Searcharxiv⌕ Search

arXiv subjects

A. V. Zasov

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

At least 19 recordsLinked to original sources

Star formation in low-density regions of galactic disks

We argue that star formation in the disks of low-surface-brightness (LSB) galaxies shares a similar nature with that occurring in the far outer regions of normal-brightness spiral galaxies, such as those with the extended ultraviolet (XUV) disks. In both cases, stars are born in gravitationally stable disks with an extremely low average gas density (on kiloparsec scales), and the efficiency of this process depends on a disk brightness in a similar way. Processes which can stimulate star formation under these conditions are shortly discussed.

astro-ph.GA↗

Galaxies with Declining Rotation Curves

A sample of 22 spiral galaxies compiled from published data is studied. The galaxy rotation curves pass through a maximum distance of more than $\sim 1$ kpc from the center with a subsequent decrease in the rotation velocity. The galaxy position in the Tully-Fisher (TF) and baryonic Tully-Fisher (BTF) diagrams show that the values of maximum rotation velocities are located on the same sequence with other galaxies, while the velocities at the disk periphery for some galaxies are significantly lower than the expected values for a given mass or luminosity. Thus, the decrease in the rotation curve can be associated with a reduced contribution of the dark halo to the rotation velocity. For seven galaxies with the longest rotation curves, the disk mass was estimated to be with the dark halo (Newtonian model) and without the halo (modified Newtonian dynamics (MOND) model). In four of the galaxies, the MOND model encounters difficulties in interpreting the rotation curve: in order to be consistent with the observations, the MOND parameter $a_0$ should differ significantly from the expected value $a_0 \sim 10^{-8}$ cm/s$^2$, while the disk mass exceeds the value based on IR photometry and maximum disk model. The conflict with MOND is the most significant for NGC 157.

astro-ph.GA↗

Dark matter in galaxies

Dark matter in galaxies, its abundance, and its distribution remain a subject of long-standing discussion, especially in view of the fact that neither dark matter particles nor dark matter bodies have yet been found. Experts' opinions range from a very large number of completely dark galaxies exist to nonbaryonic dark matter does not exist at all in any significant amounts. We discuss astronomical evidence for the existence of dark matter and its connection with visible matter and examine attempts to estimate its mass and distribution in galaxies from photometry, dynamics, gravitational lensing, and other observations (the cosmological aspects of the existence of dark matter are not considered in this review). In our view, the presence of dark matter in and around galaxies is a well-established fact. We conclude with an overview of mechanisms by which a dark halo can influence intragalactic processes.

astro-ph.GA↗

The study of two barred galaxies with curious kinematical features

We performed long-slit spectral observations of two SB-type galaxies: NGC 5347, UGC 1344. They were previously suspected as the galaxies with unusually low mass-to-light ratios (on the ground of mass estimates from the Hi linewidths), which are in conflict with their observed colours. The observations were conducted at the Russian 6-m telescope. The aim of the study was to clarify the kinematics and structure, as well as the properties of stellar populations of the galaxies. The results of observations disproved the peculiarly low mass-to-light ratios of both galaxies. The most probable reasons of underestimation of their masses are discussed. We tried to reproduce the main observed features of kinematical profiles of the galaxies in the N-body simulations of barred galaxies. We found that both galaxies possess central components of different structure. Indeed, the age and velocity dispersion of stellar population in NGC 5347 are low in its innermost part in comparison to that of the bulge or a bar, which agrees with the presence of nuclear kinematically decoupled disc. It probably was formed due to the bar which supplied the inner region with gas. The kinematical profiles of the second galaxy UGC 1344 give evidences in favour of the central peanut-shaped bulge. In spite of the different luminosities of the two galaxies, they possess nearly equal (close to solar) central stellar abundance and the flattening of the stellar metallicity gradient in the bar regions. However, in the less luminous NGC 5347 the mean stellar age is younger than that in UGC 1344.

astro-ph.GA↗

Giant molecular clouds in M 33: are they susceptible to dynamical friction?

