Amplification of spiral density waves in gaseous and dust galactic disks as a result of interaction with the halo

The effect of the discrete structure of the halo on local oscillations of a galactic disk is analyzed. Such effects have much in common with dynamical friction. Gaseous and stellar disks are considered; in both cases, some leading spiral density waves are unstable. Bending oscillations of the disk can also be unstable when the disk interacts with the halo.     

Dynamics of gaseous disks in a non-axisymmetric dark halo

The dynamics of a galactic disk in a non-axisymmetric (triaxial) dark halo is studied in detail using high-resolution, numerical, hydrodynamical models. A long-lived, two-armed spiral pattern is generated for a wide range of parameters. The spiral structure is global, and the number of turns can be two or three, depending on the model parameters. The morphology and kinematics of the spiral pattern are studied as functions of the halo and disk parameters. The spiral structure rotates slowly, and its angular velocity varies quasi-periodically. Models with differing relative halo masses, halo semi-axis ratios, distributions of matter in the disk, Mach numbers in the gaseous component, and angular rotational velocities of their halos are considered.     

Circumstellar envelopes of hot stars: Formation of extended regions of weakly ionized gas

We propose a model explaining the presence of vast regions of partially ionized gas in the interstellar medium. The circumstellar envelope of a hot star absorbs soft ionizing radiation, but transmits an appreciable fraction of the hard photons, which are absorbed much more weakly than photons with energies close to the ionization limit. For this reason, the radiation attenuated by the envelope becomes harder, and can penetrate to larger distances. For stars hotter than 50 000 K, the transition zone between the ionized and neutral gas can extend to tens or hundreds of parsecs. Thus, a region of partially ionized hydrogen, with a small gradient of the degree of ionization without a well-defined inner HII zone, can form in the interstellar medium.     

Thermoluminescence as a tool in uranium exploration

Fundamental principles of thermoluminescence (TL) and the special role of TL in uranium exploration techniques are given in this paper with examples from Australia illustrating its application.TL can be used in two ways in uranium exploration. The first usage stems from the property of minerals capable of TL, to act as dosimeters accounting for charging of available traps by ionizing radiation. The second usage relates to ionizing radiation interfering with the crystal structure of the host mineral and thereby changing the capability for TL. Studies at the Beverley and Westmoreland uranium deposits utilize this second property.At Beverley a continuous drop in TL intensity is observed upon approaching the orebody with TL intensity at a minimum within the orebody. Recognition of ore type TL glow curves and sensitized (or marginal) TL glow curves within the area surrounding the orebody have enlarged the exploration target beyond that of the orebody and its associated radiometric halo.Traverses examining the variations in glow peak intensities, ratios and temperatures across the Junnagunna orebody at Westmoreland show that the glow peak intensity of the high-temperature glow peak increases, that of the low-temperature peak fluctuates though generally decreases, the high temperature/low-temperature glow peak ratio increases upon approaching the orebody. The high-temperature glow peak temperature is at a minimum within the orebody. Similar variations detected nearby may indicate further potential exploration sites.An example from an unmineralized area at Bremer River has indicated a lack of widespread radiation sensitization indicating the unsuitability of the area for economic uranium accumulation.     

Do the galaxies NGC 936 and NGC 3198 possess massive spheroidal subsystems?

The condition for gravitational stability of the stellar disks of the galaxies NGC 936 and NGC 3198 makes maximum disk models unacceptable. We present mass estimates for these objects' spheroidal components. The mass of the dark halo of NGC 3198, within four disk radial scale lengths, exceeds its disk mass by a factor of 1.6 to 2. The masses of the disk and spheroidal subsystem (halo + bulge), within four radial scale lengths, are approximately the same for NGC 936.     

Interaction of the dark-matter cusp with the baryonic component in disk galaxies

The influence of the formation and evolution of a (disk) galaxy on the matter distribution in the dark-matter halo is considered. Calculations of the evolution of an isolated dark-matter halo were carried out with and without including a baryonic component. N-body simulations (for the dark-matter halo) and gas-dynamical numerical simulations (for the baryonic gas) were used for this analysis. Star formation, feedback, and heating and cooling of the interstellar medium were taken into account in the gas-dynamical calculations. The results of these numerical simulations with high spatial resolution indicate that 1) including the star formation resolves the so-called cusp problem (according to CDMcosmological models, the density distribution in the central regions of the dark-matter halo should have a distinct peak (cusp), which is not shown by observations); 2) the interaction of the dark matter with dynamical substructures of the stellar-gas galactic disk (spiralwaves, a bar) affects the shape of the dark-matter halo. In particular, the calculated dark-matter distribution in the plane of the disk is more symmetric when the baryonic component is taken into account.     

