arXiv subjects

Evgenii O. Vasiliev

Publications and source records attributed to Evgenii O. Vasiliev.

23 records · Page 2Linked to original sources

arXiv2014

HI absorption from the epoch of reionization and primordial magnetic fields

We study the impact of primordial magnetic fields on the HI absorption from the Epoch of Reionization. The presence of these fields result in two distinct effects: (a) the heating of the haloes from the decay of magnetic fields owing to ambipolar diffusion, and (b) an increase in the number of haloes owing to additional matter fluctuations induced by magnetic fields. We analyse both these effects and show that the latter is potentially observable because the number of haloes along of line of sight can increase by many orders of magnitude. While this effect is not strongly dependent on the magnetic field strength in the range $0.3\hbox{--}0.6$ nG, it is extremely sensitive to the magnetic field power spectral index for the near scale-free models. Therefore the detection of such absorption features could be a sensitive probe of the primordial magnetic field and its power spectrum. We discuss the detectability of these features with the ongoing and future radio interferometers. In particular, we show that LOFAR might be able to detect these absorption features at $z \simeq 10$ in less than 10 hrs of integration if the flux of the background source is 400 mJy.

astro-ph.CO↗

arXiv2013

Non-equilibrium cooling rate for a collisionally cooled metal-enriched gas

We present self-consistent calculations of non-equilibrium (time-dependent) cooling rates for a dust-free collisionally controlled gas in wide temperature ($10 K\le T\le 10^8 K$) and metallicity ($10^{-4} Z_\odot \le Z \le 2 Z_\odot$) ranges. We confirm that molecular hydrogen dominates cooling at $10^2 \simlt T\simlt 10^4$ K and $Z\simlt 10^{-3} Z_\odot$. We find that the contribution from H$_2$ into cooling rate around $T\sim (4-5)\times 10^3$ K stimulates thermal instability in the metallicity range $Z\simlt 10^{-2} Z_\odot$. Isobaric cooling rates are generally lower than isochoric ones, because the associated increase of gas density leads to both more efficient hydrogen recombination and equilibration of the fine-structure level populations. Isochoric cooling keeps the ionization fraction remains quite high at $T\simlt10^4$ K: up to $\sim0.01$ at $T\simeq 10^3$ K and $Z\simlt 0.1 Z_\odot$, and even higher at higher metallicity, contrary to isobaric cooling where it at least an order of magnitude lower. Despite this increase in ionization fraction the gas-phase formation rate of molecular hydrogen (via H$^-$) lowers with metallicity, because higher metallicity shorttens the evolution time. We implement our self-consistent cooling rates into the multi-dimensional parallel code ZEUS-MP in order to simulate evolution of a supernova remnant, and compare it with an analogous model with tabulated cooling rates published in previous works. We find significant differences between the two descriptions, which may appear, e.g., in mixing of the ejected metals in the circumstellar medium.

astro-ph.CO↗

arXiv2011

Thermal instability in the collisionally cooled gas

We have presented the non-equilibrium (time-dependent) cooling rate and ionization state calculations for a gas behind shock waves with $v \sim 50-150$ km s$^{-1}$ ($T_s \sim 0.5 - 6\times 10^5$ K). Such shock waves do not lead to the radiative precursor formation, i.e. the thermal evolution of a gas behind the shock waves are controlled by collisions only. We have found that the cooling rate in a gas behind the shock waves with $v \sim 50-120$ km s$^{-1}$ ($T_s \sim 0.5 - 3\times 10^5$ K) differs considerably from the cooling rate for a gas cooled from $T = 10^8$ K. It is well-known that a gas cooled from $T = 10^8$ K is thermally unstable for isobaric and isochoric perturbations at $T \simgt 2\times 10^4$ K. We have studied the thermal instability in a collisionally controlled gas for shock waves with $v \sim 50-150$ km s$^{-1}$. We have found that the temperature range, where the postshock gas is thermally unstable, is significantly modified and depends on both gas metallicity and ionic composition of a gas before shock wave. For $Z \simgt 0.1Z_\odot$ the temperature range, where the thermal instability criterion for isochoric perturbations is not fulfilled, widens in comparison with that for a gas cooled from $T = 10^8$ K, while that for isobaric perturbations remains almost without a change. For $Z\sim Z_\odot$ a gas behind shock waves with $v \simlt 65$ km s$^{-1}$ ($T_s \simlt 10^5$ K) is thermally stable to isochoric perturbations during full its evolution. We have shown that the transition from isobaric to isochoric cooling for a gas with $Z \simgt 0.1Z_\odot$ behind shock waves with $T_s = 0.5 - 3\times 10^5$ K proceeds at lower column density layer behind a shock wave than that for a gas cooled from $T = 10^8$ K. (abridged)

astro-ph.CO↗

arXiv2011

Nonequilibrium ionization states and cooling rates of the photoionized enriched gas

Nonequilibrium (time-dependent) cooling rates and ionization state calculations are presented for low-density gas enriched with heavy elements (metals) and photoionized by external ultraviolet/X-ray radiation. We consider a wide range of gas densities and metallicities and also two types of external radiation field: a power-law and the extragalactic background spectra. We have found that both cooling efficiencies and ionic composition of enriched photoionized gas depend significantly on the gas metallicity and density, the flux amplitude, and the shape of ionizing radiation spectrum. The cooling rates and ionic composition of gas in nonequilibrium photoionization models differ strongly (by a factor of several) from those in photoequilibrium due to overionization of the ionic states in the nonequilibrium case. The difference is maximal at low values of the ionization parameter and similar in magnitude to that between the equlibrium and nonequilibrium cooling rates in the collisionally controlled gas. In general, the nonequilibrium effects are notable at $T\simlt 10^6$ K. In this temperature range, the mismatch of the ionic states and their ratios between the photoequilibrium and the photo-nonequilibrium models reach a factor of several. The net result is that the time-dependent energy losses due to each chemical element (i.e. the contributions to the total cooling rate) differ singificantly from the photoequilibrium ones. We advocate the use of nonequilibrium cooling rates and ionic states for gas with near-solar (and above) metallicity exposed to an arbitrary ionizing radiation flux. We provide a parameter space (in terms of temperature, density, metallicity and ionizing radiation flux), where the nonequilibrium cooling rates are to be used. (abridged)

astro-ph.CO↗

arXiv2010

Carbon ionization states and the cosmic far-UV background with HeII absorption

We constrain the spectrum of the cosmic ultraviolet background radiation by fitting the observed abundance ratios carbon ions at $z\sim 2\hbox{--}3$ with those expected from different models of the background radiation. We use the recently calculated modulation of the background radiation between 3 and 4 Ryd due to resonant line absorption by intergalactic HeII, and determine the ratios of CIII to CIV expected at these redshifts, as functions of metallicity, gas density and temperature. Our analysis of the observed ratios shows that 'delayed reionization' models, which assume a large fraction of HeII at $z\sim3$, is not favored by data. Our results suggest that HeII reionization was inhomogeneous, consistent with the predictions from recent simulations.

astro-ph.CO↗