Most of giant molecular clouds (GMCs) in M 33 are connected with spiral-like gaseous arms (filaments) with the exception of the inner 2 kpc region where the link between the arms and GMCs disappears (see Tosaki et al. 2011). We check whether it may be caused by the dynamic friction retarding the clouds. Using semi-analytical model for this galaxy we calculate the dynamics of GMCs of different masses situated at different initial galactocentric distances in the disk plane. We demonstrate that the dynamical friction may really change the orbits of GMCs in the central 2 kpc-size region. However in this case the typical lifetimes of GMCs should be close to or greater than $10^8$~yr, which is larger than the usually accepted values.

astro-ph.GA↗

Review of scientific topics for Millimetron space observatory

This paper describes outstanding issues in astrophysics and cosmology that can be solved by astronomical observations in a broad spectral range from far infrared to millimeter wavelengths. The discussed problems related to the formation of stars and planets, galaxies and the interstellar medium, studies of black holes and the development of the cosmological model can be addressed by the planned space observatory Millimetron (the "Spectr-M" project) equipped with a cooled 10-m mirror. Millimetron can operate both as a single-dish telescope and as a part of a space-ground interferometer with very long baseline.

astro-ph.IM↗

Supermassive Black Holes and Nuclear Star Clusters: Connection with the Host Galaxy Kinematics and Color

We consider the relationship between the masses of the central objects in disky galaxies - supermassive black holes (SMBHs) and nuclear star clusters (NCs) - and various parameters of parent galaxies: velocity of rotation $V_{(2)}$ at $R= 2$ kpc, maximal velocity of rotation $V_{\textrm{max}}$, the indicative dynamical mass $M_{25}$, the total mass of the stellar population $M_{*}$, and the total color index ($B{-}V$). The mass of nuclear clusters $M_{\rm nc}$ correlates more closely with the kinematic parameters and total mass of galaxies than the mass of central black holes $M_{\rm bh}$, including correlation with the velocity $V_{\textrm{max}}$, which characterizes the virial mass of the dark halo. On average, lenticular galaxies are distinguished by higher masses $M_{\rm bh}$ compared to other types of galaxies with similar characteristics. The dependence of masses of central objects on the color index is bimodal: galaxies of the red group (red-sequence), which have ($B{-}V) > 0.6{-}0.7$, being mostly early-type galaxies, differ from bluer galaxies, by higher values of $M_{\rm nc}$ and $M_{\rm bh}$. The red-group galaxies have systematically higher $M_{\rm bh}$ values, even when the host-galaxy parameters are similar. In contrast, in the case of nuclear stellar clusters, the "blue" and "red" galaxies form unified sequences at the diagrams. 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 mass exceeds $10^6 {-} 10^7 M_{\odot}$ (depending on the total mass of a galaxy). Most of galaxies with $M_{\rm bh}> 10^7 M_{\odot}$ are lenticular galaxies (types S0, E/S0), whose disks are usually devoid of gas.

astro-ph.GA↗

WSRT observations and surface photometry of two unusual spiral galaxies

We discuss the results of a mass decomposition of two spiral galaxies, NGC 6824 and UGC 11919. In a previous analysis of the Hyperleda catalog, the galaxies were identified as having a peculiar dynamical $M/L$. The aim of this study is to confirm or disprove the preliminary findings, indicating a non-standard stellar initial mass function (IMF) for the galaxies. The surface photometry in B, V, and R bands was carried out with the Apache Point 0.5-m telescope and the \ion{H}{I} data cubes were obtained with the Westerbork Synthesis Radio Telescope (WSRT). Photometric profiles were decomposed into bulge and exponential disk components. Using the obtained \ion{H}{I} data cubes, rotation curves of both galaxies were constructed. Employing the photometric profiles, the mass distribution of the galaxies was decomposed into mass components: bulge, stellar disk, gas, and pseudo-isothermal dark halo. We conclude that NGC 6824 possesses a stellar disk with mass-to-light ratio $(M/L_B)_{\rm disk} = 2.5$, in agreement with its color $(B-V)_0$. On the contrary, UGC 11919 appears to have a very lightweight disk. Its dynamically estimated mass corresponds to a low stellar disk mass-to-light ratio $(M/L_B)_{\rm disk} \approx 0.5$. Under standard assumptions, this ratio does not agree with the relatively red color of the disk, while a bottom light stellar initial mass function is needed to explain the observations.

astro-ph.CO↗

The relationship between the neutral hydrogen and dark mass in the galaxies

Starting from Bosma' (1981) paper, it was demonstrated by different authors that the observed shape of rotation curves of many spiral galaxies can be explained if to assume that the radial density distribution of the dark matter is correlated with the distribution of $HI$: the column densities of the dark matter and $HI$ are proportional. We show that this correlation is obviously to be an artifact and can be explained by assuming that the gas surface density is approximately equal or in general proportional to the critical density for the local gravitational stability of gaseous layer.

astro-ph.CO↗

Interaction between collisionless galactic discs and nonaxissymmetric dark matter haloes

