arXiv subjects

Biman B. Nath

Publications and source records attributed to Biman B. Nath.

At least 37 records · Page 2Linked to original sources

arXiv2022

Cosmic rays from massive star clusters : A close look at Westerlund 1

We study the effect of cosmic ray (CR) acceleration in the massive compact star cluster Westerlund 1 in light of its recent detection in $γ$-rays. Recent observations reveal a $1/r$ radial distribution of the CR energy density. Here we theoretically investigate whether or not this profile can help to distinguish between (1) continuous CR acceleration in the star cluster stellar wind-driven shocks and (2) discrete CR acceleration in multiple supernovae shocks -- which are often debated in the literature. Using idealized two-fluid simulations and exploring different acceleration sites and diffusion coefficients, we obtain the CR energy density profile and luminosity to find the best match for the $γ$-ray observations. We find that the inferred CR energy density profiles from observations of $γ$-ray luminosity and mass can be much different from the true radial profile. CR acceleration at either the cluster core region or the wind termination shock can explain the observations, if the diffusion coefficient is $κ_{\rm cr}\sim 10^{27}$ cm$^2$ s$^{-1}$ and a fraction of $\approx 10\%-20\%$ of the shock power/post-shock pressure is deposited into the CR component. We also study the possibility of discrete supernovae (SN) explosions being responsible for CR acceleration and find that with an injection rate of 1 SN in every $\sim 0.03$ Myr, one can explain the observed $γ$-ray profile. This multiple SN scenario is consistent with X-ray observations only if the thermal conductivity is close to the Spitzer value.

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arXiv2021

A panoramic view of the circumgalactic medium in the photoionized precipitation model

We consider a model of the circumgalactic medium (CGM) in which feedback maintains a constant ratio of cooling time to freefall time throughout the halo, so that the entire CGM is marginally unstable to multiphase condensation. This 'precipitation model' is motivated by observations of multiphase gas in the cores of galaxy clusters and the halos of massive ellipticals. We derive from the model density and temperature profiles for the CGM around galaxies with masses similar to the Milky Way. After taking into consideration the geometrical position of our solar system in the Milky Way, we show that the CGM model is consistent with observed OVI, OVII, and OVIII column densities only if temperature fluctuations with a log-normal dispersion $σ_{\ln T} \sim 0.6$-$1.0$ are included. We show that OVI column densities observed around star-forming galaxies require systematically greater values of $σ_{\ln T}$ than around passive galaxies, implying a connection between star formation in the disk and the state of the CGM. Photoionization by an extra-galactic UV background does not significantly change these CGM features for galaxies like the Milky Way but has much greater and significant effects on the CGM of lower-mass galaxies.

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arXiv2021

Constraints on precipitation-limited hot halos from massive galaxies to galaxy clusters

We present constraints on a simple analytical model for hot diffuse halo gas, derived from a fit spanning two orders of magnitude in halo mass ($M_{500} \sim 10^{12.5}-10^{14.5} M_{\odot}$). The model is motivated by the observed prevalence of a precipitation limit, and its main free parameter is the central ratio of gas cooling timescale to free-fall timescale ($t_{\rm cool}/t_{\rm ff}$). We use integrated X-ray and thermal Sunyaev-Zel'dovich observations of the environments around massive galaxies, galaxy groups and clusters, averaged in halo mass bins, and obtain the best-fitting model parameters. We find $t_{\rm cool}/t_{\rm ff} \sim 50-110$, depending on the model extrapolation beyond the halo virial radius and possibly on biases present in the data-sets used in the fitting analysis. The model adequately describes the entire mass range, except for intermediate mass halos ($M_{500} \sim 10^{13.5} M_{\odot}$) which systematically fall below the model predictions. However, the best fits for $t_{\rm cool}/t_{\rm ff}$ substantially exceed the values typically derived from X-ray observations of individual systems ($t_{\rm cool}/t_{\rm ff} \sim 10-30$). We consider several explanations for those discrepancies, including X-ray selection biases and a potential anti-correlation between X-ray luminosity and the central galaxy's stellar mass.

