Intergalactic medium: X-ray background and compton parameter

A model of intergalactic medium heated by QSOs and cooled by the expansion of the universe and Compton cooling is studied in the framework of a Friedmann-Robertson-Walker universe. Cosmological evolution functions of the comoving density of QSO's as well as the case of no evolution are considered. The theoretical X-ray background spectrum (through thermal bremsstrahlung) and Comptony parameter are calculated including relativistic corrections in the electron-electron, electron-proton and electron-photon interactions. The observed X-ray background and the upper limit of the Compton parametery cobe given by the COBE satellite are used to adjust, for each value of reheating redshiftsz _c ranging from 0.1 to 5.0, the present values of the temperatureT ₀ and densityn ₀ of the intergalactic gas. Forz _c > 0.25, when the theoretical X-ray spectrum fits the observed one, the adjusted values ofT ₀ andn ₀ imply iny >y cobe. On the other hand, whenT ₀ andn ₀ are consistent withy cobe, the calculated X-ray spectrum is lower than the observed one. Unless 100% of the observed X-ray background is due to discrete sources and if the intergalactic medium contributes more than 2.5% to such background we come to the interesting result that the medium must have been heated atz _c < 1. In this case we shall have to explain the high energy rates necessary to heat the intergalactic medium. Forz _c 0.25, it is possible to find values ofT ₀ andn ₀ such that both the calculated X-ray background and the y parameter simultaneously reproduce the corresponding observed values. However, in this case, unless it could be shown to be otherwise by future observations or theoretical studies, it seems that the model of hot intergalactic medium is not plausible because of the high energies required to heat the intergalactic gas.     

The effect of intergalactic dust on the measurement of the cosmological deceleration parameterq 0

In a recent attempt to explain the cosmic microwave background without the big bang, a thermalization mechanism involving intergalactic whisker grains of graphite was proposed. The effect of absorption by the intergalactic medium in general, and of the above type in particular, on the measurement of the deceleration parameterq ₀ of the expanding universe is discussed. Its effect is shown to be comparable in magnitude but opposite to that of the luminosity evolution in galaxies. A consequential selection effect is also discussed.     

The chemical composition of matter in the early universe

The results of the computations of the chemical evolution for a galaxy cluster are presented. The matter exchange between galaxies and intergalactic medium is taken into account. Two dependences of star formation rate on time are considered: (i) monotonously decreasing dependence characteristic of elliptical galaxies, (ii) dependence having two peaks associated with creation of spiral galaxy subsystems, with suppression of star formation at the period between maxima. It is assumed that galactic ejection is due to explosions of II-type supernova with massesm5M , and that the accretion on to a galaxy depends but weakly on the time. By comparing the obtained results with total combination of available observations, it is established that the rate of gaseous exchange between a galaxy and intergalactic medium should be rather large: 0.03M _gal Gyr^–1. Besides, the activity of each type of galaxy leads to an approximately equal enrichment of intergalactic gas by new elements synthesized in the stars. The existence of a large accretion on to the Galaxy leads to the decrease of primordial deuterium abundance by a factor of no more than 2 during the galaxy evolution time. It enables us to assume that the standard Big Bang model with baryon density parameter _b 0.1 may be considered as true.     

X-ray absorption effects due to intergalactic abundance of carbon

The absorption effects at the soft X-ray and hard ultraviolet wavelengths due to some model abundance of intergalactic carbon material have been investigated for different cosmologies. Even though the local density, 2 × 10⁹ <ϱ ₀ = 1.0 × 10⁻³⁴ g cm⁻³ of the absorbing component of the intergalactic material in the form of carbon is not adequate for the thermalization of the discrete background radiation, the amount of absorption in the X-rays up to the Hubble radius is not negligible.     

Variation in the Opacity of the Intergalactic Medium During its Reionization

It is now assumed that reionization of the intergalactic medium occurred under the action of ultraviolet radiation from hot stars contained in galaxies that formed early. Regions of ionized hydrogen (H II zones) were formed around such galaxies. The effects of hydrogen recombination and of the simultaneous cosmological expansion of such regions on their opacity are considered. It is shown that regions formed at z _i < 6 are opaque to L _c radiation, while for z _i 5 the intergalactic medium should be transparent at 0 < z < 5. This conclusion is in accord with recent observational data.     

