Atomic hydrogen deficiency in galaxies of the virgo and coma clusters: A new estimation method

We propose a new method for estimating the HI deficiency in galaxies. The method is based on a semi-empirical relationship between the total mass of HI and specific angular momentum of isolated galaxies. The atomic-hydrogen deficiency is estimated for nearby spiral galaxies and for spiral galaxies in the Virgo and Coma clusters. The mean HI deficiencies determined for these samples using our method are similar to those obtained with conventional methods, although there are considerable differences in some cases. The HI deficiency in nearby galaxies does not depend on their degree of isolation, and there is no systematic discrepancy between their HI and “normal” masses. Significant HI deficiencies are observed in the Virgo and Coma clusters, out to distances of 1.5 and 3–4 Mpc from the cluster centers, respectively. At such distances, the ram pressure is too small to sweep a considerable amount of gas from the galactic disks. Either these galaxies have passed through the dense cluster center, or their gas deficiency is due to the fact that the halo had stopped accreting onto the disk when the galaxy entered the cluster.     

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.     

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.     

Galaxies with Abnormally High Gas Content in the Disk

Astronomy Reports - The content of gas in galaxies with an anomalously high relative mass of hydrogen $${{M}_{{{\text{HI}}}}}{\text{/}}{{M}_{*}}$$ for a given mass of the stellar population...     

Analysis of Numerical Algorithms for Computing Rapid Momentum Transfers between the Gas and Dust in Simulations of Circumstellar Disks

Approaches used in modern numerical simulations of the dynamics of dust and gas in circumstellar disks are tested. The gas and dust are treated like interpenetrating continuous media that can exchange momentum. A stiff coupling between the gas and dust phases is typical for such disks, with the dust stopping time much less than the characteristic dynamical time scale. This imposes high demands on the methods used to simulate the dust dynamics. A grid-based, piecewise-parabolic method is used as the basic algorithm for solving the gas-dynamical equations. Numerical solutions obtained using various methods to compute the momentum exchanges are presented for the case of monodisperse dust. Numerical solutions are obtained for shock tube problem and the propagation of sound waves in a gas–dust medium. The studied methods are compared in terms of their ability to model media with (a) an arbitrary (short or long) dust stopping time, and (b) an arbitrary dust concentration in the gas (varying the dust to gas mass ratio from 0.01 to 1). A method for computing the momentum exchange with infinite-order accuracy in time is identified, which makes it possible to satisfy the conditions (a) and (b) with minimal computational costs. A first-order method that shows similar results in the test computations is also presented. It is shown that the proposed first-order method for monodisperse dust can be extended to a regime when the dust is polydisperse; i.e., a regime represented by several fractions with different stopping times. Formulas for computing the gas and dust velocities for polydisperse dust with each fraction exchanging momentum with the gas are presented.     

Determination of the physical parameters, Lyman continua, and chemical compositions of HII regions in blue compact dwarf galaxies

A new method for determining the physical parameters, ionizing Lyman continua (Lyc) and chemical compositions of HII regions in blue compact dwarf galaxies is developed. We propose our modified NLEHII method, which is independent on the initial mass function (IMF) for the determination of the Lyc spectra of the ionizing nuclei of HII regions. This method is based on the assumptions of ionization-recombination and thermal equilibria in the HII regions. It is used to calculate the Lyc spectra for optimization photoionization models (OPhMs) of HII regions in an iterative way, since the Lyc spectrum depends on parameters that can be found from photoionization modeling. We apply this method to determine the chemical composition of an HII region in the blue compact dwarf galaxy SBS 0940+544 and, in particular, the helium mass fraction Y. Published in Russian in Astronomicheskiĭ Zhurnal. 2008, Vol. 85, No. 3, pp. 213–229. The article was translated by the authors.     

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.     

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.     

