Multicolor CCD photometry of six lenticular and spiral galaxies. Structure of the galaxies

The results of multicolor surface photometry of the S0 galaxies NGC 524, NGC 1138, and NGC 7280 and the spiral galaxies NGC 532, NGC 783, and NGC 1589 are reported. U BV RI observations were acquired with the 1.5-m telescope of the Maidanak Observatory (Uzbekistan), while JHK data were taken from the 2MASS catalog. The overall structure of the galaxies is analyzed and the galaxy images decomposed into bulge and disk components. The parameters of the galaxy components—rings, bars, spiral arms, and dust lanes—are determined. The bulge/disk decompositions based on averaged one-dimensional photometric profiles yield incorrect parameters for the bulges of the S0-Sa galaxies with bars and/or rings, whose inner regions are dominated by the radiation of the bulge.     

Multicolor CCD photometry of the dusty,giant, late-type spiral galaxy NGC 5351

The paper reports the results of BV RI surface photometry of the giant galaxy NGC 5351 based on CCD observations obtained on the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. Analysis of the structure and radial brightness distribution in the galaxy shows that NGC 5351 has a complex and, in some places, asymmetric structure. The galaxy possesses a large quantity of dust. The average internal extinction due to dust is A_V=1.2^m±0.4^m. After correcting for the effect of this dust, the parameters of the galaxy are typical of late-type spirals. The compositions of the stellar population in various parts of the galaxy are estimated using two-color diagrams. Star-forming regions in NGC 5351 are identified and studied. Most of the star-forming regions are located in the ring of the galaxy. Evolutionary modeling is used to estimate the ages of regions of violent star formation. An elliptical companion galaxy to NGC 5351 was found. The rotation curve of the galaxy is modeled and its mass estimated. The disk of NGC 5351 is self-gravitating within its optical radius.     

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.     

Polygonal structure of spiral galaxies

We have carried out numerical simulations of hydrodynamical processes occurring in the disks of spiral galaxies. The initial state of the disk is an equilibrium stellar-gaseous configuration. The spherical component is described by a standard analytical model for the gravitational potential. The behavior of the modeled disk in the presence of an external perturbation is analyzed. The results of numerical simulations of stellar-gaseous galactic disks aimed at studying the formation of polygonal structures in spiral galaxies are presented. The possible influence of spur-like formations on the appearance of polygonal structure is studied.     

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.     

Numerical modeling of transient structures in the disks of spiral galaxies

Nonstationary gas-dynamical processes occuring in the disks of spiral galaxies due to the external gravitational field have been studied using numerical simulations. A series of hydrodynamical discontinuities, including strong shock fronts and contact discontinuities, arises as a result of the nonlinear, supersonic interaction between emerging spiral formations and the flow of matter in the two-armed global morphology.     

Formation of large-scale dust lanes in spiral arms of galaxies

We consider the interaction of interstellar dust grains with a galactic shock in the gaseous component. Typical parameters of dust grains and spiral density waves imply that the formation of large-scale dust lanes at the front of a galactic shock is possible only in models taking into account a self-focusing phenomenon. In the case of an isothermal flow of interstellar gas through a spiral arm in a model with a gaseous disk of variable thickness, dust lanes can be projected onto the region of increased gas density, although this is not associated with a galactic shock. The dust density peak derived from the classical model of a galactic shock (isothermal flow and a constant thickness of the gaseous disk) is appreciably shifted downstream of the gas flow, so that it does not outline the gas density maximum.     

A correlation between dark matter and neutral hydrogen in spiral galaxies

We test the hypothesis put forward by Bosma (1981) that the surface density of dark matter is proportional to the surface density of HI, using decompositions of the rotation curves of a number of galaxies according to the THINGS, along with data for the galaxy NGC 6822. The rotation curves of these galaxies can be explained by assuming the existence of a massive gaseous disk in the absence of a dark halo, although the proportionality factor ??_dark/??_HI between the surface densities of dark matter and HI is different for different galaxies. However, there emerges the problem of the gravitational stability of galaxies whose stellar-velocity dispersions have been estimated, if the thickness of the dark-matter disk is similar to or less than the thickness of the stellar disk. The proportionality between ?? _dark and ??_HI is probably due to the fact that the radial profiles of ??_HI for galaxies with flat rotational curves are close to the critical density of a gravitationally stable gaseous layer (??_HI ?? R ^?1), and ??_dark(R) for a pseudo-isothermal halo obeys the same law.     

On the method of spiral arms monotony to determine the orientation angles of galaxies

A method is proposed for the determination of the position and inclination angles of the plane of a spiral galaxy based on the assumption that every spiral arm is a monotonic function of the radius and/or the azimuthal angle. This method may yield more accurate results than the more commonly employed isophote method, which is fraught with various drawbacks. The use of the new method is illustrated by applying it to a sample of 43 objects, and the results agree well with data from other sources.     

The stellar population and evolution of galaxies of the NGC 80 group

Seven early-type galaxies that are members of the massive X-ray group containing NGC 80 have been studied using two-dimensional spectroscopy with the 6-m telescope of the Special Astrophysical Observatory. We searched for evidence for the synchronous secular evolution of the galaxies in the group. The bulges of five of the seven galaxies appear to be old, with the average age of the bulge stars being 10–15 billion years. Signs of a relatively recent star-formation burst are observed in the small S0 galaxy IC 1548, whose average bulge age is 3 billion years and average core age is 1.5 billion years. A circumnuclear polar gas ring was also detected in this galaxy; in its outer regions, it makes a smooth transition to a gas disk that counter-rotates relative to the stars. IC 1548 probably underwent a close interaction, which resulted in its transformation from a spiral to a lenticular galaxy; the same interaction may also have induced the central burst of star formation. In the giant E0 galaxy NGC 83, a compact massive stellar-gas disk with a radius of about 2 kpc and very rapid rotation is observed, with ongoing star formation; the so-called “minor merger” is likely to have occurred there. We conclude that the NGC 80 group is in a state of formation, with the small NGC 83 subgroup “falling into” the large, old NGC 80 subgroup.     