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arXiv2020

Gamma-ray and radio background constraints on cosmic rays in Milky Way circumgalactic medium

We study the interaction of cosmic rays (CRs) with the diffuse circumgalactic gas of Milky Way (MW) galaxy that results in hadronic $γ-$ray emission and radio synchrotron emission. We aim to constrain the CR population in our circumgalactic medium (CGM) with the help of observed isotropic $γ$-ray background (IGRB), its anisotropy and radio continuum. We modify different models of CGM gas in hydrostatic equilibrium discussed in literature by including a cosmic ray population, parametrized by $η\equiv P_{\rm CR}/P_{\rm th}$. For the simplest isothermal model, while the IGRB intensity allows $η\lesssim 3$, the anisotropy resulting from the Solar system's off-center position in MW rules out all values of $η$. For the precipitation model, in which the cooling of the CGM gas is regulated with an optimum ratio of cooling time to free-fall time,while the observed IGRB intensity allows $η\lesssim 230$, the observed anisotropy allows only very large values of $η$, of order $\gtrsim 100$. The radio continuum limits $η\lesssim 400$ for precipitation model and does not constrain isothermal model, however these constraints are mitigated by synchrotron loss time being comparable to CR diffusion time scales. These bounds are relevant for current numerical simulations that indicate a significant CR population in CGM of galaxies of MW mass.

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arXiv2020

Diffuse Galactic Gamma-rays from star clusters

We demonstrate that young star clusters have a $γ$-ray surface brightness comparable to that of the diffuse Galactic emission (DGE), and estimate that their sky coverage in the direction of the inner Galaxy exceeds unity. We therefore suggest that they comprise a significant fraction of the DGE.

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arXiv2020

Role of cosmic rays in the early stages of galactic outflows

Using an idealized set-up, we investigate the dynamical role of cosmic rays (CRs) in the early stages of galactic outflows for galaxies of halo masses $10^{8}$, $10^{11}$ and $10^{12}$ $M_\odot$. The outflow is launched from a central region in the galactic disk where we consider three different constant star formation rates ($0.1$, $1$, and $10$ $M_{\odot}$ yr$^{-1}$) over a dynamical timescale of $50$ Myr. We determine the temperature distribution of the gas and find that CRs can reduce the temperature of the shocked gas, which is consistent with previous results. However, we show that CRs do not have any noticeable effect on the mass loading by the outflow. We find that CRs can reduce the size of the outflow, which contradicts previous claims of efficient dynamical impact of CRs; however, it is consistent with earlier theoretical models of cosmic ray driven blastwave as well as stellar wind. We discuss the dependence of our results on CR injection prescriptions and compare them with earlier studies. We conclude that in the early stages of galactic outflows the dynamical role of CRs is not important.

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arXiv2020

Far-infrared nebular spectral features from growing massive black holes

Supermassive black holes (BHs) and their host galaxies are interlinked by virtue of feedbacks and are thought to be co-eval across the Hubble time. This relation is highlighted by an approximate proportionality between the BH mass $M_\bullet$ and the mass of a stellar bulge $M_\ast$ of the host galaxy. However, a large spread of the ratio $M_\bullet/M_\ast$ and a considerable excess of BH mass at redshifts $z\sim8$, indicate that the coevolution of central massive BHs and stellar populations in host galaxies may have experienced variations in its intensity. These issues require a robust determination of the relevant masses (BH, stars and gas), which is difficult in the case of distant high-redshift galaxies that are unresolved. In this paper, we seek to identify spectral diagnostics that may tell us about the relative masses of the BH, the gas mass and stellar mass. We consider general features of SEDs of galaxies that harbour growing massive BHs, forming stars and interstellar/circumgalactic gas. We focus on observational manifestations of possible predominances or intermittent variations in evolutionary episodes of growing massive BHs and forming stellar populations. We consider simplified scenarios for star formation and massive BHs growth, and simple models for chemical composition of gas, for dust free gas as well as for gas with dust mass fraction of $1/3$ of the metal content. We argue that wideband multi-frequency observations (X-ray to submillimeter) of the composite emission spectra of growing BH, stellar population and nebular emission of interstellar gas are sufficient to infer their masses.

