arXiv subjects

Biman B. Nath

Publications and source records attributed to Biman B. Nath.

At least 55 records · Page 3Linked to original sources

arXiv2017

AGN feedback with the Square Kilometer Array (SKA) and implications for cluster physics and cosmology

AGN feedback is regarded as an important non-gravitational process in galaxy clusters, providing useful constraints on large-scale structure formation. It modifies the structure and energetics of the intra-cluster medium (ICM) and hence its understanding is crucially needed in order to use clusters as high precision cosmological probes. In this context, particularly keeping in mind the upcoming high quality radio data expected from radio surveys like SKA with its higher sensitivity, high spatial and spectral resolutions, we review our current understanding of AGN feedback, its cosmological implications and the impact that SKA can have in revolutionizing our understanding of AGN feedback in large-scale structures. Recent developments regarding the AGN outbursts and its possible contribution to excess entropy in the hot atmospheres of groups and clusters, its correlation with the feedback energy in ICM, quenching of cooling flows and the possible connection between cool core clusters and radio mini-halos, are discussed. We describe current major issues regarding modeling of AGN feedback and its impact on the surrounding medium. With regard to the future of AGN feedback studies, we examine the possible breakthroughs that can be expected from SKA observations. In the context of cluster cosmology, for example, we point out the importance of SKA observations for cluster mass calibration by noting that most of $z>1$ clusters discovered by eROSITA X-ray mission can be expected to be followed up through a 1000 hour SKA-1 mid programme. Moreover, approximately $1000$ radio mini halos and $\sim 2500$ radio halos at $z<0.6$ can be potentially detected by SKA1 and SKA2 and used as tracers of galaxy clusters and determination of cluster selection function.

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arXiv2017

Excess entropy and energy feedback from within cluster cores up to r$_{200}$

We estimate the "non-gravitational" entropy-injection profiles, $ΔK$, and the resultant energy feedback profiles, $ΔE$, of the intracluster medium for 17 clusters using their Planck SZ and ROSAT X-Ray observations, spanning a large radial range from $0.2r_{500}$ up to $r_{200}$. The feedback profiles are estimated by comparing the observed entropy, at fixed gas mass shells, with theoretical entropy profiles predicted from non-radiative hydrodynamic simulations. We include non-thermal pressure and gas clumping in our analysis. The inclusion of non-thermal pressure and clumping results in changing the estimates for $r_{500}$ and $r_{200}$ by 10\%-20\%. When clumpiness is not considered it leads to an under-estimation of $ΔK\approx300$ keV cm$^2$ at $r_{500}$ and $ΔK\approx1100$ keV cm$^2$ at $r_{200}$. On the other hand, neglecting non-thermal pressure results in an over-estimation of $ΔK\approx 100$ keV cm$^2$ at $r_{500}$ and under-estimation of $ΔK\approx450$ keV cm$^2$ at $r_{200}$. For the estimated feedback energy, we find that ignoring clumping leads to an under-estimation of energy per particle $ΔE\approx1$ keV at $r_{500}$ and $ΔE\approx1.5$ keV at $r_{200}$. Similarly, neglect of the non-thermal pressure results in an over-estimation of $ΔE\approx0.5$ keV at $r_{500}$ and under-estimation of $ΔE\approx0.25$ keV at $r_{200}$. We find entropy floor of $ΔK\approx300$ keV cm$^2$ is ruled out at $\approx3σ$ throughout the entire radial range and $ΔE\approx1$ keV at more than 3$σ$ beyond $r_{500}$, strongly constraining ICM pre-heating scenarios. We also demonstrate robustness of results w.r.t sample selection, X-Ray analysis procedures, entropy modeling etc.

