arXiv subjects

Biman B. Nath

Publications and source records attributed to Biman B. Nath.

At least 73 records · Page 4Linked to original sources

arXiv2015

CMB distortion from circumgalactic gas

We study the Sunyaev-Zel'dovich (SZ) distortion of the cosmic microwave background radiation (CMBR) from extensive circumgalactic gas (CGM) in massive galactic halos. Recent observations have shown that galactic halos contain a large amount of X-ray emitting gas at the virial temperature, as well as a significant amount of warm OVI absorbing gas. We consider the SZ distortion from the hot gas in those galactic halos in which the gas cooling time is longer than the halo destruction time scale. We show that the SZ distortion signal from the hot gas in these galactic halos at redshifts $z\approx 1\hbox{--}8$ can be significant at small angular scales ($\ell\sim 10^4$), and dominate over the signal from galaxy clusters. The estimated SZ signal for most massive galaxies (halo mass $\ge 10^{12.5}$ M$_\odot$) is consistent with the marginal detection by {\it Planck} at these mass scales. We also consider the SZ effect from warm circumgalactic gas. The integrated Compton distortion from the warm OVI absorbing gas is estimated to be $y\sim 10^{-8}$, which could potentially be detected by experiments planned for the near future. Finally, we study the detectability of the SZ signal from circumgalactic gas in two types of surveys, a simple extension of the SPT survey and a more futuristic cosmic variance-limited survey. We find that these surveys can easily detect the kSZ signal from CGM. With the help of a Fisher Matrix analysis, we find that it will be possible for these surveys to constrain the gas fraction in CGM, after marginalizing over cosmological parameters, to $\le 33$\%, in case of no redshift evolution of the gas fraction.

astro-ph.CO↗

arXiv2014

In a hot bubble: why does superbubble feedback work, but isolated supernovae do not?

Using idealized 1-D Eulerian hydrodynamic simulations, we contrast the behavior of isolated supernovae with the superbubbles driven by multiple, collocated supernovae. Continuous energy injection via successive supernovae going off within the hot/dilute bubble maintains a strong termination shock. This strong shock keeps the superbubble over-pressured and drives the outer shock well after it becomes radiative. Isolated supernovae, in contrast, with no further energy injection, become radiative quite early ($\lesssim 0.1$ Myr, 10s of pc), and stall at scales $\lesssim 100$ pc. We show that isolated supernovae lose almost all of their mechanical energy by a Myr, but superbubbles can retain up to $\sim 40\%$ of the input energy in form of mechanical energy over the lifetime of the star cluster (few 10s of Myr). These conclusions hold even in the presence of realistic magnetic fields and thermal conduction. We also compare various recipes for implementing supernova feedback in numerical simulations. For various feedback prescriptions we derive the spatial scale below which the energy needs to be deposited for it to couple to the interstellar medium (ISM). We show that a steady thermal wind within the superbubble appears only for a large number ($\gtrsim 10^4$) of supernovae. For smaller clusters we expect multiple internal shocks instead of a smooth, dense thermalized wind.

astro-ph.GA↗

arXiv2014

Evolution of multiple supernova remnants

(Abridged) Heating of the interstellar medium by multiple supernovae (SNe) explosions is at the heart of producing galaxy-scale outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the interstellar medium (ISM) and filling the volume with gas of high temperatures. We argue that it is important for SNe remnants to have a large filling factor {\it and} a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas ($T \ge 3 \times 10^6$ K) can percolate through the whole region after the total volume filling factor reaches a threshold of $\sim 0.3$. The efficiency of heating the gas to X-ray temperatures can be $\ge 0.1$ after this percolation epoch, which occurs after a period of $\approx 10$ Myr for a typical starburst SNe rate density of $ν_{\rm SN} \approx 10^{-9}$ pc$^{-3}$ yr$^{-1}$ and gas density of $n\approx 10$ cm$^{-3}$ in starburst nuclei regions. This matches the recent observations of a time delay of similar order between the onset of star formation and galactic outflows. The efficiency to heat gas up to X-ray temperatures ($\ge 10^{6.5}$ K) roughly scales as $ν_{\rm SN}^{0.2} n^{-0.6}$. For a typical SNe rate density and gas density in starburst nuclei, the heating efficiency is $\sim 0.15$, also consistent with previous interpretations from X-ray observations. We discuss the implications of our results with regard to observational diagnostics of ionic ratios and emission measures in starburst nuclei regions.

astro-ph.GA↗

arXiv2014

Cosmic backgrounds due to the formation of the first generation of supermassive black holes