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arXiv2019

Radio halos of star forming galaxies

We study the synchrotron radio emission from extra-planar regions of star forming galaxies. We use ideal magneto-hydrodynamical (MHD) simulations of a rotating Milky Way-type disk galaxy with distributed star formation sites for three star formation rates (SFRs) (0.3, 3, 30 M$_{\odot}$ yr$^{-1}$). From our simulations, we see emergence of galactic-scale magnetised outflows, carrying gas from the disk. We compare the morphology of the outflowing gas with hydrodynamic (HD) simulations. We look at the spatial distribution of magnetic field in the outflows. Assuming that a certain fraction of gas energy density is converted into cosmic ray energy density, and using information about the magnetic field, we obtain synchrotron emissivity throughout the simulation domain. We generate the surface brightness maps at a frequency of 1.4 GHz. The outflows are more extended in the vertical direction than radial and hence have an oblate shape. We further find that the matter right behind the outer shock, shines brighter in these maps than that above or below. To understand whether this feature can be observed, we produce vertical intensity profiles. We convolve the vertical intensity profile with the typical beam sizes of radio telescopes, for a galaxy located at 10 Mpc (similar to NGC 891) in order to estimate the radio scale height to compare with observations. We find that for our SFRs this feature will lie below the RMS noise limit of instruments. The radio scale height is found to be $\sim 300-1200$ pc , depending on the resolution of the telescope. We relate the advection speed of the outer shock with the surface density of star formation as $\rm{v}_{\rm adv} \propto Σ_{\rm SFR}^{0.3}$ which is consistent with earlier observations and analytical estimates.

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arXiv2019

Realistic modeling of wind and supernovae shocks in star clusters: addressing ${\rm ^{22}Ne/^{20}Ne}$ and other problems in Galactic cosmic rays

Cosmic ray (CR) sources leave signatures in the isotopic abundances of CRs. Current models of Galactic CRs that consider supernovae (SNe) shocks as the main sites of particle acceleration cannot satisfactorily explain the higher ${\rm ^{22}Ne/^{20}Ne}$ ratio in CRs compared to the interstellar medium. Although stellar winds from massive stars have been invoked, their contribution relative to SNe ejecta has been taken as a free parameter. Here we present a theoretical calculation of the relative contributions of wind termination shocks (WTSs) and SNe shocks in superbubbles, based on the hydrodynamics of winds in clusters, the standard stellar mass function, and stellar evolution theory. We find that the contribution of WTSs towards the total CR production is at least $25\%$, which rises to $\gtrsim 50\%$ for young ($\lesssim 10$ Myr) clusters, and explains the observed $^{22}{\rm Ne}/^{20} {\rm Ne}$ ratio. We argue that since the progenitors of apparently isolated supernovae remnants (SNRs) are born in massive star clusters, both WTS and SNe shocks can be integrated into a combined scenario of CRs being accelerated in massive clusters. This scenario is consistent with the observed ratio of SNRs to $γ$-ray bright ($L_γ\gtrsim 10^{35}$ erg s$^{-1}$) star clusters, as predicted by star cluster mass function. Moreover, WTSs can accelerate CRs to PeV energies, and solve other longstanding problems of the standard supernova paradigm of CR acceleration.