The star-formation efficiency and density of the disks of spiral galaxies

The four well studied spiral galaxies M33, M81, M100, and M101 are used to analyze the dependences of the star-formation rate (SFR) and star-formation efficiency (SFE = SFR/M _gas ) on galactocentric distance R and the photometric and some kinematic parameters of galactic disks. The dependences SFR(R) were estimated based on UV and far-infrared data using published extinction-corrected UV brightness profiles of the galaxies. The local SFE values are most closely related to the surface brightness (density) of the galactic disk at a given R, with this dependence being the same for all four galaxies (except for their central regions). In order to explain the observed disk densities in terms of a simple conservative model (“toy model”) for the evolution of the gas density, the local value of the parameter N in the Schmidt law for the disk (SFR ～ σ _gas ^N ) must not exceed unity. In this case, the observed dependences σ _gas (R) and SFE(R) can be matched assuming that accretion is occuring in the central regions of the disks.     

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).     

Restructuring and destruction of hydrocarbon dust in the interstellar medium

A model describing the main processes determining the evolution of hydrocarbon dust grains of arbitrary size under astrophysical conditions corresponding to regions of ionized hydrogen (HII regions) and supernova remnants is presented. The processes considered include aromatization and photodestruction, sputtering by electrons and ions, and shattering during collisions between grains. The model can be used to calculate the size distribution of the grains and the degree of aromatization during the evolution of HII regions and supernova remnants for a specified radiation field, relative velocity between the gas and dust, etc. The contribution of various processes to the evolution of hydrocarbon dust grains for parameters typical for the interstellar medium of our Galaxy is considered. Small grains (with fewer than 50 carbon atoms) should be fully aromatized in the interstellar medium. If larger grains initially have an aliphatic structure, this is preserved to a substantial extent. Variation in the size distribution of the grains due to collisions between grains depend appreciably on the adopted initial size distribution. With an initial distribution corresponding to that of Mathis et al. (1977), the mass fraction contributed by smaller grains tends to increase with time, while, with an initial distribution corresponding to that of Jones et al. (2013), in which the fraction of small grains is initially high, there is a general decrease in the number of grains of various sizes with time.     

Infrared emission and the destruction of dust in HII regions

The generation of infrared (IR) radiation and the observed IR-intensity distribution at wavelengths of 8, 24, and 100 µm in the ionized hydrogen region around a young, massive star is investigated. The evolution of the HII region is treated using a self-consistent chemical-dynamical model in which three dust populations are included—large silicate grains, small graphite grains, and polycyclic, aromatic hydrocarbons (PAHs). A radiative transfer model taking into account stochastic heating of small grains and macromolecules is used to model the IR spectral energy distribution. The computational results are compared with Spitzer and Herschel observations of the RCW 120 nebula. The contributions of collisions with gas particles and the radiation field of the star to stochastic heating of small grains are investigated. It is shown that a model with a homogeneous PAH content cannot reproduce the ring-like IR-intensity distribution at 8 µm. A model in which PAHs are destroyed by ultraviolet radiation of the star, generating region HII, provides a means to explain this intensity distribution. This model is in agreement with observations for realistic characteristic destruction times for the PAHs.     

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.     

Do low surface brightness galaxies have dense disks?

The disk masses of four low-surface-brightness (LSB) galaxies are estimated using the criterion of marginal gravitational stability. The constructed mass models are close to those for a maximum disk. The results show that the disks of LSB galaxies may be significantly more massive than is usually believed based on their brightnesses. In this case, their surface densities and masses are fairly typical for higher-surface-brightness spirals. Alternatively, it may be that LSB disks are dynamically overheated.     

The influence of Population III stars on the early evolution of galaxies

Numerical simulations of the chemical evolution of disk galaxies taking into account the influence of Population III stars are considered. The probability that stars with peculiar chemical compositions are present in the solar neigborhood is analyzed, and possibilities for their detection considered. For various assumptions about the slope of the initial mass function for Population III stars and the critical metallicity, the radius surrounding the Sun containing at least one such star is 10–12 pc. Such objects could be studied using modern large telescopes. The influence of Population III stars on the chemical evolution of disk galaxies is investigated. Taking into account the first stars in early stages leads to an earlier onset of chemical enrichment of the ISM and a characteristic chemical composition of the gas, but all traces of this enrichment have disappeared by the current epoch.     

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.     

