arXiv subjects

Biman B. Nath

Publications and source records attributed to Biman B. Nath.

100 records · Page 6Linked to original sources

arXiv1997

Lyman alpha absorption lines from mini pancakes

[Abridged abstract:] Recent numerical simulations show that many $\lyal$ absorption lines of column densities $\nha \la 10^{15}$ cm$^{-2}$ are produced in transient, mini pancakes. Such pancakes are modeled here, approximating the initial perturbation leading to the formation of the pancake as a single sinusoidal wave. The density and temperature profiles of the gas in the pancake are determined for $z_c \sim 3$, where $z_c$ is the collapse redshift. The $\lyal$ absorption line profiles for a line of sight through the pancake are then calculated. The absorption lines in general have wings signifying bulk motions in the gas. It is shown that the deviation from a single Voigt profile is large for small H I column density lines, in which the effect of bulk motions is large. For lines with $\nha > 10^{13}$ cm$^{-2}$, high temperature tend to wash out the signatures of bulk motion. The analytical modeling of mini pancakes associated with $\lyal$ forest lines --- with $10^{13} \la \nha \la 10^{15}$ cm$^{-2}$---gives the corresponding mass scales. It is shown here that, for typical values of cosmological parameters, absorption lines with $\nha \sim 10^{14}$ cm$^{-2}$ correspond to structures with baryonic mass of $M_b \sim 10^{10}$ M$_{\odot}$ with an overdensity of $\sim 10$ at $z \sim 3$. The value of $\nha$ can change by a factor $\sim 3$ in the course of evolution of the pancake in time. It is also shown that there is an upper limit to $\nha$ from a pancake due to the slow recombination rate and the importance of collisional ionization at high temperatures. Mini pancakes do not give rise to $\lyal$ lines with $\nha \ga 10^{14.5}$ cm$^{-2}$, for $\j21=1$ and $Ω_{IGM} \sim 0.03$.

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arXiv1995

On the interpretation of the He II absorption in the line of sight of Q0302-003

We point out the peculiarities of the line of sight of Q0302-003 which was recently used to estimate the He II opacity at $z \sim 3.285$. This line of sight contains a large void in Lyman $α$ clouds, in the region used for the He II opacity observation, and the void is thought to be caused by the ionizing radiation from a foreground quasar (Dobrzycki and Bechtold 1991). If this is true, then the radiation from the foreground quasar is more relevant in the region used for the estimation of the He II opacity than the diffuse UV background radiation. We argue that in that case the observed opacity should not be used to put constraint on the diffuse UV background radiation. Using a typical spectral index of quasars, we estimate that most of the observed He II opacity must be due to Gunn-Peterson effect and suggest a H I Gunn-Peterson test along this line of sight. We further discuss the clumpiness of the intergalactic medium in the vicinity of Q0302-003 as suggested by observations.

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arXiv1995

Radiation Dosimetry of Binary Pulsars

Companion stars exposed to high energy radiation from a primary neutron star or accreting black hole can experience significant spallation of their heavy elements, so that their atmospheres would be extremely rich in lithium, beryllium, and especially boron. In this paper we note that the detection or non-detection of these elements, and their relative abundances if detected, would provide a diagnostic of the high energy output of the primary, and possibly the shock acceleration of particles at the companion's bow shock in a pulsar wind.

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arXiv1995

DWARF GALAXIES AND THE ORIGIN OF INTRACLUSTER MEDIUM

Following the recent suggestion that it is dwarf galaxies in clusters -- as opposed to large ellipticals -- that provide the intracluster gas, we estimate the metallicity of the intracluster medium (ICM) in such a case. We derive analytical expressions for the fraction of mass of dwarf galaxies that is ejected, and estimate the metallicity of the resulting intracluster gas. We find that the metallicity resulting from this hypothesis is adequate only for clusters with low-metallicity gas. Since galactic winds from dwarf galaxies occur much earlier than those from ellipticals, due to the smaller binding energy of the gas, we show that the gas ejected by dwarf galaxies is enriched mostly by Type II supernovae, as opposed to Type I supernovae in the case of large galaxies. We further point out that the gas in small scale structures, which never cools and forms stars due to low temperatures and, consequently, large cooling time scale, gets dispersed in the process of hierarchical clustering, and is incorporated as the diffuse intracluster gas. We show that this process can provide enough hot gas to be compatible with X-ray observations in rich clusters.

