arXiv subjects

Yu. A. Shchekinov

Publications and source records attributed to Yu. A. Shchekinov.

At least 37 records · Page 2Linked to original sources

arXiv2007

Particle decay in the early universe: predictions for 21 cm

The influence of ultra-high energy cosmic rays (UHECRs) and decaying dark matter particles on the emission and absorption characteristics of neutral hydrogen in 21 cm at redshifts $z = 10-50$ is considered. In presence of UHECRs 21 cm can be seen in absorption with the brightness temperature $T_b=-(5-10)$ mK in the range $z=10-30$. Decayng particles can stimulate a 21 cm signal in emission with $T_b\sim 50-60$ mK at $z =50$, and $T_b \simeq 10$ mK at $z \sim 20$. Characteristics of the fluctuations of the brightness temperature, in particular, its power spectrum are also calculated. The maps of the power spectrum of the brightness temperature on the plane {\it wavenumber-redshift} are shown to be sensitive to the parameters of UHECRs and decaying dark matter. Observational possibilities to detect manifestations of UHECRs and/or decaying particles in 21 cm with the future radio telescopes (LOFAR, 21CMA and SKA), and to distinguish contributions from them are briefly discussed.

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arXiv2006

The influence of ultra-high-energy cosmic rays on star formation in the early universe

The presence of ultra-high-energy cosmic rays (UHECR) results in an increase in the degree of ionization in the post-recombination Universe, which stimulates the efficiency of the production of H$_2$ 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 take place. We discuss possible observational manifestations of these effects and their dependence on the parameters of UHECR.

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arXiv2006

Primordial gas cooling behind shock waves in merging halos

We investigate thermal regime of the baryons behind shock waves arising in the process of virialization of dark matter halos. We find a fraction of the shocked gas cooled by radiation of HD molecules down to the temperature of the cosmic microwave background (CMB): this fraction increases sharply from about $f_{\rm c}\sim 10^{-3}$ for dark halos of $M=5\times 10^7\msun$ to $\sim 0.1$ for halos with $M=10^8\msun$ at $z=10$. We show, however, that further increase of the mass does not lead to a significant growth of $f_{\rm c}$ -- the assymptotic value for $M\gg 10^8\msun$ is of 0.2. We estimate star formation rate associated with such shock waves, and show that it can be a small but not negligible fraction of the star formation connected with cooling by HI and H$_2$. We argue that extremely metal-poor low-mass stars in the Milky Way may have been formed from primordial gas behind such shocks.

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arXiv2005

Formation of HD molecules in merging dark matter haloes

HD molecules can be an important cooling agent of the primordial gas behind the shock waves originated through mergings of the dark matter haloes at epochs when first luminous objects were to form. We study the necessary conditions for the HD cooling to switch on in the low temperature range $T<200$ K. We show that these conditions are fulfiled in merging haloes with the total (dark matter and baryon) mass in excess of $M_{\rm cr}\sim 10^7[(1+z)/20]^{-2}\msun$. Haloes with masses $M>M_{\rm cr}$ may be the sites of low-mass star formation.

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arXiv2005

Radial transport of dust in spiral galaxies

Motivated by recent observations which detect dust at large galactocentric distances in the disks of spiral galaxies, we propose a mechanism of outward radial transport of dust by spiral stellar density waves. We consider spiral galaxies in which most of dust formation is localized inside the corotation radius. We show that in the disks of such spiral galaxies, the dust grains can travel over radial distances that exceed the corotation radius by roughly 25%. A fraction of the dust grains can be trapped on kidney-shaped stable orbits between the stellar spiral arms and thus can escape the destructive effect of supernova explosions. These grains form diffuse dusty spiral arms, which stretch 4-5 kpc from the sites of active star formation. About 10% of dust by mass injected inside corotation, can be transported over radial distances 3-4 kpc during approximately 1.0 Gyr. This is roughly an order of magnitude more efficient than can be provided by the turbulent motions.

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arXiv2005

Contribution of HD molecules in cooling of the primordial gas

We study the effects of HD molecules on thermochemical evolution of the primordial gas behind shock waves, possibly arised in the process of galaxy formation. We find the critical shock velocity when deuterium transforms efficiently into HD molecules which then dominate gas cooling. Above this velocity the shocked gas is able to cool down to the temperature of the cosmic microwave background. Under these conditions the corresponding Jeans mass depends only on redshift and initial density of baryons $M_J \propto δ_c^{-0.5} (1+z)^{0.5}$. At $z\simgt 45$ HD molecules heat shocked gas, and at larger redshift their contribution to thermal evolution becomes negligible.