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arXiv2017

How multiple supernovae overlap to form superbubbles

We explore the formation of superbubbles through energy deposition by multiple supernovae (SNe) in a uniform medium. We use total energy conserving, 3-D hydrodynamic simulations to study how SNe correlated in space and time create superbubbles. While isolated SNe fizzle out completely by $\sim 1$ Myr due to radiative losses, for a realistic cluster size it is likely that subsequent SNe go off within the hot/dilute bubble and sustain the shock till the cluster lifetime. For realistic cluster sizes, we find that the bubble remains overpressured only if, for a given $n_{g0}$, $N_{\rm OB}$ is sufficiently large. While most of the input energy is still lost radiatively, superbubbles can retain up to $\sim 5-10\%$ of the input energy in form of kinetic+thermal energy till 10 Myr for ISM density $n_{g0} \approx 1$ cm$^{-3}$. We find that the mechanical efficiency decreases for higher densities ($η_{\rm mech} \propto n_{g0}^{-2/3}$). We compare the radii and velocities of simulated supershells with observations and the classical adiabatic model. Our simulations show that the superbubbles retain only $\lesssim 10\%$ of the injected energy, thereby explaining the observed smaller size and slower expansion of supershells. We also confirm that a sufficiently large ($\gtrsim 10^4$) number of SNe is required to go off in order to create a steady wind with a stable termination shock within the superbubble. We show that the mechanical efficiency increases with increasing resolution, and that explicit diffusion is required to obtain converged results.

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arXiv2017

Lack of thermal energy in superbubbles: hint of cosmic rays?

Using analytic methods and $1$-D two-fluid simulations, we study the effect of cosmic rays (CRs) on the dynamics of interstellar superbubbles (ISBs) driven by multiple supernovae (SNe)/stellar winds in OB associations. In addition to CR advection and diffusion, our models include thermal conduction and radiative cooling. We find that CR injection at the reverse shock or within a central wind-driving region can affect the thermal profiles of ISBs and hence their X-ray properties. Even if a small fraction ($10-20\%$) of the total mechanical power is injected into CRs, a significant fraction of the ram pressure at the reverse shock can be transferred to CRs. The energy transfer becomes efficient if (1) the reverse shock gas Mach number exceeds a critical value ($M_{\rm th}\gtrsim 12$) and (2) the CR acceleration time scale $τ_{\rm acc}\sim κ_{\rm cr}/v^2$ is shorter than the dynamical time, where $κ_{\rm cr}$ is CR diffusion constant and $v$ is the upstream velocity. We show that CR affected bubbles can exhibit a volume averaged hot gas temperature $1-5\times10^{6}$ K, lower by a factor of $2-10$ than without CRs. Thus CRs can potentially solve the long-standing problem of the observed low ISB temperatures.

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arXiv2017

Evolution of clustered supernovae

We study the merging and evolution of isolated supernovae (SNe) remnants in a stellar cluster into a collective superbubble, with the help of 3-D hydrodynamic simulations. We particularly focus on the transition stage when the isolated SNe remnants gradually combine to form a superbubble. We find that when the SN rate is high ($ν_{\rm sn}\sim 10^{-9}$ pc$^{-3}$ yr$^{-1}$), the merging phase lasts for $\sim 10^4$ yr, for $n=1\hbox{--}10$ cm$^{-3}$, and the merging phase lasts for a longer time ($\sim 0.1$ Myr or more) for lower SN rates ($ν_{\rm sn}\le 10^{-10}$ pc$^{-3}$ yr$^{-1}$). During this transition phase, the growing superbubble is filled with dense and cool fragments of shells and most of the energy is radiated away during this merging process. After passing through the intermediate phase, the superbubble eventually settles on to a new power-law wind asymptote that is smaller than estimated in a continuous wind model. This results in a significant (more than {\it several times}) underestimation of the mechanical luminosity needed to feed the bubble. We determine the X-ray and H$α$ surface brightnesses as functions of time for such merging SNe in a stellar cluster and find that clusters with high SN rate shine predominantly in soft X-rays and H$α$. In particular, a low value of the volume averaged H$α$ to H$β$ ratio and its large spread can be a good indicator of the transition phase of merging SNe.