Chemical Composition of Globular Clusters of Milky Way Subsystems from Gaia DR2 Data

Results of a study of the kinematic and chemical properties of globular clusters of the Milky Way based on data from the Gaia DR2 catalog and meaurements with the Hubble Space Telescope are presented. A new method for dividing globular clusters into Galatic subsystems based on the elements of their Galactic orbits is proposed. Samples of globular clusters belonging to the bar/bulge, thick disk, and halo of the Milky Way are obtained. The mean metallicities of the globular clusters in various subsystems are calculated. The mean metallicities of globular clusters of the thick disk and halo display statistically significant differences. At the same time, no statistically significant differences are present between the mean metallicities of halo globular clusters moving in the direction of rotation of the Galactic disk and those moving in the retrograde direction. This argues against the suggestion that retrograde and prograde globular clusters have different origins.

     

BVRI surface photometry of the galaxy NGC 3726

We present BVRI surface photometry of the late-type spiral galaxy NGC 3627. The distributions of the color indices and extinction-independent Q indices show that the observed photometric asymmetry in the inner part of the galaxy, including the bar, is due to an asymmetric distribution of absorbing material. The bluest regions of star formation are located in a ring surrounding the bar. The background-subtracted color indices of individual blue knots are used to estimate the ages of young stellar aggregates. In combination with previously published photometric data, our measurements indicate that the R-band profile of the disk is rather flat in its inner part (r<50″) and becomes steeper further from its center. We estimate the mass of the disk and dark halo by decomposing the rotation curve. The mass-to-light ratio M/L _B for the stellar disk is ≈1.4. The galaxy possesses a massive dark halo; however, the mass of the disk exceeds that of the halo in the inner part of the galaxy, which displays a regular spiral structure.     

Cobalt in stars and the galaxy

An analysis of the abundance of cobalt in atmospheres of red giants, indicates they can be divided into two groups: stars with the normal [Co/Fe] abundance and those with a small [Co/Fe] excess. A comparative analysis of the spectrograms taking into account the effect of superfine splitting provides evidence for a [Co/Fe] excess in some stars. We have also detected physical and kinematical differences between these groups. Stars with a [Co/Fe] excess are related to the thick-disk population of the Galaxy. These stars are older and less massive, display lower metallicities, and have Galactic velocities corresponding to those of thick-disk objects. It is suggested that the observed pattern of a [Co/Fe] excess in the halo and thick disk reflects the chemical composition of the Galaxy at a very early stage of its evolution, when Population III objects existed. The lower abundance excess in the thick disk compared to the halo and the absence of an excess in the thin disk are due to the contributiuon from Type I supernovae at later stages of the Galaxy’s evolution. We have found that the thick disk of the Galaxy displays gradients of its cobalt and iron abundances, possibly providing evidence that the thick disk formed as a result of the collapse of a protogalactic cloud.     

The influence of ultrahigh-energy cosmic rays on star formation in the early Universe

The presence of ultrahigh-energy (UHE) cosmic rays results in an increase in the degree of ionization in the post-recombination Universe, which stimulates the efficiency of the production of H₂ molecules and the formation of the first stellar objects. As a result, the onset of the formation of the first stars is shifted to higher redshifts, and the masses of the first stellar systems decrease. As a consequence, a sufficient increase in the ionizing radiation providing the reionization of the Universe can occur. We discuss the possible observational manifestations of these effects and their dependence on the parameters of UHE cosmic rays.     

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.     

Structure of the accretion disk in the cataclysmic variable UX UMa in the transition to its quiescent state

Our analysis of BV RI light curves for the cataclysmic variable UX UMa obtained in intermediate activity states, in the transition between the active and quiescent states of the system on March 12, 1997 and May 3, 2000, shows that the shapes of these light curves cannot be adequately described using the standard hot-spot model. A model with a “hot line” near the edge of the disk and a two-armed spiral structure on the disk surface reproduces much better out-of-eclipse variations in the light curves. The presence of an extended hot line can explain the anomalous shape of the I light curve on March 12, 1997. The decrease in the observed luminosity of the system between March 12, 1997 and May 3, 2000 could be due to a decrease in the disk luminosity by a factor of 2–2.5; the higher disk luminosity on the earlier date is associated with appreciable deviations of the radial temperature distribution of the disk material from that for the standard model. The phases and depths of dips in the out-of-eclipse sections of the UX UMa light curves are due primarily to the parameters of the complex shape of the accretion disk, which has a spiral structure located mainly near its outer edge. The contribution of the spiral arms in the V filter reaches 20–50% of the total disk radiation. The crest of the first spiral wave in our model maintains its approximate position in azimuth; this structure could represent a bulge in a halo at the outer edge of the disk near orbital phases φ ～ 0.7, in the direction of the continuation of the extended shock in the disk itself. The position of the crest of the second spiral arm changes with time. This structure may represent a one-armed spiral wave near the apastron of the weakly elliptical disk. Finally, the observations testify to the presence of another spiral arm that is les clearly manifest in terms of both its luminosity and its height above the unperturbed disk surface. Thus, in an intermediate activity state of UX UMa, the surface of the accretion disk is distorted by the action of a two-armed spiral structure in the outer regions of the disk, which is asymmetric in both its luminosity and dimensions, and a bulge at the disk edge in the region of its interaction with the inflow to the disk.     