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arXiv2005

The Minimum Stellar Mass in Early Galaxies

The conditions for the fragmentation of the baryonic component during merging of dark matter halos in the early Universe are studied. We assume that the baryonic component undergoes a shock compression. The characteristic masses of protostellar molecular clouds and the minimum masses of protostars formed in these clouds decrease with increasing halo mass. This may indicate that the initial stellar mass function in more massive galaxies was shifted towards lower masses during the initial stages of their formation. This would result in an increase of the number of stars per unit halo mass, i.e., the efficiency of star formation.

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arXiv2005

The generation of low-energy cosmic rays in molecular clouds

It is argued that if cosmic rays penetrate into molecular clouds, the total energy they lose can exceed the energy from galactic supernovae shocks. It is shown that most likely galactic cosmic rays interacting with the surface layers of molecular clouds are efficiently reflected and do not penetrate into the cloud interior. Low-energy cosmic rays ($E<1$ GeV) that provide the primary ionization of the molecular cloud gas can be generated inside such clouds by multiple shocks arising due to supersonic turbulence.

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arXiv2004

Dusty plasmas on galactic scales

Interstellar dust is spread in galaxies over large scales, often far beyond the stellar disks. Several mechanisms can be responsible for carrying the dust both in the vertical and radial directions, producing in general different spatial dust distributions. In spite of the small mass fraction dust can be in some cases dynamically active, and its role in star formation process and dynamics of shock waves is still not completely understood. In this review I discuss briefly the issues connected with the large scale distribution of the interstellar dust, the impact of the dust on star formation and on dynamics of shock waves.

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arXiv2004

Primordial star formation triggered by UV photons from UHECR

The presence of ultra-high energy cosmic rays in the universe in the pre-ionization epoch enhances significantly H$_2$ kinetics in virialized halos. It results in more efficient radiative cooling and decreases the lower mass limit of the first star-forming systems. Consequently, the fraction of baryons contained in the first luminous objects and their contribution to the reionization of the universe can significantly increase in comparison with the standard scenario.

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arXiv2004

Synchrotron emission from circumstellar disks around massive stars

We argue that the interaction of stellar wind with the surface of a circumstellar accretion (or protoplanetary) disk can result in the acceleration of relativistic electrons in an external layer of the disk, and produce synchrotron radiation. Conservative estimates give a total synchrotron luminosity $L_s\sim 10^{-5}\lsun$ for a central star with $\dot M=10^{-6}\msun$ yr$^{-1}$, comparable with the value observed around the TW object in the W3(OH) region.

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arXiv2003

HI holes in galactic disks: Tracing the dark matter distribution

Multiple SN explosions in disk galaxies efficiently evacuate gas and form cavities with the sizes and shapes of the surrounding envelopes determined by the total amount of injected energy and by the initial gas distribution. Such cavities are seen as HI holes when observed in face-on galaxies. Gas hydrodynamics simulations are performed to obtain the quantitative characteristics of HI holes that could serve for the determination of the gas vertical scale height and the corresponding dark matter content and its distribution. Among these characteristics is the ratio of the maximum column density in the HI ring surrounding the hole to the background HI column density and the vertical expansion velocity of gas in the HI ring. We show that in some cases the extragalactic background ionizing radiation may produce HI holes in the outer regions of galaxies, and can account for the existence of HI holes in nearby face-on galaxies with the apparent lack of an underlying stellar population.

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arXiv2003

Numerical simulations of expanding supershells in dwarf irregular galaxies. I. Application to Holmberg I

Numerical hydrodynamical modelling of supernova-driven shell formation is performed with a purpose to reproduce a giant HI ring (diameter 1.7 kpc) in the dwarf irregular galaxy Holmberg I (Ho I). We find that the contrast in HI surface density between the central HI depression and the ring is sensitive to the shape of the gravitational potential. This circumstance can be used to constrain the total mass (including the dark matter halo) of nearly face-on dwarf irregulars. We consider two models of Ho I, which differ by an assumed mass of the dark matter halo M_h. The contrast in HI surface density between the central HI depression and the ring, as well as the lack of gas expansion in the central hole, are better reproduced by the model with a massive halo of M_h=6.0*10^9 M_sun than by that with a small halo of M_h=4.0*10^8 M_sun, implying that Ho I is halo-dominated. Assuming the halo mass of 6.0*10^9 M_sum, we determine the mechanical energy required to form the observed ring equal to (3.0 +- 0.5)*10^53 ergs, equivalent 300+-50 Type II supernovae. The inclination of Ho I is constrained to 15-20 degrees by comparing the modelled HI spectrum and channel maps with those observed.

