arXiv subjects

Mukesh Singh Bisht

Publications and source records attributed to Mukesh Singh Bisht.

5 recordsLinked to original sources

arXiv2025

Detection of 'super-virial' gas in the Circumgalactic medium of the Milky Way towards PKS 2155-304

We present the first simultaneous detection of four distinct highly ionized $z=0$ absorbing phases using Chandra and XMM-Newton grating spectra toward the blazar PKS 2155$\hbox{-}$304. We detect the MgXII K$α$ absorption line for the first time in the circumgalactic medium (CGM) of the Milky Way. Along with MgXII K$α$, we detect SiXIV K$α$ absorption, which are the tell-tale signatures of the hot 'super-virial' gas in the CGM. Both from the model-independent calculations and hybrid-ionization modeling, we infer four phases at distinct temperatures, hot 'super-virial' ($5.4^{+1.9}_{-0.8} \times 10^7$ K), warm-hot 'virial' ($1.8^{+0.3}_{-0.2} \times 10^6$ K), warm 'sub-virial' ($2.2\pm 0.5 \times 10^5$ K), and cool phase ($<1.7 \times 10^5$ K). The warm-hot and hot phases are $α$-enhanced, and [C/O] and [Ne/O] are super-solar in the warm-hot phase, while [Mg/O] and [Si/O] are super-solar in the hot phase. The low-ionization lines are blue-shifted (v$_{\rm los} \approx -100$ km s$^{-1}$), whereas the high-ionization lines are red-shifted. It suggests a scenario of infalling sub-virial, quasi-static virial, and outflowing super-virial phases along this sightline. Earlier studies on individual sightlines were confined to the Northern Hemisphere. Our sightline is located in the Southern hemisphere, demonstrating that hot super-virial gas is also present at Southern Galactic latitudes as well. This confirms a more widespread distribution of the super-virial gas across both hemispheres.

astro-ph.GA↗

arXiv2025

Misty, patchy, and turbulent: constraining the cool circumgalactic medium with mCC

The circumgalactic medium (CGM) is the largest baryon reservoir around galaxies, but its extent, mass, and temperature distribution remain uncertain. We propose that cool gas ($\sim 10^4$ K) in the CGM resides in clumpy structures referred to as cloud complexes (CCs) rather than uniformly filling the entire CGM volume. Each CC contains a mist of tiny cool cloudlets dispersed in a warm/hot medium ($\sim 10^5$ - $10^6$~K). Modeling CCs in the mist limit (unit area covering fraction within a CC) simplifies the calculation of observables like ion absorption columns, equivalent widths, compared to modeling individual cloudlets from first principles. Through Monte Carlo realizations of CCs, we explore how CC properties affect the observed variation in observables. We find that a power-law distribution of CCs ($dN_{\rm CC}/dR \propto R^{-1}$) with a total of $\sim 10^3$ CCs each with a radius of $\sim 10$ kpc and total cool gas mass of $\sim 10^{10} M_\odot$ reproduces MgII column density and equivalent width distribution trends with impact parameter for the COS-Halos sample (Werk+ 2013). We further show that the area-averaged MgII column density, combined with the area covering fraction, provides a robust proxy for estimating the cool CGM mass, independent of other model parameters. Modeling a larger number of (smaller size) cloudlets within a CC shows that line blending from individual cloudlets results in turbulent broadening on the CC scale. This work presents a practical framework for linking CGM models with observations of a multiphase CGM, illuminating the distribution of cool gas in galaxy halos.

astro-ph.GA↗

arXiv2024

Can supernova from runaway stars mimic the signs of absorbing `super-virial' gas?

The recent detection of large column density absorption lines from highly ionized gas in a few directions through the circumgalactic medium (CGM) of the Milky Way (MW) has been puzzling. The inferred temperature from these absorption lines far exceeds the virial temperature of the MW, and the column densities are also too large to be easily explained. In this paper, we propose a novel idea to explain these observations and claim that they may not have originated from the CGM, but from a totally different type of source, namely, stellar ejecta from supernovae (SNe) above the Galactic disk that happen to lie in the line of sight to the background quasars. About $\sim 20\%$ of massive OB stars (progenitors of core-collapse supernovae) are known to be runaway stars that have high ejection velocities near the Galactic plane and can end up exploding as SNe above the Galactic disk. We show that the associated reverse shock in the supernova remnant in the early non-radiative phase can heat the ejecta to temperatures of $\gtrsim 10^7\,{\rm K}$ and can naturally explain the observed high column density of ions in the observed `super-virial' phase along with $α$-enriched super-solar abundance that is typical of core-collapse supernovae. However, SNe from runaway stars has a covering fraction of $\lesssim 0.7 \%$ and thus can only explain the observations along limited sightlines.

astro-ph.GA↗

arXiv2024

On the origin of the $10^7$ K hot emitting gas in the Circumgalactic medium of the Milky Way

The presence of the $\approx 10^6$ K gas in the circumgalactic medium of the Milky Way has been well established. However, the location and the origin of the newly discovered hot gas at `super-virial' temperatures of $\approx 10^7$ K have been puzzling. This hot gas has been detected in both absorption and emission; here we focus on the emitting gas only. We show that both the `virial' and the `super-virial' temperature gas as observed in \emph{emission} occupy disk-like extraplanar regions, in addition to the diffuse virial temperature gas filling the halo of the Milky Way. We perform idealized hydrodynamical simulations to show that the $\approx 10^7$ K emitting gas is likely to be produced by stellar feedback in and around the Galactic disk. We further show that the emitting gas at both super-virial and virial temperatures in the extraplanar regions is metal enriched and is not in hydrostatic equilibrium with the halo but is continuously evolving.

astro-ph.GA↗

arXiv2024

Beyond radial profiles: Using log-normal distributions to model the multiphase circumgalactic medium

Recent observations and simulations reveal that the circumgalactic medium (CGM) surrounding galaxies is multiphase, with the gas temperatures spanning a wide range at most radii, $\sim 10^4\ {\rm K}$ to the virial temperature ($\sim 10^6$ K for Milky Way). Traditional CGM models using simple density profiles are inadequate at reproducing observations that indicate a broad temperature range. Alternatively, a model based on probability distribution functions (PDFs) with parameters motivated by simulations can better match multi-wavelength observations. In this work, we use log-normal distributions, commonly seen in the simulations of the multiphase interstellar and circumgalactic media, to model the multiphase CGM. We generalize the isothermal background model by Faerman et al. 2017 to include more general CGM profiles. We extend the existing probabilistic models from 1D-PDFs in temperature to 2D-PDFs in density-temperature phase space and constrain its parameters using a Milky Way-like {\tt Illustris TNG50-1} halo. We generate various synthetic observables such as column densities of different ions, UV/X-ray spectra, and dispersion and emission measures. X-ray and radio (Fast Radio Burst) observations mainly constrain the hot gas properties. However, interpreting cold/warm phase diagnostics is not straightforward since these phases are patchy, with inherent variability in intercepting these clouds along arbitrary lines of sight. We provide a tabulated comparison of model predictions with observations and plan to expand this into a comprehensive compilation of models and data. Our modeling provides a simple analytic framework that is useful for describing important aspects of the multiphase CGM.

astro-ph.GA↗