HI content in galactic disks: The role of gravitational instability

We examine the dependence of the total hydrogen mass M _HI in late-type star-forming galaxies on rotation velocity V _rot and optical size D ₂₅ or radial scale length R ₀ of the disk for two samples of galaxies: (i) isolated galaxies (AMIGA) and (ii) galaxies with edge-on disks (flat galaxies according to Karachentsev et al.). M _HI given in the HYPERLEDA database for flat galaxies have turned out to be, on average, overestimated by ~0.2 dex compared to isolated galaxies with similar V _rot or D ₂₅, which is apparently due to an overestimation of the self-absorption in the HI line. The hydrogen mass in the galaxies of both samples closely correlates with the total specific angular momentum of the galactic disk J, which is proportional to V _rot D ₂₅ or V _rot R ₀, with the low-surface-brightness galaxies lying along the common V _rot R ₀ sequence. We discuss the possibility of explaining the relationship between M _HI and V _rot D ₂₅ by assuming that the gas mass in the disk is regulated by the marginal gravitational stability condition for the gas layer. Comparison of the observed and theoretically expected dependences leads us to conclude that either the gravitational stability corresponds to higher values of the Toomre parameter than is usually assumed, or the threshold stability condition formost galaxies was fulfilled only in the past, when the gasmass in the disks was a factor of 2–4 higher than that at present (except for the galaxies with an anomalously high observed HI content). The latter condition requires that for most galaxies the conversion of gas into stars be not compensated by the external accretion of gas onto the disk.     

Stellar velocity dispersion and mass estimation for galactic disks

Available velocity dispersion estimates for the old stellar population of galactic disks at galactocentric distances r?2L (where L is the photometric radial scale length of the disk) are used to determine the threshold local surface density of disks that are stable against gravitational perturbations. The mass of the disk M_d calculated under the assumption of its marginal stability is compared with the total mass M_t and luminosity L _B of the galaxy within r=4L. We corroborate the conclusion that a substantial fraction of the mass in galaxies is probably located in their dark halos. The ratio of the radial velocity dispersion to the circular velocity increases along the sequence of galactic color indices and decreases from the early to late morphological types. For most of the galaxies with large color indices (B–V)₀>0.75, which mainly belong to the S0 type, the velocity dispersion exceeds significantly the threshold value required for the disk to be stable. The reverse situation is true for spiral galaxies: the ratios M_d/L_B for these agree well with those expected for evolving stellar systems with the observed color indices. This suggests that the disks of spiral galaxies underwent no significant dynamical heating after they reached a quasi-equilibrium stable state.     

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 and the Galaxy 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 the molecular gas in the galaxies under consideration.     

H<Emphasis Type="Italic">α</Emphasis> survey of the local volume: Isolated southern galaxies

We present our Hα observations of 11 isolated southern galaxies: SDIG, PGC 51659, E 222-010, E 272-025, E 137-018, IC 4662, Sag DIG, IC 5052, IC 5152, UGCA 438, and E 149-003, with distances from 1 to 7 Mpc. We have determined the total Hα fluxes from these galaxies. The star formation rates in these galaxies range from 10^?1 (IC 4662) to 10^?4 M _⊙ yr^?1 (SDIG) and the gas depletion time at the observed star formation rates lies within the range from 1/6 to 24 Hubble times H ₀ ^?1 .     

Mechanisms of the vertical secular heating of a stellar disk

We investigate the nonlinear growth stages of the bending instability in stellar disks with exponential radial density profiles. We found that the unstable modes are global (the wavelengths are larger than the disk scale lengths) and that the instability saturation level is much higher than that following from a linear criterion. The instability saturation time scales are of the order of one billion years or more. For this reason, the bending instability can play an important role in the secular heating of a stellar disk in the z direction. In an extensive series of numerical N-body simulations with a high spatial resolution, we were able to scan in detail the space of key parameters (the initial disk thickness z₀, the Toomre parameter Q, and the ratio of dark halo mass to disk mass M_h/M_d). We revealed three distinct mechanisms of disk heating in the z direction: bending instability of the entire disk, bending instability of the bar, and heating on vertical inhomogeneities in the distribution of stellar matter.     

