arXiv subjects

Biman B. Nath

Publications and source records attributed to Biman B. Nath.

At least 91 records · Page 5Linked to original sources

arXiv2010

Extended X-ray emission from radio galaxy cocoons

We study the emission of X-rays from lobes of FR-II radio galaxies by inverse Compton scattering of microwave background photons. Using a simple model that takes into account injection of relativistic electrons, their energy losses through adiabatic expansion, synchrotron and inverse Compton emission, and also the stopping of the jet after a certain time, we study the evolution of the total X-ray power, the surface brightness, angular size of the X-ray bright region and the X-ray photon index, as functions of time and cocoon size, and compare the predictions with observations. We find that the radio power drops rapidly after the stopping of the jet, with a shorter time-scale than the X-ray power. The X-ray spectrum initially hardens until the jet stops because the steepening of electron spectrum is mitigated by the injection of fresh particles, for electrons with $γ\ge 10^3$. This happens because of the concurrence of two times scales, that of the typical jet lifetimes and cooling due to inverse Compton scattering ($\sim 10^{7\hbox{--}8}$ yr), of electrons responsible for scattering CMB photons into keV range photons (with $γ\sim \sqrt{1 \, {\rm keV}/ kT_{CMB}}$). Another finding is that the ratio of the X-ray to radio power is a robust parameter that varies mostly with redshift and ambient density, but is weakly dependent on other parameters. We also determine the time-averaged ratio of X-ray to radio luminosities (at 1 keV and 151 MHz) and find that it scales with redshift as $\propto (1+z)^{3.8}$, for typical values of parameters. We then estimate the X-ray luminosity function of FR-II radio galaxies and estimate the number of these diffuse X-ray bright objects above a flux limit of $\sim 3 \times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$ to be $\sim 25$ deg$^{-2}$.

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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.

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arXiv2009

Starburst--driven galactic outflows

We propose a model of starburst--driven galactic outflows whose dynamics depends on both radiation and thermal pressure. Standard models of thermal pressure--driven winds fail to explain some key observations of outflows at low and high redshift galaxies. We discuss a scenario in which radiation pressure from massive stars in a stellar population drive a shell of gas and dust. Subsequent supernova (SN) explosions from the most massive stars in this population then drive stellar ejecta outward in a rarefied medium, making it collide with the radiation pressure driven shell. The collision imparts renewed momentum to the shell, and the resulting re-acceleration makes the shell Rayleigh-Taylor unstable, fragmenting the shell. We show that the speed of these ballistic fragments can explain some recently observed correlations in Lyman break galaxies between wind speed, reddening and star formation rate.

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arXiv2008

Primordial magnetic fields and formation of molecular hydrogen

We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen. We show that, for magnetic field strengths in the range $2 \times 10^{-10} {\rm G} \la B_0 \la 2 \times 10^{-9} {\rm G}$, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universe.

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arXiv2008

Dynamical friction of radio galaxies in galaxy clusters

The distribution of luminous radio galaxies in galaxy clusters has been observed to be concentrated in the inner region. We consider the role of dynamical friction of massive galaxies ($M\sim 10^{12.5}$ M$_{\odot}$), assuming them to be hosts of luminous radio galaxies, and show that beginning with a Navarro-Frenk-White density profile of a cluster of mass $M_{cl}\sim 10^{15}$ M$_{\odot}$ of concentration $c\sim 5$ and collapsing at $z\sim 1$, the density profile of radio galaxies evolve to a profile of concentration $c \sim 25$, as observed, in a time scale of $t\sim 3\hbox{--}5$ Gyr.

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arXiv2006

Limits on the AGN activities in X-ray underluminous galaxy groups

We have observed four X-ray underluminous groups of galaxies using the Giant Meterwave RadioTelescope. The groups NGC 524, 720, 3607, and 4697 are underluminous in relation to the extrapolation of the Lx - T relation from rich clusters and do not show any evidence of current AGN activities that can account for such a departure. The GMRT observations carried out at low frequencies (235 and 610 MHz) were aimed at detecting low surface brightness, steep-spectrum sources indicative of past AGN activities in these groups. No such radio emissions were detected in any of these four groups. The corresponding upper limits on the total energy in relativistic particles is about 3 X 10$^{57}$ erg. This value is more than a factor of 100 less than that required to account for the decreased X-ray luminosities (or, enhanced entropies) of these four groups in the AGN-heating scenario. Alternatively, the AGN activity must have ceased about 4 Gyr ago, allowing the relativistic particles to diffuse out to such a large extent (about 250 kpc) that their radio emission could have been undetected by the current observations. If the latter scenario is correct, the ICM was pre-heated before the assembly of galaxy clusters.