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arXiv2019

Cosmological implications of the composite spectra of galactic X-ray binaries constructed using MAXI data

We have investigated the long term average spectral properties of galactic X-ray binaries in the energy range of 3-20 keV, using long term monitoring data from MAXI -Gas Slit Camera (GSC). These long term average spectra are used to construct separately the composite spectra of galactic High Mass X-ray binaries (HMXBs) and Low Mass X-ray binaries (LMXBs). These composite spectra can be described empirically with piece-wise power-law with three components. X-rays from HMXBs are considered as important contributors to heating and ionization of neutral hydrogen in the intergalactic medium during the Epoch of Reionization. Using the above empirical form of the composite HMXB spectra extrapolated to lower energies as an input, we have studied the impact of these sources on the 21-cm signal using the outputs of N-body simulation and 1D radiative transfer. The heating due to the composite spectrum is less patchy compared to power-law spectrum with a spectral index $α= 1.5$, used in previous studies. The amplitude of the heating peak of large scale power spectrum, when plotted as a function of the redshift, is less for the composite spectrum.

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arXiv2019

Launching of hot gas outflow by disc-wide supernova explosions

Galactic gas outflows are driven by stellar feedback with dominant contribution from supernovae (SN) explosions. The question of whether the energy deposited by SNe initiates a large scale outflow or gas circulation on smaller scales -- between discs and intermediate haloes, depends on SN rate and their distribution in space and time. We consider here gas circulation by disc-wide unclustered SNe with galactic star formation rate in the range from {$\simeq 6\times 10^{-4}$ to $\simeq 6\times 10^{-2}~M_\odot$~yr$^{-1}$~kpc$^{-2}$, corresponding to mid-to-high star formation observed in galaxies. We show that such disc-wide SN explosion regime can form circulation of warm ($T\sim 10^4$ K) and cold ($T<10^3$ K) phases within a few gas scale heights, and elevation of hot {($T>10^5$ K)} gas at higher ($z>1$ kpc) heights. We found that the threshold energy input rate for hot gas outflows with disc-wide supernovae explosions is estimated to be of the order $\sim 4\times 10^{-4}$~erg~s$^{-1}$~cm$^{-2}$. We discuss the observational manifestations of such phenomena in optical and X-ray bands. In particular, we found that for face-on galaxies with SF ($Σ_{_{\rm SF}}>0.02~M_\odot$~yr$^{-1}$~kpc$^{-2}$), the line profiles of ions typical for warm gas show a double-peak shape, corresponding to out-of-plane outflows. In the X-ray bands, galaxies with high SF rates ($Σ_{_{\rm SF}}>0.006~M_\odot$~yr$^{-1}$~kpc$^{-2}$) can be bright, with a smooth surface brightness in low-energy bands ($0.1\hbox{--}0.3$~keV) and patchy at higher energies ($1.6\hbox{--}8.3$~keV).}

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arXiv2018

Radio background and IGM heating due to Pop III supernovae explosions

We consider the synchrotron emission from high energy electrons accelerated in supernova explosions of massive Population III stars in high redshift minihaloes of mass $10^{5\hbox {--}7} \rm M_\odot$. We show the resulting intensity of radio background from this process can be substantial, which could potentially explain the recently reported EDGES result, if not for the associated heating of the IGM by CR protons which are also produced at the same time. The trade-off between the radio background and heating is such that the 21 cm brightness temperature cannot be larger than $\vert ΔT_{21}\vert \sim 0.25$ K. The radio background and heating are both produced by energetic particles, although one by energetic electrons and the other by energetic protons. The two competing processes, production of radio background and heating of IGM by Pop III supernovae, determine the depth of the trough in the 21 cm brightness temperature which can be observed in future experiments and used as a test of this scenario.