The observed structure of extremely distant galaxies

We have identified 22 galaxies with photometric redshifts z_ph=5–7 in the northern and southern Hubble Space Telescope deep fields. An analysis of the images of these objects shows that they are asymmetric and very compact (～1 kpc) structures with high surface brightness and absolute magnitudes of M_B≈?20^m. The average spectral energy distribution for these galaxies agrees with the distributions for galaxies with active star formation. The star formation rate in galaxies with z_ph=5–7 was estimated from their luminosity at λ=1500 Å to be ~30 M_⊙yr^?1. The spatial density of these objects is close to the current spatial density of bright galaxies. All the above properties of the distant galaxies considered are very similar to those of the so-called Lyman break galaxies (LBGs) with z ～ 3–4. The similarity between the objects considered and LBGs suggests that at z ～6, we observe the progenitors of present-day galaxies that form duringmergers of protogalactic objects and that undergo intense starbursts.     

Ultra-flat galaxies selected from RFGC catalog. III. Star formation rate

We examine the star formation properties of galaxies with very thin disks selected from the Revised FlatGalaxy Catalog (RFGC). The sample contains 333 ultra-flat galaxies (UFG) at high Galactic latitudes, |b| > 10?, with a blue major angular diameter of a ≥ 1.'2, blue and red apparent axial ratios of (a/b)_b > 10, (a/b)_r > 8.5 and radial velocities within 10 000 kms^?1. As a control sample for them we use a population of 722 more thick RFGC galaxies with (a/b)_b > 7, situated in the same volume. The UFG distribution over the sky indicates them as a population of quite isolated galaxies.We found that the specific star formation rate, sSFR _FUV, determined via the FUV GALEX flux, increases steadily from the early type to late type disks for both the UFG and RFGC–UFG samples, showing no significant mutual difference within each morphological type T. The population of UFG disks has the average HI-mass-to-stellarmass ratio by (0.25 ± 0.03) dex higher than that of RFGC–UFG galaxies. Being compared with arbitrary orientated disks of the same type, the ultra-flat edge-on galaxies reveal that their total HI mass is hidden by self-absorption on the average by approximately 0.20 dex.We demonstrate that using the robust stellar mass estimate via 〈B?K〉-color and galaxy type T for the thin disks, together with a nowaday accounting for internal extinction, yields their sSFR quantities definitely lying below the limit of ?9.4 dex (yr^?1). The collected observational data on UFG disks imply that their average star formation rate in the past has been approximately three times the current SFR. The UFG galaxies have also sufficient amount of gas to support their observed SFR over the following nearly 9 Gyrs.     

A photometric study of faint galaxies in the field of GRB 000926

We present our B, V, R_c, and I_c observations of a \(3'.6 \times 3'\) field centered on the host galaxy of GRB 000926 (α_2000.0=17^h04^m11^s, \(\delta _{2000.0} = + 51^ \circ 47'9\mathop .\limits^{''} 8\)). The observations were carried out on the 6-m Special Astrophysical Observatory telescope using the SCORPIO instrument. The catalog of galaxies detected in this field includes 264 objects for which the signal-to-noise ratio is larger than 5 in each photometric band. The following limiting magnitudes in the catalog correspond to this limitation: 26.6 (B), 25.7 (V), 25.8 (R), and 24.5 (I). The differential galaxy counts are in good agreement with previously published CCD observations of deep fields. We estimated the photometric redshifts for all of the cataloged objects and studied the color variations of the galaxies with z. For luminous spiral galaxies with M(B)z～1.     

HUDF 1619—A candidate for polar-ring galaxies in the Hubble Ultra Deep Field

A galaxy that is a good candidate for polar-ring galaxies has been detected in the Hubble Ultra Deep Field (HUDF). The galaxy HUDF 1619 (V ≈ 25 ^m , z ～ 1) is the most distant object of this type known to date. A large-scale structure crosses the highly warped disk of the main galaxy seen almost edge-on at an angle of about 70°. The luminosity of this structure (the possible polar ring) reaches ～1/3 of the luminosity of the central galaxy. A strong absorption lane is seen in the region where this structure is projected onto the disk of the central object. There are two galaxies of comparable luminosity adjacent to HUDF 1619 (in projection). One of them may be the donor galaxy the interaction with which gave rise to the ring structure.     