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arXiv2005

The Radiative Transport of Dust in Primordial Galaxies and Second-Generation Star Formation

We investigate the radiative transport of dust in primordial galaxies in the presence of the UV radiation field from the first metal-free stars. We find that dust created in the first supernova (SN) explosions can be driven through the interior of the SN remnant to accumulate in the SN shells, where second-generation stars may form from compressed cooling gas. This scenario requires metal-free stars to form continuously over timescales of up to 10 Myr, consistent with recent estimates. Silicate and graphite grains, as well as iron-bearing magnetites, are transported to the shells for reasonable parameter assumptions, but their relative yields from primordial SNe is an important factor in the resulting abundance ratios. We compare the results of segregated grain transport with the current nucleosynthetic data on extremely metal-poor Galactic halo stars. Fossil signatures of this process may already have been detected in those iron-poor stars with enhanced carbon and silicate elements such as magnesium, silicon and oxygen. We discuss the implications of our results for the transition from first- to second-generation star formation in primordial galaxies, and the role played by the radiative transport of dust in this process.

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arXiv2005

Cosmic rays, lithium abundance and excess entropy in galaxy clusters

We consider the production of $^6$Li in spallation reactions by cosmic rays in order to explain the observed abundance in halo metal-poor stars. We show that heating of ambient gas by cosmic rays is an inevitable consequence of this process, and estimate the energy input required to reproduce the observed abundance of $^6$Li/H$\sim 10^{-11}$ to be of order a few hundred eV per particle. We draw attention to the possibility that this could explain the excess entropy in gas in galaxy groups and clusters. The evolution of $^6$Li and the accompanying heating of gas is calculated for structures collapsing at the present epoch with injection of cosmic rays at high redshift. We determine the energy required to explain the abundance of $^6$Li at $z \sim 2$ corresponding to the formation epoch of halo metal-poor stars, and also an increased entropy level of $\sim 300$ keV cm$^2$ necessary to explain X-ray observations of clusters. The energy budget for this process is consistent with the expected energy output of radio-loud AGNs, and the diffusion length scale of cosmic-ray protons responsible for heating is comparable to the size of regions with excess entropy. We also discuss the constraints imposed by the extragalactic gamma-ray background.

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arXiv2005

Self-regulated black hole accretion, the M-sigma relation, and the growth of bulges in galaxies

We argue that the velocity dispersions and masses of galactic bulges and spheroids are byproducts of the feedback that regulates rapid black hole growth in protogalaxies. We suggest that the feedback energy liberated by accretion must pass through the accreting material, in an energy-conserving flux close-in and a momentum-conserving flux further out. If the inflowing gas dominates the gravitational potential outside the Bondi radius, feedback from Eddington-limited accretion drives the density profile of the gas to that of a singular isothermal sphere. We find that the velocity dispersion associated with the isothermal potential, sigma, increases with time as the black hole mass M grows, in such a way that M is proportional to sigma^4. The coefficient of this proportionality depends on the radius at which the flow switches from energy conserving to momentum conserving, and gives the observed M-sigma relation if the transition occurs at ~100 Schwarzschild radii. We associate this transition with radiative cooling and show that bremsstrahlung, strongly boosted by inverse Compton scattering in a two-temperature (T_p >> T_e) plasma, leads to a transition at the desired radius. According to this picture, bulge masses M_b are insensitive to the virial masses of their dark matter haloes, but correlate linearly with black hole mass. Our analytic model also explains the M_b-sigma (Faber-Jackson) relation as a relic of black hole accretion. The model naturally explains why the M-sigma relation has less scatter than either the M-M_b (Magorrian) or the Faber-Jackson relation. It suggests that the M-sigma relation could extend down to very low velocity dispersions, and predicts that the relation should not evolve with redshift.