The statistics of black holes and their masses strongly suggests that their mass distribution has a cutoff towards lower masses near $3 \times 10^{6}$ M$_{\odot}$. This is consistent with a classical formation mechanism from the agglomeration of the first massive stars in the universe. However, when the masses of the stars approach $10^{6}$ M$_{\odot}$, the stars become unstable and collapse, possibly forming the first generation of cosmological black holes. Here we speculate that the claimed detection of an isotropic radio background may constitute evidence of the formation of these first supermassive black holes, since their data are compatible in spectrum and intensity with synchrotron emission from the remnants. The model proposed fulfills all observational conditions for the background, in terms of single-source strength, number of sources, far-infrared and gamma-ray emission. The observed high energy neutrino flux is consistent with our calculations in flux and spectrum. The proposal described in this paper may also explain the early formation and growth of massive bulge-less disk galaxies as derived from the massive, gaseous shell formed during the explosion prior to the formation of a supermassive black hole.

astro-ph.CO↗

arXiv2013

Interaction of galactic wind with halo gas and the origin of multiphase extraplanar material

We study the interaction of galactic wind with hot halo gas using hydrodynamical simulations. We find that the outcome of this interaction depends crucially on the wind injection density and velocity. Various phases of the extraplanar media such as high velocity clouds (HVCs), outflowing clouds, and OVI regions can originate in the interaction zones of wind with the halo gas, depending on the injection velocity and density. In our simulations the size of the HVCs is of the order of 100 pc. The total mass contained in the clouds is 10^5 -- 10^7 M_sun and they have a normal distribution of velocities in the galactic standard of rest frame, similar to HVCs. For high injection density and velocity, a significant number of clouds move outwards and resemble the case of cold neutral outflows. Furthermore a 10^5 -- 10^6 K phase is formed in our simulations which has a column density ~ 10^{18} cm^{-2}, and resembles the observed OVI regions. The injection velocity and density are linked with the mass loading factor of the outflow, efficiency of energy injection due to supernovae and the SFR. Comparison of the predicted morphology of extraplanar gas with observations can serve as a useful diagnostic for constraining feedback efficiency of outflows.

astro-ph.CO↗

arXiv2013

Average spectral properties of galactic X-ray binaries with 3 years of MAXI data

The energy spectra of X-ray binaries (XRBs) have been investigated during the last few decades with many observatories in different energy bands and with different energy resolutions. However, these studies are carried out in selected states of XRBs like during outbursts, transitions, quiescent states, and are always done in limited time windows of pointed observations. It is now possible to investigate the long term averaged spectra of a large number of X-ray binaries with the all sky monitor MAXI, which also has a broad energy band. Here we present the average spectral behaviour of a few representative XRBs. The long term averaged spectrum of Cyg X-1 is described by a sum of two power-laws having $Γ_{1}$ ~ 2.8 and $Γ_{2}$ ~ 1.2, along with a multi color disk blackbody having an inner disk temperature of 0.5 keV, GX 301-2 is described by a power-law with a high energy cut-off at $E_{c}$ ~ 15 keV and a blackbody component at 0.2 keV and that of Aql X-1 is described by a multi color disk blackbody at 2 keV and a power-law of $Γ$ ~ 2.2. We have also constructed the combined X-ray spectrum of the X-ray binaries in the Milky Way, which can be compared to the XRBs spectra of other galaxies observed with Chandra and XMM-Newton. These measurements are also relevant to investigate the X-ray interaction with the ISM and its contribution to the ionising X-ray background in the early universe.

astro-ph.HE↗

arXiv2013

The Galactic Wind Haze and its $γ$-spectrum

We study the possibility that the gamma ray emission in the Fermi bubbles observed is produced by cosmic ray electrons with a spectrum similar to Galactic cosmic rays. We argue that the cosmic ray electrons steepen near 1 TeV from $E^{-3}$ to about $E^{-4.2}$, and are partially secondaries derived from the knee-feature of normal cosmic rays. We speculate that the observed feature at $\sim 130$ GeV could essentially be due to inverse Compton emission off a pair-production peak on top of a turn-off in the $γ$ ray spectrum at $\sim 130$ GeV. It suggests that the knee of normal cosmic rays is the same everywhere in the Galaxy. A consequence could be that all supernovae contributing give the same cosmic ray spectrum, with the knee feature given by common stellar properties; in fact, this is consistent with the supernova theory proposed by Bisnovatyi-Kogan (1970), a magneto-rotational mechanism, if massive stars converge to common properties in terms of rotation and magnetic fields just before they explode.