Outskirts of Galaxy Clusters A1139, A1314, A1656, A2040, A 2052, A2107: Star-Formation Rate

We investigate the variation of the fraction of galaxies with suppressed star formation (M_K < ?21 _. ^m 5) and early-type galaxies (frac_E) of the “red sequence” along the projected radius in six galaxy clusters:Coma (A1656), A1139, and A1314 in the Leo supercluster region (z ≈ 0.037) and A2040, A2052, A2107 in the Hercules supercluster region (z ≈ 0.036). According to SDSS (DR10) data, frac_E is the highest in the central regions of galaxy clusters and it is, on the average, equal to 0.62 ± 0.03, whereas in the 2–3R/R_200c interval and beyond the R_sp ≈ 0.95 ± 0.04 R_200m radius that we inferred from the observed profile frac_E is minimal and equal to 0.25 ± 0.02. This value coincides with the estimate frac_E = 0.24 ± 0.01 that we inferred for field galaxies located between the Hercules and Leo superclusters at the same redshifts. We show that the fraction of galaxies with suppressed star formation decreases continuously with cluster radius from 0.87 ± 0.02 in central regions down to 0.43 ± 0.03 in the 2–3 R/R_200c interval and beyond R_sp, but remains, on the average, higher than 26% than the corresponding fraction for field objects. This decrease is especially conspicuous in the galaxy mass interval log M_* [M_⊙] = 9.5–10. We found that galaxies with ongoing star formation have average clustercentric distances 1.5–2.5 R/R_200c and that their radial-velocity dispersions are higher than those of galaxies with suppressed star formation.     

2D spectroscopy of candidate polar-ring galaxies: I. The pair of galaxies UGC 5600/09

Observations of the pair of galaxies VV 330 with the SCORPIO multimode instrument on the 6-m Special Astrophysical Observatory telescope are presented. Large-scale velocity fields of the ionized gas in Hα and brightness distributions in continuum and Hα have been constructed for both galaxies with the help of a scanning Fabry-Perot interferometer. Long-slit spectroscopy is used to study the stellar kinematics. Analysis of the data obtained has revealed a complex structure in each of the pair components. Three kinematic subsystems have been identified in UGC 5600: a stellar disk, an “inner gas ring” turned with respect to the disk through ～80°, and an outer gas disk. The stellar and outer gas disks are noncoplanar. Possible scenarios for the formation of the observed multicomponent kinematic galactic structure are considered, including the case where the large-scale velocity field of the gas is represented by the kinematic model of a disk with a warp. The velocity field in the second galaxy of the pair, UGC 5609, is more regular. A joint analysis of the data on the photometric structure and the velocity field has shown that this is probably a late-type spiral galaxy whose shape is distorted by the gravitational interaction, possibly, with UGC 5600.     

Voids in the SDSS galaxy survey

Using the method of searching for arbitrary shaped voids in the distribution of volume-limited samples of galaxies from the DR5 SDSS survey, we have identified voids and investigated their characteristics and the change in these characteristics with decreasing M _lim (from ?19.7 to ?21.2, H ₀ = 100 km s^?1 Mpc^?1)—the upper limit on the absolute magnitude of the galaxies involved in the construction of voids. The total volume of the 50 largest voids increases with decreasing M _lim with a break near M* = ?20.44—the characteristic value of the luminosity function for SDSS galaxies. The mean overdensity in voids increases with decreasing M _lim also with a weak break near M*. The exponent of the dependence of the volume of a void on its rank increases significantly with decreasing M _lim starting from M _lim ～ ?20.4 in the characteristic range of volumes, which reflects the tendency for greater clustering of brighter galaxies. The averaged profile of the galaxy overdensity in voids has a similar pattern almost at all M _lim. The galaxies mostly tend to gravitate toward the void boundaries and to avoid the central void regions; the overdensity profile is flat in the intermediate range of distances from the void boundaries. The axial ratios of the ellipsoids equivalent to the voids are, on average, retained with changing M _lim and correspond to elongated and nonoblate void shapes, but some of the voids can change their shape significantly. The directions of the greatest void elongations change chaotically and are distributed randomly at a given M _lim. The void centers show correlations reflecting the correlations of the galaxy distribution on scales (35–70)h ^?1 Mpc. The galaxy distribution in the identified voids is nonrandom—groups and filaments can be identified. We have compared the properties of the galaxies in voids (in our case, the voids are determined by the galaxies with absolute magnitudes M _abs < M _lim = ?20.44, except for the isolated galaxies) and galaxies in structures identified using the minimum spanning tree. A bimodal color distribution of the galaxies in voids has been obtained. A noticeable difference is observed in the mean color indices and star formation rates per unit stellar mass of the galaxies in dense regions (structures)—as expected, the galaxies in voids are, on average, bluer and have higher log (SFR/M _star). These tendencies become stronger toward the central void regions.     