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arXiv2004

Entropy of the intracluster medium at high redshift

Recent observations of entropy of intracluster medium (ICM) at high redshift are not consistent with a redshift independent entropy floor. In order to interpret these observations, we study models in which the entropy of ICM is enhanced at the epoch of cluster formation (z_f), which then passively evolves as a result of radiative cooling until the epoch of observation (z_o). We confirm that a z-independent entropy floor is incompatible with the observations. We find that models in which energy deposition into the ICM increases with redshift (scaling approximately as $\sim M_cl^0.6 (1+z_f)^2.2$) are consistent with observations of low and high redshift clusters. Possible sources of such non-gravitational heating are briefly discussed.

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arXiv2003

Implications of the Universal Temperature Profile for Galaxy Clusters

We study the X-ray cluster gas density distribution in hydrostatic equilibrium using the universal temperature profile obtained from recent simulations involving only gravitational processes. If this temperature profile is an indicator of the influence of gravitational processes alone on the intracluster medium, then the comparison of various X-ray parameters expected from this profile and the observed data would point towards any additional physics that may be required. We compare the entropy at 0.1 R_{200} and R_{500}, the scaled entropy profile, the gas fraction at 0.3 R_{200} and the gas fraction profile with recent observations and discuss the implications of this temperature profile in light of these data. We find that the entropy imparted to the gas from gravitational processes alone is larger than previously thought. The entropy at R_{500} for rich clusters is consistent with data, whereas the entropy at 0.1R_{200} is still less than the observed values. We also find that the gas fraction in the inner region of clusters, expected from gravitational processes alone, is smaller than previously thought but larger than the observed data. It does show a trend with the emission-weighted temperature ( ) as shown by data. We therefore find that the role of any additional non-gravitational process influencing the physical state of ICM would have to be revised in light of these findings.

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arXiv2003

Suppression of thermal conduction in non-cooling flow clusters

Recent X-ray observations have revealed a universal temperature profile of the intracluster gas of non-cooling flow clusters which is flat for r \le 0.2 r_{180}. Numerical simulations, however, obtain a steeper temperature profile in the inner region. We study the effect of thermal conduction on the intracluster gas in non-cooling flow clusters in light of these observations, using the steep temperature profiles obtained by authors of numerical simulations. We find that given 10^{10} yr for the intracluster gas to evolve, thermal conduction should be suppressed from the classical value by a factor \sim 10^{-3} in order to explain the observations.

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arXiv2002

Cooling flows and the entropy of the intragroup medium

We study steady, homogeneous and subsonic cooling flows in poor clusters of galaxies in light of the recent proposal that radiative cooling of the intracluster gas can explain the observations of the `entropy floor' and other related X-ray observations. We study a family of cooling flow solutions parameterized by the mass flux rate, and then determine the resulting entropy and the X-ray luminosity. We find that cooling flows with mass flux rates in excess of 1000 M_{solar} per yr are required to explain the observations of entropy and X-ray luminosity. In view of the observed lack of such large flows in rich clusters, our calculations suggest that heating sources are needed, in addition to the effect of radiative cooling, to explain the observations.

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arXiv2001

Using HI to probe large scale structures at z ~ 3

The redshifted 1420 MHz emission from the HI in unresolved damped Lyman-αclouds at high z will appear as a background radiation in low frequency radio observations. This holds the possibility of a new tool for studying the universe at high-z, using the mean brightness temperature to probe the HI content and its fluctuations to probe the power spectrum. Existing estimates of the HI density at z~3 imply a mean brightness temperature of 1 mK at 320 Mhz. The cross-correlation between the temperature fluctuations across different frequencies and sight lines is predicted to vary from 10^{-7} K^2 to 10^{-8} K^2 over intervals corresponding to spatial scales from 10 Mpc to 40 Mpc for some of the currently favoured cosmological models. Comparing this with the expected sensitivity of the GMRT, we find that this can be detected with \~10 hrs of integration, provided we can distinguish it from the galactic and extragalactic foregrounds which will swamp this signal. We discuss a strategy based on the very distinct spectral properties of the foregrounds as against the HI emission, possibly allowing the removal of the foregrounds from the observed maps.