Using $N$-body simulations ($N\sim 10^6 - 10^7$), we examine how a non-axisymmetric dark halo affects the dynamical evolution of the structure in collisionless (stellar) discs. We demonstrate how the model parameters such as mass of the halo, initial conditions in the disc and the halo axes ratio affect morphology and kinematics of the stellar discs. We show that a non-axisymmetric halo can generate a large-scale spiral density pattern in the embedded stellar disc. The pattern is observed in the disc for many periods of its revolution, even if the disc is gravitationally over-stable. The growth of the spiral arms is not accompanied by significant dynamical heating of the disc, irrelevant to its initial parameters. We also investigate transformation of the dark halo's shape driven by the long-lived spiral pattern in the disc . We show that the analysis of the velocity field in the stellar disc and in the spiral pattern gives us a possibility to figure out the spatial orientation of the triaxial-shaped dark halo and to measure the triaxiality.

astro-ph.CO↗

Kinematics and Stellar Disk Modeling of Lenticular Galaxies

We present the results of spectroscopic observations of three S0-Sa galaxies: NGC338, NGC 3245, and NGC 5440 at the SAO RAS 6-m BTA telescope. The radial distributions of the line-of-sight velocities and radial velocity dispersions of stars and ionized gas were obtained, and rotation curves of galaxies were computed. We construct the numerical dynamic N-body galaxy models with N > 10^6 point masses. The models include three components: a "live" bulge, a collisionless disk, dynamically evolving to the marginally stable state, and a pseudo-isothermal dark halo. The estimates of radial velocities and velocity dispersions of stars obtained from observations are compared with model estimates, projected onto the line of sight. We show that the disks of NGC 5440 and the outer regions of NGC 338 are dynamically overheated. Taking into account the previously obtained observations, we conclude that the dynamic heating of the disk is present in a large number of early-type disk galaxies, and it seems to ensue from the external effects. The estimates of the disk mass and relative mass of the dark halo are given for seven galaxies, observed at the BTA.

astro-ph.GA↗

Do Disk Galaxies with Abnormally Low Mass-to-Light Ratios Exist?

We performed the photometric B, V and R observations of nine disk galaxies that were suspected in having abnormally low total mass-to-light (M/L) ratios for their observed color indices. We use our surface photometry data to analyze the possible reasons for the anomalous M/L. We infer that in most cases this is a result of errors in photometry or rotational velocity, however for some galaxies we cannot exclude the real peculiarities of the galactic stellar population. The comparison of the photometric and dynamical mass estimates in the disk shows that the low M/L values for a given color of disks are probably real for a few our galaxies: NGC 4826 (Sab), NGC 5347 (Sab), and NGC 6814 (Sb). The small number of such galaxies suggests that the stellar initial mass function is indeed universal, and that only a small fraction of galaxies may have a non-typical low-mass star depleted initial mass function. Such galaxies require more careful studies for understanding their star formation history.

astro-ph.CO↗

On the mass and the density of stellar disk of M33

The disk surface density of the nearby spiral galaxy M33 is estimated assuming that it is marginally stable against gravitational perturbations. For this purpose we used the radial profile of line-of-sight velocity dispersion of the disk planetary nebulae obtained by Ciardullo et al. (2004). The surface density profile we obtained is characterized by the radial scalelength which is close to the photometrical one and is in a good agreement with the rotation curve of M33 and with the mass-to-light ratio corresponding to the observed color indices. However at the galactocentric distance $r>7$ kpc the dynamical overheating of the disk remains quite possible. A thickness of the stellar disk of M33 should increase outwards. The dark halo mass exceeds the mass of the disk at $r>$ 7 kpc. The obtained radial profile of the disk surface density and the radial gradient of $O/H$ are used to calculate the effective oxygen yield $Y_{eff}$ in the frame of the instantaneous recycling approximation. It is shown that $Y_{eff}$ increases with radius which may indicate that the role of accretion of metal-poor gas in the chemical evolution of interstellar medium decreases outwards.