Color redshifts and the age of the stellar population of distant RC radio galaxies

BV RI data are presented for the majority of steep-spectrum objects in the RC catalog with m _R <23.5^m. Previously developed programs are applied to these data to estimate the redshifts and ages of the stellar systems of the host galaxies. Applying this program to the color data (BV RI JHK) for distant radio galaxies with spectroscopic redshifts indicates that this approach provides accurate estimates of the redshifts of such radio galaxies, close to those obtained using field galaxies (～20%). The age estimates are much less trustworthy, but a lower limit to the ages of objects that are not very distant (z<1.5) can be determined with certainty. We have identi fied several galaxies whose formal ages exceed the age of the Universe at the corresponding z in simple Cold Dark Matter models for the Universe. The possibility of using such objects to elucidate the role of “dark energy” is discussed. This paradox disappears in models with cosmological constants (Λ terms) equal to 0.6–0.8.     

The stellar structure of irregular galaxies. Face-on galaxies

Stellar photometry of nearby irregular galaxies of the Local Group is used to identify and study the young and old stellar populations of these galaxies. An analysis of the spatial distributions of stars of different ages in face-on galaxies shows that the young stellar populations in irregular galaxies are concentrated toward the center, and form local inhomogeneities in star-forming regions, while the old stellar populations—red giants—form extended structures around the irregular galaxies. The sizes of these structures exceed the visible sizes of the galaxies at the 25^m/arcsec² isophote by a factor of two to three. The surface density of the red giants decreases exponentially from the center toward the edge, similar to the disk components in spiral galaxies.     

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

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

A catalog of galaxies in and around the cluster A1367. Spectral studies of several galaxies

We present a catalog of galaxies in and around the cluster A1367, together with the results of a spectroscopic study of eight faint galaxies projected onto the central part of the cluster. The observations were carried out with the Boller and Chivens spectrograph of the 2m telescope of the Guillermo Haro Observatory (Cananea, Mexico). Redshifts of eight galaxies were derived from both emission and absorption lines; the redshift for one of these, derived from Hα, SII, OIII, and Hβ emission lines, is z = 0.015. The spectrum of this galaxy displays no absorption lines at z = 0.026, testifying that it is located between the observer and A1367.     

<Emphasis Type="Italic">UBV RI</Emphasis> CCD photometry of the nearby low-surface-brightness galaxy NGC 5585

The structure and composition of the stellar population in the low-surface-brightness galaxy NGC 5585 is studied using UBV RI CCD photometry. The observations were obtained on the 1.5-m telescope of the Maidanak Observatory of the Astronomical Institute of the Academy of Sciences of Uzbekistan under conditions with seeing 1.2″–1.8″. A two-dimensional decomposition of the galaxy emission into bulge and disk components is carried out. Both components have low surface brightnessess. The Sersic parameter for the bulge is n = 1.2–1.6. The effective radius of the bulge in R and I is equal to the scale length for the brightness decrease in the disk, and comprises 30″–40″ (0.8–1.1 kpc). The spiral arms seem to form a bar, but the centers of the bar and ring do not coincide with the center of NGC 5585. A powerful star-forming region is observed 3.2″ (100 pc) from the galactic center, whose radiation swamps the nucleus in the U and B filters. Based on the positions of the various components of the galaxy in two-color diagrams, it is concluded that NGC 5585 has a complex star-forming history that may be different at different distances from the center.     

Stellar subsystems of the galaxy NGC 2366

Hubble Space Telescope archive data are used to perform photometry of stars in seven fields at the center and periphery of the galaxy NGC 2366. The variation of the number density of stars of various ages with galactocentric radius and along the minor axis of the galaxy are determined. The boundaries of the thin and thick disks of the galaxy are found. The inferred sizes of the subsystems of NGC 2366 (Z _thin = 4 kpc and Z _thick = 8 kpc for the thin and thick disks, respectively) are more typical for spiral galaxies. Evidence for a stellar halo is found at the periphery of NGC 2366 beyond the thick disk of the galaxy.     

Modeling the rotation curves of spiral galaxies in a thin-disk model with a cosmological term that depends linearly on the Ricci scalar

The rotation curves of spiral galaxies are modeled in the case when the cosmological term depends linearly on the Ricci scalar. Themodel galaxy is approximated using a thin disk with an exponential radial distribution of the matter density. This model is used to estimate the free parameter of the theory, and to construct model rotation curves for several selected galaxies. The results obtained are in satisfactory agreement with observations.     

The distribution of urban population and income: Explorations using six asian cases

Urban concentration (or primacy) and inequality (in size distribution of income) are expected to follow bell shaped curves through the development process. Spatial convergence (through investments in transportation etc.) is expected to precede income convergence. Using longitudinal data from six Asian countries (Japan, Taiwan, Malaysia, the Philippines, Sri Lanka and India) this paper shows that (i) the bell shapes for urban concentration and income inequality generally hold, and (ii) the temporal relationship between the curve peaks is determined by geographical factors (for urban concentration); income inequality is seen to be more policy amenable.     

Evolution of galaxies and the Tully-Fisher relation

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