<Emphasis Type="Italic">GALEX</Emphasis> and star formation

Wide-field far-UV (FUV, 1344–1786 Å) and near-UV (NUV, 1771–2831 Å) imaging from GALEX provides a deep, comprehensive view of the young stellar populations in hundreds of nearby galaxies, shedding new light on the process of star formation (SF) in different environments, and on the interplay between dust and SF. GALEX’s FUV-NUV color is extremely sensitive to stellar populations of ages up to a few hundred Myrs, unambiguously probing their presence and enabling age-dating and stellar mass estimate, together with the characterization of interstellar dust extinction. The deep sensitivity, combined with the wide field-of-view, made possible in particular the discovery and characterization of star formation in extremely low-density, diffuse gas environments such as outer galaxy disks, tidal tails, low-surface-brightness galaxies (LSB) and dwarf Irregular galaxies, and of rejuvenation episodes in early-type galaxies. Such results provide several missing links for interpreting galaxy classes in an evolutionary context, extend our knowledge of the star-formation process to previously unexplored conditions, constrain models of galaxy disk formation, and clarify the mutual role of dust and star formation. We review a variety of star-forming environments studied by GALEX, and provide some model analysis tools useful for interpretation of GALEX measurements, and potentially as basic science planning tools for next-generation UV instruments.     

Photometry of the low-luminosity spiral galaxy NGC 4136

Multicolor BVRI surface photometry of the low-luminosity (M _V ≈?18^m) spiral galaxy NGC 4136 is presented. The photometric parameters of its components and the color distribution over the galactic disk are estimated. The color indices and the corresponding effective ages are determined for the brightest star-forming regions. The disk-to-dark halo mass ratio is derived from the measured rotation curve of the galaxy. The disk mass dominates within the optical boundaries of the galaxy, so its disk can be considered as a self-gravitating system.     

Observational study of the candidate polar-ring galaxies NGC 304 and NGC 7625

We present the results of our photometric (BV R) and spectroscopic CCD observations of NGC 304 and NGC 7625, candidate polar-ring galaxies, performed with the 6-m Special Astrophysical Observatory telescope. For NGC 304, such a study has been carried out for the first time. We have obtained basic integrated characteristics of the galaxies and determined their morphological types (S0 for NGC 304 and Sa for NGC 7625). The absolute magnitudes of the galaxies, M _B = ?20^m.81 for NGC 304 and M _B = ?19^m.34 for NGC7625, are indicative of their fairly high luminosities. The disk and bulge parameters have been determined forNGC 304 (µ₀ = 20^m.60, h = 3.86 kpc, µ _e = 21^m.59, r _e = 1.26 kpc in the B band); these correspond to the parameters of S0-type objects. The rotation velocity for NGC 304 (200 km s^?1) reaches its maximum at a galactocentric distance of 3.1 kpc, which yields a mass estimate for the galaxy of 2.8 × 10¹⁰ \(\mathcal{M}_ \odot \). The observed photometric features at the center of NGC 304 indicate that it may have an inner ring structure, although we have failed to confirm the existence of two kinematic systems based on our spectroscopic observations. In NGC 7625, the disk makes a dominant contribution to the total brightness. The derived integrated color indices (B-V = 0^m.81 and V-R = 0^m.61) agree with previous determinations of other authors. We have estimated the учештсешщт in the inner galactic regions. In the outer regions, we have detected structures with bluer colors (B-V = 0^m.60), which may be indicative of a polar ring with a minor stellar component.     

Pre-and post-outburst parameters for the components of the symbiotic star V1329 Cyg

We present new UB V observations of the symbiotic nova V 1329 Cyg. Based on all our UB V observations of a uniform system, we redetermined the orbital period of the binary and estimated the magnitudes and luminosities of its components. We show that the pre-outburst visual luminosities of the red giant and the hot star were almost equal and that the rapid irregular photographic variability of the star was caused by the nonstationary behavior of the hot component. The outburst amplitude of the hot component (subdwarf) in 1964 was found to be ～2^m in the V band, which is typical of ordinary symbiotic stars. We estimated the continuum luminosity of the gaseous component that appeared after the outburst. In the V band, it was almost 1^m fainter than the flared hot star. Structurally, the gaseous component is an ionized gaseous disk comparable in size to an M giant.     