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arXiv1998

Contribution of Galaxies to the Background Hydrogen-Ionizing Flux

We estimate the evolution of the contribution of galaxies to the cosmic background flux at $912 Å$ by means of a semi-analytic model of galaxy formation and evolution. Such a modelling has been quite successful in reproducing the optical properties of galaxies. We assume hereafter the high-redshift damped Lyman-$α$ (DLA) systems to be the progenitors of present day galaxies, and we design a series of models which are consistent with the evolution of cosmic comoving emissivities in the available near infrared (NIR), optical, ultraviolet (UV), and far infrared (FIR) bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman-$α$ systems (DLA). We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for $0 \simeq z \le 4$. We take into account the absorption of Lyman-limit photons by HI and dust in the interstellar medium (ISM) of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by HI in the ISM is neglected. The ISM HI absorption results in a severe diminishing of this flux--by almost three orders of magnitude at high redshifts to between one and two orders at $z \simeq 0$. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at $z \simeq 0$.

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arXiv1998

Black Hole Energy Release to the Gaseous Universe

We estimate the energy release of black hole formation to the intra-cluster medium of the Coma cluster and find that this is comparable to the present day energy content. Therefore an energetic and maybe hydrostatic influence is possible. Our calculations rely on the assumption of an universal black hole to galaxy mass ratio (more exactly: spheroidal mass component of the stellar population), for which there is growing evidence. On a cosmological scale, there is also an energy release of black hole formation comparable to what is expected to be present within the thermal gas, caused by the process of structure formation. This indicates an important dynamical influence, neglected by present day structure formation simulations. This estimate of cosmological black hole energy release is independent of the black hole to galaxy mass ratio, but consistent with its value.

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arXiv1997

On The Source of Ionization of The Intergalactic Medium at $z \sim 2.4$

We use the recent detection of He II absorption at $z=2.2-2.6$ in the line of sight of the quasar HS1700+64 to put bounds on the sources of ionization. We find that given the uncertainty in $\tauhgp$ and the model of absorption in the intergalactic medium (IGM), a wide range of possible sources of ionization is still allowed by the observations. We show that a significant contribution from star forming galaxies is allowed and is consistent with the proximity effect. In the case of photoionization by the UV background radiation, the contribution of star forming galaxies to the intensity at $912 > Å$ can be within a range of $\sim 1 - 15$ times that of the quasars. We also investigate the case of a collisionally ionized intergalactic medium. We show that although collisional ionization can be a dominant source of ionization at $z \sim 2.4$ (taking into account the upper limit on Compton y-parameter of the microwave background radiation), the thermal state of the IGM cannot yet be determined. We also consider Sciama's model of radiatively decaying neutrinos and show that the model of decaying neutrino is consistent with the observations.

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arXiv1997

On The Origin Of Metallicity In Lyman-Alpha Forest Systems

We investigate the hypothesis that $\lyal$ absorption lines arise in two populations of halos --- minihalos of small circular velocity ($V_c \la 55$ km s$^{-1}$) in which star formation and metal production are inhibited by photoionization of the UV background radiation, and large galactic halos ($55 \la V_c \la 250$ km s$^{-1}$) which possess stars and metals. Based on the model of $\lyal$-absorbing gas confined in both populations of halos, we attempt to explain the recent observations of (1) associations of visible galaxies with $\lyal$ lines at low redshifts $z \la 1$, and (2) metal lines associated with a non-negligible fraction of low H I column density $\lyal$ lines at $z \sim 3$. For galactic halos, we find that photoionized gas clouds confined in the pressure of ambient hot gas can produce $\lyal$ absorptions with H I column density as low as $10^{14}$ cm$^{-2}$, and that the impact parameter of a sightline for such absorptions matches well with the observed radius of gaseous envelope in a typical luminous galaxy. Using the Press- Schechter prescription for the mass function of halos, we also show that the fraction of $\lyal$ lines with associated metal lines can be understood in terms of the fraction of $\lyal$ absorbers that are associated with galactic halos. In particular, the reported fraction of $\sim 0.5--0.75$ at $z \sim 3$ is reproduced when the boundary value of $V_c$ to separate mini or galactic halos is $40 \sim 60$ km s$^{-1}$, which is consistent with the theoretical prediction of galaxy formation under photoionization. .....

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