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arXiv2017

Clues to the origin of Fermi Bubbles from OVIII/OVII line ratio

We constrain the origin of Fermi Bubbles using 2D hydrodynamical simulations of both star formation driven and black hole accretion driven wind models. We compare our results with recent observations of OVIII to OVII line ratio within and near Fermi Bubbles. Our results suggest that independent of the driving mechanisms, a low luminosity ($\mathcal{L} \sim 0.7-1\times 10^{41}$ erg s$^{-1}$) energy injection best reproduces the observed line ratio for which the shock temperature is $\approx 3\times 10^6$ K. Assuming the Galactic halo temperature to be $2\times 10^6$K, we estimate the shock velocity to be $\sim 300$ km s$^{-1}$ for a weak shock. The corresponding estimated age of the Fermi bubbles is $\sim 15-25$ Myr. Such an event can be produced either by a star formation rate of $\sim 0.5$ M$_\odot$ yr$^{-1}$ at the Galactic centre or a very low luminosity jet/accretion wind arising from the central black hole. Our analysis rules out any activity that generates an average mechanical luminosity $\gtrsim 10^{41}$ \ergps as a possible origin of the Fermi Bubbles.

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arXiv2017

Constraining the X-ray AGN halo occupation distribution: implications for eROSITA

The X-ray emission from active galactic nucleus (AGN) is a major component of extragalactic X-ray sky. In this paper, we use the X-ray luminosity function (XLF) and halo occupation distribution (HOD) formalism to construct a halo model for the X-ray emission from AGNs. Verifying that the two inputs (XLF and HOD) are in agreement with each other, we compute the auto-correlation power spectrum in the soft X-ray band (0.5-2 keV) due to the AGNs potentially resolved by eROSITA (extended ROentgen Survey with an Imaging Telescope Array) mission and explore the redshift and mass dependence of the power spectrum. Studying the relative contribution of the Poisson and the clustering terms to the total power, we find that at multipoles $l\lesssim 1000$ (i.e. large scales), the clustering term is larger than the Poisson term. We also forecast the potential of X-ray auto-correlation power spectrum and X-ray-lensing cross-correlation power spectrum using eROSITA and eROSITA-LSST (Large Synoptic Survey Telescope) surveys, respectively, to constrain the HOD parameters and their redshift evolution. In addition, we compute the power spectrum of the AGNs lying below the flux resolution limit of eROSITA, which is essential to understand in order to extract the X-ray signal from the hot diffuse gas present in galaxies and clusters.

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arXiv2016

Little evidence for entropy and energy excess beyond $r_{500}$ - An end to ICM preheating?

Non-gravitational feedback affects the nature of the intra-cluster medium (ICM). X-ray cooling of the ICM and in situ energy feedback from AGN's and SNe as well as {\it preheating} of the gas at epochs preceding the formation of clusters are proposed mechanisms for such feedback. While cooling and AGN feedbacks are dominant in cluster cores, the signatures of a preheated ICM are expected to be present even at large radii. To estimate the degree of preheating, with minimum confusion from AGN feedback/cooling, we study the excess entropy and non-gravitational energy profiles upto $r_{200}$ for a sample of 17 galaxy clusters using joint data sets of {\it Planck} SZ pressure and {\it ROSAT/PSPC} gas density profiles. The canonical value of preheating entropy floor of $\gtrsim 300$ keV cm$^2$, needed in order to match cluster scalings, is ruled out at $\approx 3σ$. We also show that the feedback energy of 1 keV/particle is ruled out at 5.2$σ$ beyond $r_{500}$. Our analysis takes both non-thermal pressure and clumping into account which can be important in outer regions. Our results based on the direct probe of the ICM in the outermost regions do not support any significant preheating.

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arXiv2016

The Nature and Origin of Ultra-High Energy Cosmic Ray Particles

We outline two concepts to explain Ultra High Energy Cosmic Rays (UHECRs), one based on radio galaxies and their relativistic jets and terminal hot spots, and one based on relativistic Super-Novae (SNe) or Gamma Ray Bursts (GRBs) in starburst galaxies, one matching the arrival direction data in the South (the radio galaxy Cen A) and one in the North (the starburst galaxy M82). Ubiquitous neutrino emission follows accompanied by compact TeV photon emission, detectable more easily if the direction is towards Earth. The ejection of UHECRs is last. We have observed particles up to ZeV, neutrinos up to PeV, photons up to TeV, 30 - 300 Hz GW events, and hope to detect soon of order Hz to mHz GW events. Energy turnover in single low frequency GW events may be of order 10^63 erg. How can we further test these concepts? First of all by associating individual UHECR events, or directional groups of events, with chemical composition in both the Telescope Array (TA) Coll. and the Auger Coll. data. Second by identifying more TeV to PeV neutrinos with recent SMBH mergers. Third by detecting the order < mHz GW events of SMBH binaries, and identifying the galaxies host to the stellar BH mergers and their GW events in the range up to 300 Hz. Fourth by finally detecting the formation of the first generation of SMBHs and their mergers, surely a spectacular discovery.