A model of low-mass neutron stars with a quark core

We consider an equation of state that leads to a first-order phase transition from the nucleon state to the quark state with a transition parameter λ>3/2 (λ=ρ_Q/(ρ_N+P₀/c²)) in superdense nuclear matter. Our calculations of integrated parameters for superdense stars using this equation of state show that on the stable branch of the dependence of stellar mass on central pressure dM/dP_c>0) in the range of low masses, a new local maximum with M_max=0.082_⊙ and R=1251 km appears after the formation of a toothlike kink (M=0.08M_⊙, R=205 km) attributable to quark production. For such a star, the mass and radius of the quark core are M_core=0.005M_⊙ and R_core=1.73 km, respectively. In the model under consideration, mass accretion can result in two successive transitions to a quark-core neutron star with energy release similar to a supernova explosion: initially, a low-mass star with a quark core is formed; the subsequent accretion leads to configurations with a radius of ～1000 km; and, finally, the second catastrophic restructuring gives rise to a star with a radius of ～100 km.     

H I observations of flat galaxies

We present the HI observations of 94 flat spiral galaxies from RFGC (Revised Flat Galaxy Catalog) and 14 galaxies from 2MFGC (the 2MASS selected Flat Galaxy Catalog) performed with the 100-m radio telescope in Effelsberg (Germany). HI fluxes, heliocentric radial velocities, and HI line widths are given for 65 detected galaxies. We present a mosaic of HI profiles. We calculated some of the global parameters of the galaxies and analyzed the linear correlations between them. The ratios of the total (indicative) masses of the galaxies to their luminosities lie within the range 0.4–8.2 with a mean of 3.8 (M_⊙/L_⊙), and the mean mass fraction of neutral hydrogen is 13%. Upper limits are given for the radio fluxes from 43 undetected galaxies.     

Infrared luminosities of local-volume galaxies

Based on data from the Two-Micrometer All-Sky Survey (2MASS), we analyzed the infrared properties of 451 Local-Volume galaxies at distances D ≤ 10 Mpc. We determined the K-band luminosity function of the galaxies in the range of absolute magnitudes from ?25^m to ?11^m. The local luminosity density within 8 Mpc is 6.8 × 10⁸L_⊙ Mpc^?3, a factor of 1.5 ± 0.1 higher than the global mean K-band luminosity density. We determined the ratios of the virial mass to the K-band luminosity for nearby groups and clusters of galaxies. In the luminosity range from 5 × 10¹⁰ to 2 × 10¹³L_⊙, the dependence log(M/L_K) ∝ (0.27 ± 0.03) log L_K with a dispersion of ～0.1 comparable to the measurement errors of the masses and luminosities of the systems of galaxies holds for the groups and clusters of galaxies. The ensemble-averaged ratio, 〈M/L_K〉 ? (20–25) M_⊙/L_⊙, was found to be much smaller than the expected global ratio, (80–90)M_⊙/L_⊙, in the standard model with Ω_m = 0.27. This discrepancy can be eliminated if the bulk of the dark matter in the Universe is not associated with galaxies and their systems.     

Ultra-flat galaxies selected from RFGC catalog. II. Orbital estimates of halo masses

We used the Revised Flat Galaxy Catalog (RFGC) to select 817 ultra-flat (UF) edge-on disk galaxies with blue and red apparent axial ratios of (a/b)_B > 10.0 and (a/b)_R > 8.5. The sample covering the whole sky, except the Milky Way zone, contains 490 UF galaxies with measured radial velocities. Our inspection of the neighboring galaxies around them revealed only 30 companions with radial velocity difference of | ΔV |< 500 kms^?1 inside the projected separation of R_p < 250 kpc. Wherein, the wider area around the UF galaxy within R_p < 750 kpc contains no other neighbors brighter than the UF galaxy itself in the same velocity span. The resulting sample galaxies mostly belong to the morphological types Sc, Scd, Sd. They have a moderate rotation velocity curve amplitude of about 120 km s^?1 and a moderate K-band luminosity of about 10¹⁰L_⊙. The median difference of radial velocities of their companions is 87 km s^?1, yielding the median orbital mass estimate of about 5 × 10¹¹M_⊙. Excluding six probable non-isolated pairs, we obtained a typical halo-mass-to-stellar-mass of UF galaxies of about 30, what is almost the same one as in the principal spiral galaxies, like M31 and M81 in the nearest groups. We also note that ultra-flat galaxies look two times less “dusty” than other spirals of the same luminosity.