astro-ph.HE↗

arXiv2013

Conditions for supernovae driven galactic winds

We point out that the commonly assumed condition for galactic outflows, that supernovae (SNe) heating is efficient in the central regions of starburst galaxies, suffers from invalid assumptions. We show that a large filling factor of hot ($\ge 10^6$ K) gas is difficult to achieve through SNe heating, irrespective of the initial gas temperature and density, and of its being uniform or clumpy. We instead suggest that correlated supernovae from OB associations in molecular clouds in the central region can drive powerful outflows if the molecular surface density is $> 10^3$ M$_{\odot}$ pc$^{-2}$.

astro-ph.CO↗

arXiv2013

AGN feedback and entropy injection in galaxy cluster cores

The non-gravitational energy feedback is of crucial importance in modeling/simulating clusters to be used as cosmological probes. AGNs are, arguably, of primary importance in injecting energy in the cluster cores. We make the first estimate of non-gravitational energy {\it profiles} in galaxy cluster cores (and beyond) from observational data. Comparing the observed entropy profiles within $r_{500}$, from the Representative {\it XMM-Newton} Cluster Structure Survey (REXCESS), to simulated entropy profiles from both AMR and SPH non-radiative simulations, we estimate the amount of non-gravitational energy, $E_{\rm ICM}$, contained in the ICM. Adding the radiative losses we estimate the total energy feedback, $E_{\rm Feedback}$, into the clusters. The profiles for the energy deposition, $ΔE_{\rm ICM}(x)$, in the inner regions differ for Cool-Core (CC) and Non Cool-Core (NCC) clusters, decreasing after accounting for the radiative cooling. The total feedback energy scales with the mean spectroscopic temperature as $E_{\rm Feedback} \propto T_{\rm sp}^{2.52 \pm0.08}$ and $E_{\rm Feedback} \propto T_{\rm sp}^{2.17 \pm 0.11}$, when compared with the baseline SPH and AMR profiles respectively. The scatter in the two cases is 15% and 23%, respectively. The mean non-gravitational energy per particle within $r_{500}$, is $ε_{\rm ICM} = {2.8} \pm {0.8}$ keV for the SPH theoretical relation and $ε_{\rm ICM} = {1.7} \pm {0.9}$ keV for the AMR theoretical relation. We use the {\it NRAO/VLA Sky Survey} (NVSS) source catalog to determine the radio luminosity, $L_R$, at 1.4 GHz of the central source(s) of our sample. For $T_{\rm sp} > 3$ keV, the $E_{\rm Feedback}$ correlates with $L_R$. We show that AGNs could provide a significant component of the feedback. (Abridged)

astro-ph.CO↗

arXiv2013

Superbubble breakout and galactic winds from disk galaxies

We study the conditions for disk galaxies to produce superbubbles that can break out of the disk and produce a galactic wind. We argue that the threshold surface density of supernovae rate for seeding a wind depends on the ability of superbubble energetics to compensate for radiative cooling. We first adapt Kompaneets formalism for expanding bubbles in a stratified medium to the case of continuous energy injection and include the effects of radiative cooling in the shell. With the help of hydrodynamic simulations, we then study the evolution of superbubbles evolving in stratified disks with typical disk parameters. We identify two crucial energy injection rates that differ in their effects, the corresponding breakout ranging from being gentle to a vigorous one. (a) Superbubbles that break out of the disk with a Mach number of order 2-3 correspond to an energy injection rate of order 10^{-4} erg cm^{-2} s^{-1}, which is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (b) A larger energy injection threshold, of order 10^{-3} erg cm^{-2} s^{-1}, or equivalently, a star formation surface density of \sim 0.1 solar mass yr^{-1} kpc^{-2}, corresponds to superbubbles with a Mach number \sim 5-10. While the milder superbubbles can be produced by large OB associations, the latter kind requires super-starclusters. These derived conditions compare well with observations of disk galaxies with winds and the existence of multiphase halo gas. Furthermore, we find that contrary to the general belief that superbubbles fragment through Rayleigh-Taylor (RT) instability when they reach a vertical height of order the scale height, the superbubbles are first affected by thermal instability for typical disk parameters and that RT instability takes over when the shells reach a distance of approximately twice the scale height.