Studies of Star-forming Complexes in the Galaxies NGC 628, NGC 2976, and NGC 3351

The parameters of the radiation of interstellar matter in star-forming complexes in the high-metallicity galaxies NGC 628, NGC 2976, and NGC 3351, which have different morphological types, are analyzed. The relationship between the emission in Hα and in lines of CO and HI is considered, as well as the relationship between Hα and the emission of dust in the infrared range (IR). The fluxes and surface brightnesses in the UV and IR correlate well with the Hα emission. The HI emission also correlates well with Hα, while the correlation between the CO and Hα emission is much weaker. The ratio of the fluxes at 8 and 24 µm decreases with increasing Hα flux. This may be due to changes in the properties of the dust ensemble (a decrease in the mass fraction of polycyclic aromatic hydrocarbons) or to changing excitation conditions. Analysis of the kinematics of the CO lines shows that the CO flux grows with increasing velocity scatter ΔV when ΔV ? 70 km/s. Preliminary evidence for the existence of star-forming complexes with higher values of ΔV is provided, when the increase in the velocity scatter is accompanied by a decrease in the CO luminosity of the complex.

     

Chemodynamical evolution of gas near an expanding HII region

A self-consistent model for the chemical-dynamical evolution of a region of ionized hydrogen around a massive young star and of the surrounding molecular gas is presented. The model includes all main chemical and physical processes, namely the photoionization of atomic hydrogen, photodissociation of molecular hydrogen and other molecules, and the evaporation of molecules from the mantles of dust particles. Heating and cooling processes are taken into account in the temperature calculations, including cooling in molecular and atomic lines. The hydrodynamical equations were solved using the Zeus2D hydrodynamical software package. This model is used to analyze the expansion of a region of ionized hydrogen around massive stars (effective temperature of 30 000 and 40 000 K) in a medium with various initial density distributions. The competition between evaporation from dust mantles and the photodissociation of molecules results in the formation of a transition layer between the hot HII region and cool quiescent medium, characterized by high abundances of molecules in the gas phase. The thickness of the transition layer is different for different molecules. Since there is a velocity gradient along the transition layer, and the maxima in the distributions of different molecules are at different distances from the star, observations of molecular emission lines should reveal distinction in shifts of lines of different molecules relative to the velocity of the quiescent gas. Such shifts have indeed been detected during molecular observations of the region of ionized hydrogen Sh2-235. For an initial gas density of 10³ cm^?3, the increase in the abundances of H₂O and H₂CO in the transition layer after desorption from dust occurs gradually rather than in a jump-like fashion; therefore, the concept of a “evaporation front” can be used only formally. In addition, the distances between the evaporation fronts for different molecules are significant. At higher initial gas densities (10⁴ cm^?3), sharp evaporation fronts are formed for the different molecules, which are close to each other and to the shock front. In this case, it is possible to speak of a single evaporation front for CO, H₂O, and H₂CO.     

Relation between the parameters of dust and of molecular and atomic gas in extragalactic star-forming regions

The relationships between atomic and molecular hydrogen and dust of various sizes in extragalactic star-forming regions are considered, based on observational data from the Spitzer and Herschel infrared space telescopes, the Very Large Array (atomic hydrogen emission) and IRAM (CO emission). The source sample consists of approximately 300 star-forming regions in 11 nearby galaxies. Aperture photometry has been applied to measure the fluxes in eight infrared bands (3.6, 4.5, 5.8, 8, 24, 70, 100, and 160 μm), the atomic hydrogen 21 cm line, and CO (2–1) line. The parameters of the dust in the starforming regions were determined via synthetic-spectra fitting, such as the total dust mass, the fraction of polycyclic aromatic hydrocarbons (PAHs), etc. Comparison of the observed fluxes with the measured parameters shows that the relationships between atomic hydrogen, molecular hydrogen, and dust are different in low- and high-metallicity regions. Low-metallicity regions contain more atomic gas, but less molecular gas and dust, including PAHs. The mass of dust constitutes about 1% of the mass of molecular gas in all regions considered. Fluxes produced by atomic and molecular gas do not correlate with the parameters of the stellar radiation, whereas the dust fluxes grow with increasing mean intensity of stellar radiation and the fraction of enhanced stellar radiation. The ratio of the fluxes at 8 and 24 μm, which characterizes the PAH content, decreases with increasing intensity of the stellar radiation, possibly indicating evolutionary variations of the PAH content. The results confirm that the contribution of the 24 μm emission to the total IR luminosity of extragalactic star-forming regions does not depend on the metallicity.