiral pattern is shown; their sizes and the localization of their centers are consistent with the results of the analytic theory and of numerical simulations. Besides the anticyclones, the existence of cyclones in residual velocity fields of spiral galaxies is predicted. In the reference frame rotating with the spiral pattern these cyclones have to reveal themselves in galaxies where a radial gradient of azimuthal residual velocity is steeper than that of the rotation velocity (abridged).

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arXiv2000

The determination of mass of stellar disks of galaxies from the kinematic data

Different ways of the determination of masses of galactic disks, based on the kinematic data, are briefly discussed. The analysis of the rotation curves which reach maximum inside of a disk, and N-body modeling, reproducing the rotation curves and stellar velocity dispersion of real galaxies, enable to conclude that the mass of a disk is usually significantly less than the total mass of spherical components (bulge + dark halo) inside of optical borders of a galaxy, although the exceptions also exist.

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arXiv2000

Gas motions in the plane of the spiral galaxy NGC 3631

The velocity field of the nearly face-on galaxy NGC 3631, derived from observations in the H alpha line and the HI radio line, is analysed to study perturbations related to the spiral structure of the galaxy. We confirm our previous conclusion that the line-of-sight velocity field gives evidence of the wave nature of the observed two-armed spiral structure. Fourier analysis of the observed velocity field is used to determine the location of corotation of the spiral structure of this galaxy, and the radius of corotation R_c is found to be about 42 arcsec, or 3.2 kpc. The vector velocity field of the gas in the plane of the disc is restored, and taking into account that we previously investigated vertical motions, we now have a full 3D gaseous velocity field of the galaxy. We show clear evidence of the existence of two anticyclonic and four cyclonic vortices near corotation in a frame of reference rotating with the spiral pattern. The centres of the anticyclones lie between the observed spiral arms. The cyclones lie close to the observed spirals, but their centres are shifted from the maxima in brightness.

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arXiv2000

Possibly interacting Vorontsov-Velyaminov galaxies. I. Observations of VV432, VV543 and VV747

Among the galaxies which were included in the Atlas and Catalogue of Interacting galaxies by Vorontsov-Velyaminov (hereafter VV) as multiple systems ("nests", "chains" and similarly looking systems), there are many objects, where the interaction is not evident. Some of them are single objects, including low-mass galaxies with active star formation (SF). In this work we present the description of observations and results of the long-slit spectrophotometry with the Russian 6-m telescope of three VV-galaxies looking like double or multiple systems, and HI-observations of one of them in order to elucidate their nature, determine their metallicity, kinematic properties and the evolution status. Galaxies VV432 and VV747 are found to be dwarf systems with low oxygen abundance (O/H = 1/22 and 1/12 of the solar value, respectively). Their velocity curves indicate quite slow rotation with respective maximum velocities of about 60 and 80 km/s, in agreement with their low luminosities. The distance to VV432 is rather uncertain. If it is a member of Virgo Cluster, this is the most metal-deficient known galaxy of this aggregate. For galaxy VV543 the measured emission-line redshift 0.047 appeared ten times larger than it was given in the original paper and is cited in databases. This "system" evidently represents an optical pair of two galaxies with large velocity difference. The fainter western component is an HII-galaxy, while the brighter one is an absorption-line early-type galaxy with the radial velocity being 1600 km/s lower.

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arXiv2000

Giant Cyclones in Gaseous Discs of Spiral Galaxies

We report detection of giant cyclonic vortices in the gaseous disc of the spiral galaxy NGC 3631 in the reference frame rotating with the spiral pattern. A presence of such structures was predicted by the authors for galaxies, where the radial gradient of the perturbed velocity exceeds that of the rotational velocity. This situation really takes place in NGC 3631.

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