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arXiv2016

Molecular outflows in starburst nuclei

Recent observations have detected molecular outflows in a few nearby starburst nuclei. We discuss the physical processes at work in such an environment in order to outline a scenario that can explain the observed parameters of the phenomenon, such as the molecular mass, speed and size of the outflows. We show that outflows triggered by OB associations, with $N_{OB}\ge 10^5$ (corresponding to a star formation rate (SFR)$\ge 1$ M$_{\odot}$ yr$^{-1}$ in the nuclear region), in a stratified disk with mid-plane density $n_0\sim 200\hbox{--}1000$ cm$^{-3}$ and scale height $z_0\ge 200 (n_0/10^2 \, {\rm cm}^{-3})^{-3/5}$ pc, can form molecules in a cool dense and expanding shell. The associated molecular mass is $\ge 10^7$ M$_\odot$ at a distance of a few hundred pc, with a speed of several tens of km s$^{-1}$. We show that a SFR surface density of $10 \le Σ_{SFR} \le 50$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$ favours the production of molecular outflows, consistent with observed values.

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arXiv2016

How radiation affects superbubbles: Through momentum injection in early phase and photo-heating thereafter

Energetic winds and radiation from massive star clusters push the surrounding gas and blow superbubbles in the interstellar medium (ISM). Using 1-D hydrodynamic simulations, we study the role of radiation in the dynamics of superbubbles driven by a young star cluster of mass $10^{6}$ M$_{\odot}$. We have considered a realistic time evolution of the mechanical power as well as radiation power of the star cluster, and detailed heating and cooling processes. We find that the ratio of the radiation pressure on the shell (shocked ISM) to the thermal pressure ($\sim10^{7}$ K) of the shocked wind region is almost independent of the ambient density, and it is greater than unity before $\lesssim 1$ Myr. We explore the parameter space of density and dust opacity of the ambient medium, and find that the size of the hot gas ($\sim$ 10$^{7}$ K) cavity is insensitive to the dust opacity ($σ_{d}\approx(0.1-1.5)\times 10^{-21}$ cm$^{2}$), but the structure of the photoionized ($\sim10^4$ K) gas depends on it. Most of the radiative losses occur at $\sim10^{4}$ K, with sub-dominant losses at $\lesssim 10^3$ K and $\sim10^{6}-10^{8}$ K. The superbubbles can retain as high as $\sim 10\%$ of its input energy, for an ambient density of $10^{3}\,m{\rm_{H}\,cm^{-3}}$. We discuss the role of ionization parameter and recombination-averaged density in understanding the dominant feedback mechanism. Finally, we compare our results with the observations of 30 Doradus.

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arXiv2016

Suppression of galactic outflows by cosmological infall and circumgalactic medium

We investigate the relative importance of two galactic outflow suppression mechanisms : a) Cosmological infall of the intergalactic gas onto the galaxy, and b) the existence of a hot circumgalactic medium (CGM). Considering only radial motion, the infall reduces the speed of outflowing gas and even halts the outflow, depending on the mass and redshift of the galaxy. For star forming galaxies there exists an upper mass limit beyond which outflows are suppressed by the gravitational field of the galaxy. We find that infall can reduce this upper mass limit approximately by a factor of two (independent of the redshift). Massive galaxies ($\gtrsim 10^{12} M_{\odot}$) host large reservoir of hot, diffuse CGM around the central part of the galaxy. The CGM acts as a barrier between the infalling and outflowing gas and provides an additional source of outflow suppression. We find that at low redshifts ($z\lesssim3.5$), the CGM is more effective than the infall in suppressing the outflows. Together, these two processes give a mass range in which galaxies are unable to have effective outflows. We also discuss the impact of outflow suppression on the enrichment history of the galaxy and its environment.