astro-ph.GA↗

arXiv2013

Signature of outflows in strong MgII absorbers in quasar sightlines

We report a correlation between velocity offset (beta = v/c) of strong Mg II absorption systems and the bolometric luminosity (L_bol) of quasars in SDSS-DR7. We find that, beta shows a power law increase with L_bol, with a slope (~ 1/4). We find that such a relation of beta with L_bol is expected for outflows driven by scattering of black hole radiation by dust grains, and which are launched from the innermost dust survival radius. Our results indicate that a significant fraction of the strong MgII absorbers, in the range of beta = (0--0.4) may be associated with the quasars themselves.

astro-ph.CO↗

arXiv2012

Supernovae and AGN driven galactic outflows

We present analytical solutions for winds from galaxies with NFW dark matter halo. We consider winds driven by energy and mass injection from multiple supernovae, as well as momentum injection due to radiation from a central black hole. We find that the wind dynamics depends on three velocity scales: (a) v_star \sim (\dot{E} / 2 \dot{M})^{1/2} describes the effect of starburst activity, with \dot{E}, \dot{M} as energy and mass injection rate in a central region of radius R; (b) \vbh ~ (G\mbh / 2 R)^{1/2} for the effect of a central black hole of mass \mbh on gas at distance R and (c) v_{s} =(GM_h/ 2Cr_s)^{1/2} which is closely related to the galaxy circular velocity, with C as a function of halo concentration parameter. We find the wind terminal speed to be 2 (v_star^2 +1.5(Γ-1) \vbh^2 -v_s^2)^{1/2}, where Γis the ratio of force due to radiation pressure to gravity of the central black hole. We also find that: (a) winds from quiescent star forming galaxies cannot escape from 10^{11.5} \le M_h \le 10^{12.5}Msun galaxies, (b) circumgalactic gas at large distances should be present for galaxies in this mass range, (c) for an escaping wind, the wind speed in low to intermediate mass galaxies is ~ 400--1000 km/s, consistent with observed X-ray temperatures; (d) winds from massive galaxies with AGN have speeds \gtrsim 1000 km/s. We also find that the ratio [2 v_star ^2 -(1 -Γ) \vbh^2]/ v_c^2 dictates the amount of gas lost. Used in conjunction with an appropriate relation between \mbh and M_h, and an appropriate opacity of dust grains in IR (K band), this ratio becomes minimum at a certain halo mass scale (M_h ~ 10^{12--12.5} Msun) that signifies the cross-over of AGN domination in outflow properties from starburst activity at lower masses. We find that stellar mass for massive galaxies scales as M_star \propto M_h^{0.26},and for low mass galaxies, M_star \propto M_h^{5/3}.

astro-ph.CO↗

arXiv2012

Spectrum and ionization rate of low energy Galactic cosmic rays

We consider the rate of ionization of diffuse and molecular clouds in the interstellar medium by Galactic cosmic rays (GCR) in order to constrain its low energy spectrum. We extrapolate the GCR spectrum obtained from PAMELA at high energies ($\ge 200$ GeV/ nucleon) and a recently derived GCR proton flux at $1\hbox{--}200$ GeV from observations of gamma rays from molecular clouds, and find that the observed average Galactic ionization rate can be reconciled with this GCR spectrum if there is a low energy cutoff for protons at $10\hbox{--}100$ MeV. We also identify the flattening below a few GeV as being due to (a) decrease of the diffusion coefficient and dominance of convective loss at low energy and (b) the expected break in energy spectrum for a constant spectral index in momentum. We show that the inferred CR proton spectrum of $Φ\propto E_{kin}^{-1.7\pm0.2}$ for $E_{kin} \le$ few GeV, is consistent with a power-law spectrum in momentum $p^{-2.45\pm0.4}$, which we identify as the spectrum at source. Diffusion loss at higher energies then introduces a steepening by $E^{-α}$ with $α\sim 1/3$, making it consistent with high energy measurements.

astro-ph.GA↗

arXiv2012

Energy Deposition Profiles and Entropy in Galaxy Clusters

We report the results of our study of fractional entropy enhancement in the intra-cluster medium (ICM) of the clusters from the representative XMM-Newton cluster structure survey (REXCESS). We compare the observed entropy profile of these clusters with that expected for the ICM without any feedback, as well as with the introduction of preheating and entropy change due to gas cooling. We make the first estimate of the total, as well as radial, non-gravitational energy deposition up to r500 for a large, nearly flux-limited, sample of clusters. We find that the total energy deposition corresponding to the entropy enhancement is proportional to the cluster temperature (and hence mass), and that the energy deposition per particle as a function of gas mass shows a similar profile in all clusters, with its being more pronounced in the central region than in the outer region. Our results support models of entropy enhancement through AGN feedback.