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arXiv2018

Correlations of the feedback energy and BCG radio luminosity in galaxy clusters

We study the excess entropy and the corresponding non-gravitational feedback energy ($E_{feedback}$) in the intra-cluster medium (ICM) by considering a sample of 38 galaxy clusters using Chandra X-ray and NRAO VLA Sky Survey (NVSS)/Giant Metrewave Radio Telescope (GMRT) radio observations. We find moderate correlation of the feedback energy and brightest cluster galaxy (BCG) radio luminosity ($L_R$) with the various cluster thermal properties. We show conclusively that the active galactic nucleus (AGN) is more effective in transferring feedback energy to the ICM in less massive clusters. We find that within $0.3r_{500}$, the feedback energy correlates with cluster temperature as $E_{feedback}\propto T_{obs}^{0.98\pm0.37}$. Moreover, for radio detected BCG sample we find that BCG radio luminosity at 1.4 GHz scales with gas mass as $L_R\propto m_{g,obs}^{ 1.76\pm0.71}$ and with X-ray luminosity as $L_R\propto L_{X,obs}^{0.94\pm0.35}$. Finally, we discuss the implications of our results with regard to feedback in clusters.

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arXiv2018

X-ray and SZ constraints on the properties of hot CGM

We use observations of stacked X-ray luminosity and Sunyaev-Zel'dovich (SZ) signal from a cosmological sample of $\sim 80,000$ and $104,000$ massive galaxies, respectively, with $ 10^{12.6}\lesssim M_{500} \lesssim 10^{13} M_{\odot}$ and mean redshift, z $\sim$ 0.1 - 0.14 to constrain the hot Circumgalactic Medium (CGM) density and temperature. The X-ray luminosities constrain the density and hot CGM mass, while the SZ signal helps in breaking the density-temperature degeneracy. We consider a simple power-law density distribution ($n_e \propto r^{-3β}$) as well as a hydrostatic hot halo model, with the gas assumed to be isothermal in both cases. The datasets are best described by the mean hot CGM profile $\propto r^{-1.2}$, which is shallower than an NFW profile. For halo virial mass $\sim 10^{12}$ - $10^{13} M_{\odot}$, the hot CGM contains $\sim$ 20 - 30\% of galactic baryonic mass for the power-law model and 4 - 11\% for the hydrostatic halo model, within the virial radii. For the power-law model, the hot CGM profile broadly agrees with observations of the Milky Way. The mean hot CGM mass is comparable to or larger than the mass contained in other phases of the CGM for $L^*$ galaxies.

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arXiv2018

Cosmic ray heating of intergalactic medium: patchy or uniform?

We study the heating of the intergalactic medium (IGM) surrounding high redshift star forming galaxies due to cosmic rays (CR). We take into account the diffusion of low energy cosmic rays and study the patchiness of the resulting heating. We discuss the case of IGM heating around a high redshift minihalo ($z\sim 10\hbox{--}20$, M$\sim 10^5\hbox{--}10^7$ M$_\odot$),and put an upper limit on the diffusion coefficient $D\le 1\times 10^{26}$ cm$^2$ s$^{-1}$ for the heating to be inhomogeneous at $z\sim 10$ and $D\le 5\hbox{--}6 \times 10^{26}$ cm$^2$ s$^{-1}$ at $z\sim 20$. For typical values of $D$, our results suggest uniform heating by CR at high redshift, although there are uncertainties in magnetic field and other CR parameters. We also discuss two cases with continuous star formation, one in which the star formation rate (SFR) of a galaxy is high enough to make the IGM in the vicinity photoionized, and another in which the SFR is low enough to keep it neutral but high enough to cause significant heating by cosmic ray protons. In the neutral case (low SFR), we find that the resulting heating can make the gas hotter than the cosmic microwave background (CMB) radiation for $D < 10^{30}$ cm$^2$ s$^{-1}$, within a few kpc of the galaxy, and unlikely to be probed by near future radio observations. In the case of photoionized IGM (high SFR), the resulting heating of the gas in the vicinity of high redshift ($z\sim 4)$ galaxies of mass $\ge 10^{12}$ M$_\odot$ can suppress gas infall into the galaxy. At lower redshifts ($z\sim 0$), an SFR of $\sim 1$ M$_\odot$ yr$^{-1}$ can suppress the infall into galaxies of mass $\le 10^{10}$ M$_\odot$.