Pregalactic metal enrichment

I review recent results concerning the metal enrichment of the intergalactic medium and give arguments which allow to conclude that the heavy elements detected in the Lyα forest absorbers must have been produced and injected by a population of protogalaxies with typical mass ≈ 10⁸ M_⊙ at redshift z ≈ 10. In addition, I will discuss the existence of a metallicity-induced shift in the Initial Mass Function of first stars from an extrely top-heavy distribution to a `normal' one and bring observational support to this idea. This revised version was published online in August 2006 with corrections to the Cover Date.     

Is the intergalactic magnetic field primordial?

We consider the various methods used to constrain the possible field strength of the present day intergalactic field and findB ₀(G)<10^–10 as a probable upper bound. It is suggested that the observed intergalactic magnetic field might not be primordial in origin but rather the result of magnetic flux leakage from galaxies and clusters of galaxies.     

Intergalactic extinction and the deceleration parameter

The deceleration parameter q₀ is calculated from the relation between apparent magnitudes m of the brightest galaxies in clusters and their redshifts z considering an intergalactic extinction. The calculation is valid for a Friedman universe, homogeneously filled with dust grains, assuming the extinction to be 0.5 mag at z = 1 and a λ⁻¹-law of extinction (according to Oleak and Schmidt 1976). Using the m,z-values of Kristian, Sandage, and Westphal (1978) a formal value of q₀ ≈︁ 2.1 is obtained instead of q₀ ≈︁ 1.6 without consideration of intergalactic extinction.     

The chemical evolution of a spiral galaxy disks

The results of computing the evolution of a disk subsystem of a spiral galaxy taking into account radial gradients of chemical elements (D,⁴He,¹²C,¹³C,¹⁴N,¹⁶O,⁷Li, Fe) are presented. It is shown that flow of intergalactic matter accreting on to the galaxy results in smoothing the radial gradients of gas and chemical elements, especially at the latest stage of the evolution. The final gradients depend weakly on the age of the galaxy and value of the Hubble constanth. The best agreement between calculated and observed abundance of⁴He is revealed if the primordial yield isY ₀=0.25. The small decrease of the primordial deuterium abundance confirms the conclusion that the baryon density parameter is small: _b h ₅₀ ² 1. Therefore, the nature of dark matter should be non-baryonic.     

The mass distribution in the galaxy cluster Abell 2744

The mass distribution for the galaxy cluster Abell 2744 (z = 0.308) is investigated on the base of the archival X-ray data of the Chandra observatory. The temperature of the hot gas in the cluster (kT = 9.82_−0.41^+0.43 keV) and the cluster total mass (M ₂₀₀ = 2.22_−0.12^+0.13 × 10¹⁵ M _⊙) for the radius R ₂₀₀ = 2.38_−0.31^+0.36 Mpc are estimated. The density and mass profiles for the intergalactic gas and dark matter are obtained. The fractions of the intergalactic gas and dark matter in the total mass of the cluster are 15.4_−1.3^+1.3% and 84.6_−1.3^+1.4%, respectively.     

First stars. I. Evolution without mass loss

The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M _⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M _⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10⁻⁶ and 10⁻⁹, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.     

Massive stars and the intergalactic medium

We review the results of an X-ray (ASCA) survey of intergalactic medium metallicities in 33 groups and clusters of galaxies. The observed picture could be explained either by adopting top heavy IMF for galaxies observed at high environment overdensities or by metal-rich SN II driven winds.     

Elemental abundances for stellars-process studies

Nuclear reaction network calculations of heavy element enhancements are tabulated for different values of the mean neutron exposure ₀. These tables are useful for comparison with observeds-process abundance patterns which appear to span a wide range in ₀.     

On the existence and the density of intergalactic dust

A relation between the redshift z of QSO's and their colour index (B–V)' corrected for line emission and galactic absorption is interpreted in terms of an intergalactic selective extinction. The observed amount of extinction corresponds to a density of intergalactic dust grains of about 10⁻³⁴ to 10⁻³³ g cm⁻³ at z = 0. The life-time of these particles are estimated to be longer than the Hubble -time at present. But at large z the life-time of the grains considerably depends on the flux density of cosmic rays. Some implications of the existence of intergalactic dust are discussed.     