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arXiv2016

Diffuse X-ray emission from star forming galaxies

We study the diffuse X-ray luminosity ($L_X$) of star forming galaxies using 2-D axisymmetric hydrodynamical simulations and analytical considerations of supernovae (SNe) driven galactic outflows. We find that the mass loading of the outflows, a crucial parameter for determining the X-ray luminosity, is constrained by the availability of gas in the central star forming region, and a competition between cooling and expansion. We show that the allowed range of the mass loading factor can explain the observed scaling of $L_X$ with star formation rate (SFR) as $L_X \propto$ SFR$^2$ for SFR $\gtrsim 1$ M$_\odot$yr$^{-1}$, and a flatter relation at low SFRs. We also show that the emission from the hot circumgalactic medium (CGM) in the halo of massive galaxies can explain the sub-linear behaviour of the $L_X-$SFR relation as well as a large scatter in the diffuse X-ray emission for low SFRs ($\lesssim$ few M$_\odot$yr$^{-1}$). Our results point out that galaxies with small SFRs and large diffuse X-ray luminosities are excellent candidates for detection of the elusive CGM.

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arXiv2015

Probing the circumgalactic baryons through cross-correlations

We study the cross-correlation of distribution of galaxies, the Sunyaev-Zel'dovich (SZ) and X-ray power spectra of galaxies from current and upcoming surveys and show these to be excellent probes of the nature, i.e. extent, evolution and energetics, of the circumgalactic medium (CGM). The SZ-galaxy cross-power spectrum, especially at large multipoles, depends on the steepness of the pressure profile of the CGM. This property of the SZ signal can, thus, be used to constrain the pressure profile of the CGM. The X-ray cross power spectrum also has a similar shape. However, it is much more sensitive to the underlying density profile. We forecast the detectability of the cross-correlated galaxy distribution, SZ and X-ray signals by combining South Pole Telescope-Dark Energy Survey (SPT-DES) and eROSITA-DES/eROSITA-LSST (extended ROentgen Survey with an Imaging Telescope Array-Large Synoptic Survey Telescope) surveys, respectively. We find that, for the SPT-DES survey, the signal-to-noise ratio (SNR) peaks at high mass and redshift with SNR $\sim 9$ around $M_h\sim 10^{13} h^{-1} M_{\odot}$ and $z\sim 1.5\hbox{--} 2$ for flat density and temperature profiles. The SNR peaks at $\sim 6 (12 )$ for the eROSITA-DES (eROSITA-LSST) surveys. We also perform a Fisher matrix analysis to find the constraint on the gas fraction in the CGM in the presence or absence of an unknown redshift evolution of the gas fraction. Finally, we demonstrate that the cross-correlated SZ-galaxy and X-ray-galaxy power spectrum can be used as powerful probes of the CGM energetics and potentially discriminate between different feedback models recently proposed in the literature; for example, one can distinguish a `no active galactic nuclei feedback' scenario from a CGM energized by `fixed-velocity hot winds' at greater than $3σ$.

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arXiv2015

Multi-wavelength features of Fermi Bubbles as signatures of a Galactic wind

Using hydrodynamical simulations, we show for the first time that an episode of star formation in the center of the Milky Way, with a star-formation-rate (SFR) $\sim 0.5$ M$_\odot$ yr$^{-1}$ for $\sim 30$ Myr, can produce bubbles that resemble the Fermi Bubbles (FBs), when viewed from the solar position. The morphology, extent and multi-wavelength observations of FBs, especially X-rays, constrain various physical parameters such as SFR, age, and the circum-galactic medium (CGM) density. We show that the interaction of the CGM with the Galactic wind driven by a star formation in the central region can explain the observed surface brightness and morphological features of X-rays associated with the Fermi Bubbles. Furthermore, assuming that cosmic ray electrons are accelerated {\it in situ} by shocks and/or turbulence, the brightness and morphology of gamma-ray emission and the microwave haze can be explained. The kinematics of the cold and warm clumps in our model also matches with recent observations of absorption lines through the bubbles.