astro-ph.CO↗

arXiv2012

Simulation of radiation driven wind from disc galaxies

We present 2-D hydrodynamic simulation of rotating galactic winds driven by radiation. We study the structure and dynamics of the cool and/or warm component($T \simeq 10^4$ K) which is mixed with dust. We have taken into account the total gravity of a galactic system that consists of a disc, a bulge and a dark matter halo. We find that the combined effect of gravity and radiation pressure from a realistic disc drives the gas away to a distance of $\sim 5$ kpc in $\sim 37$ Myr for typical galactic parameters. The outflow speed increases rapidly with the disc Eddington parameter $Γ_0(=κI/(2 c G Σ)$) for $Γ_0 \ge 1.5$. We find that the rotation speed of the outflowing gas is $\lesssim 100$ km s$^{-1}$. The wind is confined in a cone which mostly consist of low angular momentum gas lifted from the central region.

astro-ph.GA↗

arXiv2012

The roles of radiation and ram pressure in driving galactic winds

We study gaseous outflows from disk galaxies driven by the combined effects of ram pressure on cold gas clouds and radiation pressure on dust grains. Taking into account the gravity due to disk, bulge and dark matter halo, and assuming continuous star formation in the disk, we show that radiation or ram pressure alone is not sufficient to drive escaping winds from disk galaxies, and that both processes contribute. We show that in the parameter space of star formation rate (SFR) and rotation speed of galaxies, the wind speed in galaxies with rotation speed $v_c\le 200$ km s$^{-1}$ and SFR $\le 100$ M$_{\odot}$ yr$^{-1}$, has a larger contribution from ram pressure, and that in high mass galaxies with large SFR, radiation from the disk has a greater role in driving galactic winds. The ratio of wind speed to circular speed can be approximated as ${v_w \over v_c} \sim 10^{0.7} \, [{\rm SFR\over 50 \, {\rm M}_{\odot} \, {\rm yr}^{-1}}] ^{0.4} \ [{v_c\over 120\, km/s}]^ {-1.25}$. We show that this conclusion is borne out by observations of galactic winds at low and high redshift and also of circumgalactic gas. We also estimate the mass loading factors under the combined effect of ram and radiation pressure, and show that the ratio of mass loss rate to SFR scales roughly as $v_c^{-1} Σ_g^{-1}$, where $Σ_g$ is the gas column density in the disk.

astro-ph.CO↗

arXiv2011

Dust-driven wind from disk galaxies

We study gaseous outflows from disc galaxies driven by radiation pressure on dust grains. We include the effect of bulge and dark matter halo and show that the existence of such an outflow implies a maximum value of disc mass-to-light ratio. We show that the terminal wind speed is proportional to the disc rotation speed in the limit of a cold gaseous outflow, and that in general there is a contribution from the gas sound speed. Using the mean opacity of dust grains and the evolution of the luminosity of a simple stellar population, we then show that the ratio of the wind terminal speed ($v_\infty$) to the galaxy rotation speed ($v_c$) ranges between $2 \hbox{--} 3$ for a period of $\sim 10$ Myr after a burst of star formation, after which it rapidly decays. This result is independent of any free parameter and depends only on the luminosity of the stellar population and on the relation between disc and dark matter halo parameters. We briefly discuss the possible implications of our results.

astro-ph.CO↗

arXiv2011

Intracluster gas pressure, entropy injection and redshift evolution

We study the effect of entropy injection in the intracluster medium (ICM) in light of the recent observationally determined universal pressure profile of the ICM. Beginning with a power-law entropy profile that is expected in the absence of any feedback, we show that a simple universal prescription of entropy injection results in the final, observed universal pressure profile. This simple prescription has two components, one associated with an overall increase in entropy and another associated with injection in the central parts of the cluster. Importantly, both the components of entropy injection are needed to produce the final universal pressure profile. This is indicative of a need of both preheating the ICM as well {\it in situ} AGN/SNe heating. We demonstrate the usefulness of the method by extending the calculations to clusters at high redshift, and predict redshift evolution of cluster scaling relations that can be tested against data. We show that the self-similar evolution of the universal pressure profile is equivalent to a negative evolution of entropy injection with redshift, with a scaling $S_{inj} \propto (1+z)^{-0.8} S_{inj}(z=0)$. We also show the current observational data are indicative of the entropy injection decreasing with redshift.

astro-ph.CO↗