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arXiv2018

Constraining cosmic ray acceleration in young star clusters using multi-wavelength observations

We use $1$D and $3$D two-fluid cosmic ray (CR) hydrodynamic simulations to investigate the role of CRs in the vicinity of a compact young star cluster. We model a self-gravitating cloud (density profile $ρ\propto r^{-1}$), include important thermal and non-thermal processes, and explore two different CR injection scenarios. We show that if internal shocks in the wind-driving region are the main site for CR acceleration, then the resulting $γ$-ray luminosity ($L_{\rm γ}$) can reach $\approx 5\%$ of the mechanical luminosity ($L_{\rm w}$), independent of the fraction of wind energy ($\sim 1-20\%$) injected into CRs. In contrast, if the forward/reverse shock of a bubble is the injection site then $L_{\rm γ}$ increases linearly with the CR injection fraction, as expected analytically. We find that the X-ray luminosity ($L_{\rm x}$) in the forward/reverse shock injection scenario is $\gtrsim 10^{-3} L_{\rm w}$, which is $\sim 10$ times larger than in the central wind-driving injection case. We predict the corresponding range of the synchrotron radio luminosity. We show how multi-wavelength observations can constrain the CR parameters. Comparing the predicted multi-wavelength luminosities with those of 30 Doradus we identify the reverse shock as the most probable CR injection site, and that thermal conduction is important. We do not find significant dynamical impact of CRs in our models.

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arXiv2018

Extra-planar X-ray emission from disc-wide outflows in spiral galaxies

We study the effects of mass and energy injection due to OB associations spread across the rotating disc of a Milky Way-type galaxy, with the help of 3D hydrodynamic simulations. We compare the resulting X-ray emission with that produced from the injection of mass and energy from a central region. We find that the predicted X-ray image shows a filamentary structure that arises even in the absence of disc gas inhomogeneity. This structure stems from warm clumps made of disc material being lifted by the injected gas. We show that as much as half of the total X-ray emission comes from regions surrounding warm clumps that are made of a mix of disk and injected gas. This scenario has the potential to explain the origin of the observed extra-planar X-ray emission around star forming galaxies and can be used to understand the observed sublinear relation between the $L_X$ and SFR. We quantify the mass contained in these `bow-shock' regions. We also show that the top-most region of the outer shock above the central area emits harder X-rays than the rest. Further, we find that the mass distribution in different temperature ranges is bimodal, peaking at $10^4\hbox{-}10^5$ K (in warm clumps) and $10^6\hbox{-}10^7$ K (X-ray emitting gas). The mass loading factor is found to decrease with increasing SFR, consistent with previous theoretical estimates and simulations.

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arXiv2017

The mysterious $6565\,Å$ absorption feature of the Galactic halo

We consider various possible scenarios to explain the recent observation of a claimed broad H$α$ absorption in our Galactic halo with peak optical depth $τ\simeq 0.01$ and equivalent width $W \simeq 0.17 \, \rm Å$. We show that the absorbed feature cannot arise from the circumgalactic and ISM H$α$ absorption. As the observed absorption feature is quite broad $Δλ\simeq 30 \, \rm Å$, we also consider CNO lines that lie close to H$α$ as possible alternatives to explain the feature. We show such lines could also not account for the observed feature. Instead, we suggest that it can arise from diffuse interstellar bands (DIBs) carriers or polyaromatic hydrocarbons (PAHs) absorption. While we identify several such lines close to the H$α$ transition, we are unable to determine the molecule responsible for the observed feature, partly because of selection effects that prevents us from identifying DIBs/PAHs features close to H$α$ using local observations. Deep integration on a few extragalactic sources with high spectral resolution might allow us to distinguish between different possible explanations.

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