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arXiv2015

Narrow escape: how ionizing photons escape from disc galaxies

In this paper we calculate the escape fraction ($f_{\rm esc}$) of ionizing photons from starburst galaxies. Using 2-D axisymmetric hydrodynamic simulations, we study superbubbles created by overlapping supernovae in OB associations. We calculate the escape fraction of ionizing photons from the center of the disk along different angles through the superbubble and the gas disk. After convolving with the luminosity function of OB associations, we show that the ionizing photons escape within a cone of $\sim 40 ^\circ$, consistent with observations of nearby galaxies. The evolution of the escape fraction with time shows that it falls initially as cold gas is accumulated in a dense shell. After the shell crosses a few scale heights and fragments, the escape fraction through the polar regions rises again. The angle-averaged escape fraction cannot exceed $\sim [1- \cos (1 \, {\rm radian})] = 0.5$ from geometrical considerations (using the emission cone opening angle). We calculate the dependence of the time- and angle-averaged escape fraction on the mid-plane disk gas density (in the range $n_0=0.15-50$ cm $^{-3}$) and the disk scale height (between $z_0=10-600$ pc). We find that the escape fraction is related to the disk parameters (the mid-plane disk density and scale height) roughly so that $f_{\rm esc}^αn_0^2 z_0^3$ (with $α\approx 2.2$) is a constant. For disks with a given WNM temperature, massive disks have lower escape fraction than low mass galaxies. For Milky Way ISM parameters, we find $f_{\rm esc}\sim 5\%$, and it increases to $\approx 10\%$ for a galaxy ten times less massive. We discuss the possible effects of clumpiness of the ISM on the estimate of the escape fraction and the implications of our results for the reionization of the universe.

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arXiv2015

The role of 3-body H$_2$ formation in the fragmentation of primordial gas

It has been shown that the behaviour of primordial gas collapsing in a dark matter minihalo can depend on the adopted choice of 3-body H$_2$ formation rate. The uncertainties in this rate span two orders of magnitude in the current literature, and so it remains a source of uncertainty in our knowledge of population III star formation. Here we investigate how the amount of fragmentation in primordial gas depends on the adopted 3-body rate. We present the results of calculations that follow the chemical and thermal evolution of primordial gas as it collapses in two dark matter minihalos. Our results on the effect of 3-body rate on the evolution until the first protostar forms agree well with previous studies. However, our modified version of GADGET-2 SPH also includes sink particles, which allows us to follow the initial evolution of the accretion disc that builds up on the centre of each halo, and capture the fragmentation in gas as well as its dependence on the adopted 3-body H$_2$ formation rate. We find that the fragmentation behaviour of the gas is only marginally effected by the choice of 3-body rate co-efficient, and that halo-to-halo differences are of equal importance in affecting the final mass distribution of stars.

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arXiv2015

Long way to go: how outflows from large galaxies propagate through the hot halo gas

Using hydrodynamic simulations, we study the mass loss due to supernova-driven outflows from Milky Way type disk galaxies, paying particular attention to the effect of the extended hot halo gas. We find that the total mass loss at inner radii scales roughly linearly with total mass of stars formed, and that the mass loading factor at the virial radius can be several times its value at inner radii because of the swept up hot halo gas. The temperature distribution of the outflowing material in the inner region ($\sim $10 kpc) is bimodal in nature, peaking at $10^5$ K and $10^{6.5}$ K, responsible for optical and X-ray emission, respectively. The contribution of cold/warm gas with temperature $\le 10^{5.5}$ K to the outflow rate within 10 kpc is $\approx 0.3\hbox{--}0.5$. The warm mass loading factor, $η_{3e5}$ ($T\le 3 \times 10^5$ K) is related to the mass loading factor at the virial radius ($η_{v}$) as $η_{v} \approx 25\, η_{3e5}\, \left(\mbox{SFR}/{\rm M}_\odot{\rm yr}^{-1} \right)^{-0.15}$ for a baryon fraction of 0.1 and a starburst period of 50 Myr. We also discuss the effect of multiple bursts that are separated by both short and long periods. The outflow speed at the virial radius is close to the sound speed in the hot halo, $\lesssim 200$ km s$^{-1}$. We identify two `sequences' of outflowing cold gas at small scales: a fast ($\approx 500$ km~s$^{-1}$) sequence, driven by the unshocked free-wind; and a slow sequence ($\approx \pm 100$ km s$^{-1}$) at the conical interface of the superwind and the hot halo.

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