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.     

Disk wind in young binaries and its impact on the infrared excesses of young stars

We consider the effect of binarity of young stars on the spectral energy distribution of the IR radiation from circumstellar dust. The formation of a common dust envelope in a binary system with a low-mass secondary component is strongly affected by the disk wind from the secondary. The small velocities in peripheral areas of the wind are such that it can be partially or entirely captured by the primary, even when the distance between the components is up to several astronomical units. As a result, an envelope with a rather complex spatial and kinematic structure is formed. Its mass is many orders of magnitude smaller than that of the accretion disk around the binary. However, the thermal radiation emitted by dust particles of the envelope can be comparable to the total radiation of the accretion disk. This result is discussed in the context of the deficit at near-IR wavelengths (2–10µm) in current models for accretion disks around young stars.     

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.     

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.     

Neutron-capture elements in halo,thick-disk,and thin-disk stars: Neodymium

We have derived the LTE neodymium abundances in 60 cool stars with metallicities [Fe/H] from 0.25 to ?1.71 by applying a synthetic-spectrum analysis to spectroscopic observations of NdII lines with a resolution of λ/Δλ?60 000 and signal-to-noise ratios of 100–200. We have improved the atomic parameters of NdII and blending lines by analyzing the corresponding line pro files in the solar spectrum. Neodymium is overabundant with respect to iron in halo stars, [Nd/Fe]=0.33±0.09, with the [Nd/Fe] ratio decreasing systematically with metallicity when [Fe/H]>?1. This reflects an onset of efficient iron production in type I supernovae during the formation of the thick disk. The [Nd/Ba] and [Nd/Eu] abundance ratios behave differently in halo, thick-disk, and thin-disk stars. The observed abundance ratios in halo stars, [Nd/Ba]=0.34±0.08 and [Nd/Eu]=?0.27±0.05, agree within the errors with the ratios of the elemental yields for the r-process. These results support the conclusion of other authors based on analyses of other elements that the r-process played the dominant role in the synthesis of heavy elements during the formation of the halo. The [Nd/Ba] and [Nd/Eu] ratios for thick-disk stars are almost independent of metallicity ([Nd/Ba]=0.28(±0.03)?0.01(±0.04) [Fe/H] and [Nd/Eu]=?0.13(±0.03)+0.05(±0.04) [Fe/H]) but are smaller in absolute value than the corresponding ratios for halo stars, suggesting that the synthesis of s-process nuclei started during the formation of the thick disk. The s-process is estimated to have contributed ?30% of the neodymium produced during this stage of the evolution of the Galaxy. The [Nd/Ba] ratio decreases abruptly by 0.17 dex in the transition from the thick to the thin disk. The systematic decrease of [Nd/Ba] and increase of [Nd/Eu] with increasing metallicity of thin-disk stars point toward a dominant role of the s-process in the synthesis of heavy elements during this epoch.     

Possible mechanisms for the polarization of the radiation of Active Galactic Nuclei and quasars

Two mechanisms for the formation of polarized radiation in Active Galactic Nuclei and quasars are considered-synchrotron radiation from a jet and the emergence of radiation from a magnetized, optically thick accretion disk (the Milne problem in a magnetized atmosphere). These mechanisms are applied to the polarization of objects studied in [1]. We have shown that the polarization of some sources cannot be explained as synchrotron radiation. The second mechanism is more general and, in principle, can be applied to the polarized radiation of all the sources in [1].     

The influence of the decay of OB associations on the evolution of dwarf galaxies

The ejection of stars from spheroidal and disk dwarf galaxies resulting from the decay of OB associations is studied. This has substantial observational consequences for disk galaxies with escape velocities up to 20 km/s, or dynamical masses up to 10⁸ M _⊙. The ejection of stars can (i) reduce the abundances of the products of Type Ia supernovae and, to a lesser degree, Type II supernovae, in disk stars, (ii) chemically enrich the galactic halo and intergalactic medium, (iii) lead to the loss of 50% of the stellar mass in galaxies with masses ∼10⁷ M _⊙ and the loss of all stars in systems with masses ≲10⁵ M _⊙, (iv) increase the mass-to-luminosity ratio of the galaxy.