Numerical models of the galactic dynamo

Axisymmetric αω-dynamo models are investigated numerically for the galactic dynamo. Contrasting to the eigenvalue formulation of the problem by STIX (1976), an initial-value formulation is developed in a manner which is a generalization of the approach to the solar dynamo by JEPPS (1975). It is found, for STIX's model, that the critical dynamo numbers, P_c, obtained by this approach do not agree with those obtained by STIX . In order to resolve this disagreement SOWARD (1977) has evaluated an asymptotic formula for P_c which confirm the results presented here. Having established this approach, the dependence of the models upon boundary conditions and the relevant astrophysical parameters is investigated, and an attempt is made to simulate nonlinear effects. Finally a comparison is made between predictions of the dynamo models and the observed radiation of certain external galaxies which provides insight into the nature of the intergalactic medium.     

Infall of gas from intergalactic space and soft x-ray background

As the origin of the soft X-ray background, emission of soft X-rays from shocks occurred in the accretion of intergalactic gas onto the Galaxy is studied. Infall of discrete gas clouds cannot explain the diffuse component of soft X-rays. If intergalactic gas rich in heavy elements as the cosmic abundance continously flows into the Galaxy and forms a standing shock surrounding the Galaxy, the line emissions by heavy elements from the shocked gas explain the soft X-ray background. Formation of the high velocity cloud by thermal instability in the shocked gas is also discussed briefly.     

Cosmic ray origin above 1018 eV: Galactic or extragalactic?

An analysis is made of the implications of assuming that suprathermal dust grains (a3×10⁸ cm) of intergalactic origin may acquire cosmic ray energies as high as 10²⁰ eV. These dust grains may attain suprathermal energy (v _g3×10⁸ cm s^–1) by the Fermi process. Initially the dust grains are accelerated by the radiation pressure against the drag of the ambient gas of the medium, but once these dust grains attain a terminal velocity (U10⁵ cm s^–1), then they may be expelled out of the galactic region into the intergalactic medium and finally acquire high energy     

On the evolution of a system of intergalactic clouds

The presence of L _a -forests in the spectra of quasars is considered as proof of the fragmentary structure of the intergalactic medium. The masses of the clumps (of clouds) will increase as they merge together. Once the radial concentration of neutral hydrogen in a cloud attains its critical value, star formation begins, and the cloud turns into a galaxy. Certain physical properties of clouds are considered and a new approach to the investigation of the evolution of systems of clouds is proposed.Translated from Astrofizika, Vol. 37, No. 1, pp. 35–42, January–March, 1994.     

Helium contamination from the progenitor stars of planetary nebulae: The He/H radial gradient and the ΔY / ΔZ enrichment ratio

In this work, two aspects of the chemical evolution of ⁴He in the Galaxy are considered on the basis of a sample of disk planetary nebulae (PN). First, an application of corrections owing to the contamination of ⁴He from the evolution of the progenitor stars shows that the He/H abundance by number of atoms is reduced by 0.012 to 0.015 in average, leading to an essentially flat He/H radial distribution. Second, a determination of the helium to heavy element enrichment ratio using the same corrections leads to values in the range 2.8 < ΔY / ΔZ < 3.6 for Y _p = 0.23 and 2.0 < ΔY / ΔZ < 2.8 for Y _p = 0.24, in good agreement with recent independent determinations and theoretical models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spektrum und Anisotropiegrad der 3 °K-Strahlung bei anisotroper kosmologischer Expansion

Two theorems concerning the propagation of electromagnetic radiation which interacts with matter through elastic electron scattering in arbitrary gravitational fields allow to draw conclusions relevant to the thermal history of the 3 °K microwave background: Provided, a thermalization at some instant t = t₁ in the past has established at this time a Planckian photon distribution function (possibly with the temperature T depending on the propagation direction and on space coordinates), then (i) the Planckian distribution is preserved in a first approximation, if the deviation of the actual distribution from an isotropic Planckian remains always small; and (ii) in the Rayleigh-Jeans domain the Planckian shape is preserved independent of the degree of radiation anisotropy. Due to thermalizing action of electron scattering in ionized intergalactic and pre-galactic matter the observed degree of anisotropy in the 3 °K microwave background may be smaller as is expected for collisionless radiation propagating in a given geometry. To study this effect, the equation of radiative transfer with an electron scattering term is integrated in an anisotropic universe of the Heckmann-Schücking type (Bianchi type I), starting from an early optically thick epoch (corresponding to a radiation temperature T = 5000 °K) prior to plasma recombination. With the quadrupole anisotropy of the order of 2 · 10⁻³ found by Partridge and Wilkinson in the Rayleigh-Jeans domain, metric anisotropy parameters ranging from a = 380 to 900 years are derived, if the re-ionization of intergalactic matter sets in at redshift values ranging from z_r = 0 to z_r = 8.