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arXiv1994

Intergalactic Matter and Cocoons of Radio Galaxies

The cocoons surrounding powerful radio sources can be extensive if the jet that feeds the cocoon is light and supersonic. They have been shown to remain overpressured with respect to the ambient medium for most of the life time of the sources. The observed lobes of the radio sources form parts of these extensive cocoons. We show that observations of the lobes of giant radio sources allow one to estimate the {\it density} of the intergalactic medium (IGM) in which the lobes are embedded. We estimate the IGM density to be of the order of a few percent of the closure density of the universe. We further calculate the radio power of the overpressured cocoon as a function of time and the ambient density.

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arXiv1994

Gamma Line Radiation Emission from Stellar Winds in Orion Complex: A test for cosmic ray origin theory

We consider $γ$-ray emission from heavy nuclei that are accelerated in the shocks of stellar winds and are excited in interactions with the ambient matter in the Orion complex. We show that the recent detection of $γ$-ray lines in the Orion complex by COMPTEL telescope can be explained by such a scenario, assuming cosmic abundances. The scenario is consistent with recent models of cosmic ray acceleration in stellar winds of massive stars.

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arXiv1994

Effects of Photoionization on Galaxy Formation

We investigate the effects of the observed UV background radiation on galaxy formation. Photoionization by UV radiation decreases the cooling rate of the gas in halos, so that objects with only large density contrasts can self-shield against the background radiation, thereby allowing the shielded neutral cores to cool and form stars. In the context of the CDM model, we use the criterion that self-shielding is essential for star formation to calculate the mass function of galaxies based on both Press-Schechter and the peaks formalisms. The ionizing UV radiation results in the inhibition of galaxy formation --- we show the decrease in the number density of the galaxies quantitatively. We also find that the merging in general is made inefficient by the UV photons through photoionization of the gas in the bigger system into which small objects are incorporated. The latter means that, in a merging dominated region, where the number density at the low-mass end ($M_b \la 10^{10}M_{\odot}$) is usually expected to decrease with time, the trend is {\it reversed} (the number of low-mass galaxies is more at lower redshifts $z$) due to the decreasing UV flux with time after $z \sim 2$. We further discuss the implication of our results for the number counts of galaxies and possible evolution of the luminosity function of the galaxies.

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arXiv1993

Cosmic ray ionization of the interstellar medium

We consider a cosmic ray spectrum that is a power law in momentum down to a cutoff and derive a lower cutoff corresponding to $E_{kin} \sim (30-60)$ MeV from the observed ionization rates in nearby diffuse clouds. While the real spectra of cosmic rays may not be so simple, we argue that one expects a substantial change in the spectra at such energies and that, at first approximation, a power law spectra with a lower cutoff is appropriate. Such a description of the cosmic rays in the interstellar medium is not only theoretically more attractive than the spectra used in the literature, but is also supported by recent observations.

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arXiv1993

Did Cosmic Rays Reionize the Intergalactic Medium?

We investigate the role of cosmic rays from young galaxies in heating and ionizing the intergalactic medium (IGM) at high redshift. Using the IRAS observations at $60 μm$, we estimate the cosmic ray luminosity density at the present epoch. We consider various forms of luminosity evolution in redshift and calculate (a) the thresholds corresponding to the upper limits of Gunn-Peterson optical depth, (b) the Compton $y$ parameter for an IGM heated by cosmic rays and compare with the upper limits from COBE measurements and (c) an estimated limit from the integral of metal enrichment. We show that certain models, with rather strong evolution and early formation of galaxies, allow reionization of the IGM, consistent with all known constraints.

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arXiv1992

Constraints on radiative decay of the 17-keV neutrino from COBE Measurements

It is shown that, for a nontrivial radiative decay channel of the 17-keV neutrino, the photons would distort the microwave background radiation through ionization of the universe. The constraint on the branching ratio of such decays from COBE measurements is found to be more stringent than that from other considerations. The limit on the branching ratio in terms of the Compton $y$ parameter is $B_γ< 1.5 \times 10^{-7} ({τ_ν\over 10^{11} sec})^{0.45} ({y \over 10^{-3}})^{1.11} h^{-1}$ for an $Ω=1, Ω_b=0.1$ universe.

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