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arXiv2002

Metal enrichment of the high-z intergalactic medium

Recent observations show the presence of metals in low-density Ly$α$ forest absorbers at high redshift ($z\sim 3$). It remains still far from being clearly understood what mechanisms spread metals over Mpc scales from the parent galaxies, whether metals are homogeneously distributed in the intergalactic medium (IGM), how metallicity of the IGM does evolve. These questions are briefly addressed in this review.

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arXiv2001

The Influence of the Gravitational Acceleration on the Supernova-Driven P arker Instability

Within a framework of 2D MHD simulations, we explore the dynamical regimes initi ated by a supernova explosion in a magnetized stratified ISM. We concentrate on the formation of large scale magnetic structures and outflows connected with the Parker instability. For the sake of simplicity we show only models with a fixed explosion energy corresponding to a single SN occuring in host galaxies with different fixed values of the gravitational acceleration $g$ and different ratios of specific heats. We show that in general depending on these two parameters, three different regimes are possible: a) a slowly growing Parker instability on time scales much longer than the galac tic rotation period for small $g$, b) the Parker instability growing at roughly the rotation period, which for ratios of specific heats larger than 1 is accompanied byan outflow resulting from the explosion for intermediate $g$, and c) a rapidly g rowing instability and a strong blowout flow for large $g$. By means of numerical simul ations and analytical estimates we show that the explosion energy and gravitational acceleration which separate the three regimes scale as $Eg^2\sim $const in the 2D case. We expect that in the 3D case this scaling law is $Eg^3\sim $const. Our simulations demonstrate furthermore, that a single SN explosion can lead to the growth of multiple Parker loops in the disc and large scale magnetic field loops in the halo, extending over 2-3 kpc horizontally and up to 3 kpc vertically abov e the midplane of the disc.

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arXiv2001

Bubbles in galactic haloes

We briefly discuss a possible interconnection of vertical HI structures observed in the Milky Way Galaxy with large scale blow-outs caused by the explosions of multiple clustered SNe. We argue that the observed OB associations can produce only about 60 such events, or approximately one chimney per 3 kpc$^2$ within the solar circle. We also discuss the overall properties of HI shells in nearby face-on galaxies and the distribution of H$α$ and dust in edge-on galaxies. We argue that the presence of dust in galactic haloes may indicate that radiation pressure is the most probable mechanism capable of transporting dust to large heights above the galactic plane. In order to make this possible, the galactic magnetic field must have a strong vertical component. We mention that SNe explosions can initiate the Parker instability which in turn creates large scale magnetic loops with a strong vertical component. Recent observations of nearby edge-on galaxies favour this suggestion.

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arXiv1999

H2 dark matter in the galactic halo from EGRET

We present a model for the interpretation of the gamma-ray background emission from the Galactic halo, which we attribute to interaction of high-energy cosmic rays with dense molecular clumps. In a wide range of clump parameters we calculate the expected gamma-photon flux, taking into account for the first time optical depth effects. This allows us to derive new constraints on masses and sizes of possible molecular clumps and their contribution to the galactic surface density. The observed gamma-ray distribution can be explained best by models with a flattened halo distribution of axis ratio ~ 0.3. If optical depth effects are important, the clumps must have radii of ~ 6 AU and masses of ~ 10^{-3} M_sun. This would result in a total mass of ~ 2 10^{11} M_sun for such clumps and contribute with Sigma ~ 140 M_sun pc^{-2} to the local surface density.

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arXiv1998

Observational consequences of baryonic gaseous dark matter

Possible observational consequences of dark matter in the Galaxy in the form of dense molecular gas clouds - clumpuscules of masses $M_c\sim 10^{-3} \msun$ and radii $R_c\sim 3\times 10^{13}$ cm - are considered. Recent models of the extreme scattering events - refraction of radio-waves from quasars in dense plasma clumps in the Galactic halo - definitely show on such clouds as possible dark matter candidate. We argue that collisions of such clumpuscules are quite frequent: around $1-10 \msun$ a year can be ejected in the interstellar medium due to collisions. Optical continuum and 21 cm emissions from post-collisional gas are found to be observable. We show that clumpuscules can form around O stars HII regions of sizes $R\sim 30$ pc and emission measure $EM\simeq 20 cm^{-6}$ pc, and can also be observable in $H_α$ emission. Evaporation of clumpuscules by external ionizing radiation can be a substantial mass source. From requirement that the total mass input on the Hubble time cannot exceed the luminous mass in the Galaxy, typical radius of clouds is constrained as $R_c < 3.5\times 10^{12}$ cm, and their contribution to surface mass density as 50 $\msun pc^{-2}$. It is argued that dissipation of kinetic energy of the gas ejected by such clouds can be an efficient energy source for the Galactic halo.

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