astro-ph.CO↗

Supermassive Black Holes and Kinematics of Disc Galaxies

The statistical relations between the masses of supermassive black holes (SMBHs) in disk galaxies and the kinematic properties of their host galaxies are analyzed. We use the radial velocity profiles for several galaxies obtained earlier at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences parallel with the data for other galaxies taken from the literature. We demonstrate that the SMBH masses correlate well with the velocities of rotation of disks at a fixed distance R \approx 1 kpc (V1), which characterize the mean density of the central region of the galaxy. The SMBH masses correlate appreciably weaker with the asymptotic velocity at large distances from the center and with the angular velocity at the optical radius R_{25}. We suggest that the growth of the SMBH occurs inside of the forming "classical" bulge during a monolithic collapse of gas in the central kpc-size region of the protogalaxy. We have also found a correlation between the SMBH mass and the total (indicative) mass of the galaxy M_{25} within the optical radius R_{25}, which includes both baryonic and "dark" mass. The masses of the nuclear star clusters in early-type disk galaxies (based on the catalog of Seth et al.) are also scaled with the dynamical mass M_{25}, whereas the correlations with the luminosity and velocity of rotation of galaxies are practically absent for them. For a given M_{25} the masses of the nuclear clusters are, on average, nearly order of magnitude higher in S0-Sbc galaxies than in late-type galaxies.

astro-ph.CO↗

Gas density and star formation in the rarified regions of discs of normal and LSB galaxies

We calculated the radial profiles of the azimuthally averaged midplane gas volume density $ρ_g$ for 11 high surface brightness (HSB) spiral galaxies, 7 low surface brightness (LSB) galaxies and 3 S0 galaxies assuming their gaseous layers to be in the equilibrium state in the plane of marginally stable stellar discs. We compared the surface star formation rate ($Σ_{SFR}$) and star formation efficiency ($SFE=Σ_{SFR}/Σ_{gas}$) with $ρ_g$ and stellar surface density $Σ_s$ assuming the latter to be proportional to disc surface brightness. Both HSB and LSB galaxies follow a single sequence $Σ_{SFR}-ρ_{g}$ and $SFE-Σ_s$ or $SFE-ρ_s$. It means that the conditions of star formation are similar in the outer discs of normal spiral galaxies and in the inner regions of LSB galaxies if their stellar discs have similar densities. The relationship between $SFE$ and $ρ_s$ is close to the law $SFE$ $\sim ρ_s^{1/2}$ expected in the theoretical model of self-regulated star formation proposed by Ostriker et al.\cite{OstrikerAll10}. The alternative explanation is to propose that $SFE$ is proportional to a frequency of vertical oscillation of gas clouds around the disc midplane. In the most rarified regions of LSB galaxies the efficiency of star formation is nearly independent on gas and stellar disc densities being higher in the mean than it is expected from the extrapolation of the power law fit for HSB sample galaxies. Evidently in these regions with extremely low $ρ_{g}$ SFE depends on local density fluctuations rather than on the azimuthally averaged disc parameters.

astro-ph.CO↗

Gravitational stability and dynamical overheating of stellar disks of galaxies

We use the marginal stability condition for galactic disks and the stellar velocity dispersion data published by different authors to place upper limits on the disk local surface density at two radial scalelengths $R=2h$. Extrapolating these estimates, we constrain the total mass of the disks and compare these estimates to those based on the photometry and color of stellar populations. The comparison reveals that the stellar disks of most of spiral galaxies in our sample cannot be substantially overheated and are therefore unlikely to have experienced a significant merging event in their history. The same conclusion applies to some, but not all of the S0 galaxies we consider. However, a substantial part of the early type galaxies do show the stellar velocity dispersion well in excess of the gravitational stability threshold suggesting a major merger event in the past. We find dynamically overheated disks among both seemingly isolated galaxies and those forming pairs. The ratio of the marginal stability disk mass estimate to the total galaxy mass within four radial scalelengths remains within a range of 0.4---0.8. We see no evidence for a noticeable running of this ratio with either the morphological type or color index.

astro-ph.CO↗

Galactic disks and their evolution

We consider the key problems related to measuring the mass of stellar disks and dark halos in galaxies and to explaining the observed properties of disks formed in massive dark halos.

astro-ph.CO↗

Spiral galaxies with non-typical mass-to-light ratios

Total mass-to-light ratio M/L_B for S0 - Irr galaxies, where M is the dynamical mass within the optical radius R_25, increases systematically with (B-V)_0 color, but slower than that is predicted by stellar population evolution models. It shows that the mean ratio between dark halo and stellar masses is higher for more "blue" galaxies. However some galaxies don't follow this general trend. The properties of galaxies with extremely high and extremely low values of M/L_B ratios are compared, and different factors, accounting for the extremes, are analyzed. The conclusion is that in some cases too high or too low M/L_B ratios are associated with observational errors, in other cases - with non-typical dark halo mass fraction, or with peculiarities of disc stellar population. Particularly, discs of some galaxies with low M/L_B ratios turn out to be unusually "light" for their luminosity and colors, which indicates a substantial deficit of low mass stars as the most probable cause of low M/L_B.

astro-ph.CO↗