arXiv subjects

Kartick C. Sarkar

Publications and source records attributed to Kartick C. Sarkar.

18 recordsLinked to original sources

arXiv2026

Fast modelling of ionization balance in the intergalactic medium: I- implications for the IGM metallicity

Ionization balance in the intergalactic medium (IGM) is central to the interpretation of quasar absorption spectra, linking observed ionic columns to the underlying gas density, temperature, metallicity, and ionizing radiation field. Because ionization, recombination, and cooling timescales can be comparable to the timescales over which the ultraviolet background (UVB) and gas thermodynamic state evolve, ion populations may retain a strong memory of their past history. To this end, we present a fast, metals-inclusive, zero-dimensional framework for modeling the redshift evolution of the IGM. The model follows the coupled thermal and ionization evolution of a Lagrangian gas parcel in a redshift-dependent UVB, solving stiff, time-dependent rate equations for H, He, and 107 metal ions while self-consistently evolving the temperature through photoheating and standard cooling processes. We validate the framework against full three-dimensional hydrodynamical non-equilibrium calculations and find that it reproduces the thermal and ionization histories of the IGM with good accuracy over a wide redshift range, including the heating associated with $\rm He_{\,\rm II}$ reionization. As an application, we predict the cosmic $\rm C_{\,\rm IV}$ density parameter, $Ω_{\rm CIV}$, and use it to infer the origin of metal ions in the IGM and the corresponding metallicities from observational measurements, obtaining values broadly consistent with literature constraints. The framework is well suited for rapid parameter studies of how reionization timing, UVB spectral hardness, self-shielding, and UVB inhomogeneity shape the thermal and ionization history of the IGM and the resulting metal-line observables.

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arXiv2026

A Massive Galaxy at the Edge of Feedback-Free Efficiency

The efficiency with which galaxies convert their available baryonic reservoir into stars sets a fundamental ceiling on stellar mass assembly in the early Universe and encodes the cumulative effect of stellar feedback. in this paper, we report the measurement of star-formation efficiency (SFE) of a photometrically and spectroscopically vetted reference sample at $z=9-10$, out of which 31 are spectroscopically confirmed. Among them, we highlight a spectroscopically confirmed galaxy, \texttt{UNCOVER 3686} at $z = 9.31$, which has a stellar mass $M_\star = 10^{9.55}$ M$_\odot$ and physical properties comparable to the predictions of the feedback-free starburst (FFB) scenario. We use this galaxy as an anchor for the first direct observational test of whether galaxy at cosmic dawn with physical properties predicted in the FFB theory reach the maximum baryon conversion efficiencies predicted by the feedback-free starburst scenario. We find the SFE for this galaxy to lie between $\approx 20\%$ and $60\%$, a factor of 2 to 6 above empirical model predictions. The compact morphology ($R_e = 0.45$ kpc), young stellar age ($\sim 160$ Myr), low metallicity $(Z_\star/Z_\odot \approx 0.2)$, and inferred gas density $(n_{\rm gas} \sim 3\times 10^{3}$ cm$^{-3}$) of this galaxy are consistent with feedback-free (FFB) galaxy formation conditions. We conclude that UNCOVER 3686 is an excellent candidate for an FFB galaxy in which the global star formation efficiency approaches the theoretical limits due to weak stellar feedback.

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arXiv2026

Non-Equilibrium Ionisation in Photoionised Haloes: Implications for Shock Stability and Absorption-Line Signatures

We investigate the impact of nonequilibrium ionisation (NEI) and the metagalactic radiation-field on the thermal evolution, virial shock stability, and absorption signatures of gas surrounding galaxies. Using 1D, spherically symmetric hydrodynamical simulations with an extended version of the hydra code, we follow dark-matter growth, gas dynamics, time-dependent ionisation and cooling in the presence of the UV background. We explicitly track all ions of H, He, C, N, O, Ne, Mg, Si, S, and Fe in haloes of mass 1e11-1e13Msun from z=100 to z=0. Without a UV background, NEI enhances post-shock cooling due to underionised gas, reducing pressure support and raising the minimum mass for stable shock formation. Including the UV background pre-ionises the IGM, suppressing NEI, and restoring the CIE threshold. The IGM temperatures deviate from thermal equilibrium due to adiabatic expansion and collapse, while ionisation remains close to equilibrium in the presence of a UV background, except in transient rapidly cooling regions where NEI occurs. We compute absorption columns of OVI, CIV, and HI, showing that a photoionised IGM may produce substantial warm-ion columns extending beyond Rvir, including OVI column densities comparable to observed values. Our models indicate weak halo-mass dependence and extended distributions. We also find that z>~3 haloes can produce CIV (NCIV~1e13-15cm^-2) and HI (NHI~1e15-17cm^-2) columns out to ~10Rvir. Our results highlight the role of the UV background in regulating the thermal state and observable signatures of the gas surrounding galaxies, and emphasize the importance of accounting for IGM contributions when interpreting CGM absorption-line observations.

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arXiv2025

A 13-Billion-Year View of Galaxy Growth: Metallicity Gradient Evolution from the Local Universe to $z=9$ with JWST and Archival Surveys

The galaxy gas-phase metallicity gradients have been extensively studied over the past four decades, both in the local and high-redshift universe, as they trace the baryon cycle and growth of galaxies. With the unprecedented spatial resolution and sensitivity of JWST, it is now possible to measure metallicity and its radial gradients out to redshifts as high as $z = 9$. Here, we present a sample of 455 spectroscopically confirmed galaxies from redshifts $1.7 \lesssim z \lesssim 9$ that are spatially resolved on sub-kiloparsec (kpc) scales by deep JWST NIRCam or NIRISS Wide Field Slitless Spectroscopy (WFSS). Synthesizing these new JWST observations with legacy observations from the literature, we observe that at redshift $z > 5$, galaxy centers are more metal-rich, exhibiting negative metallicity gradients of $\sim-0.4$ dex kpc$^{-1}$. These gradients flatten over time, reaching near-zero around $z \approx 2$, coinciding with the peak of the cosmic star formation rate. Beyond this point, the gradients become negative again at lower redshifts approaching $z=0$. This evolution likely reflects transitions in galaxy formation modes: an inside-out growth phase dominated by intense central star formation with inefficient feedback and limited gas mixing during ``cosmic dawn", enhanced gas mixing due to feedback-driven wind and gas accretion at ``cosmic noon", and a later phase of slow evolution and reduced feedback toward the present day. These physical processes, including gas accretion and feedback, not only regulate star and galaxy formation on a cosmic scale but also shape the evolutionary pathways of individual galaxies over cosmic time.

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arXiv2025

Origin of the ~150 kpc radio filament in galaxy ESO 137-006

Sensitive wide-field radio surveys have started uncovering many filamentary structures associated with the jets and lobes of radio galaxies, radio relics in galaxy clusters, and tailed galaxies. Although limited theoretical investigations on the origin of the filamentary structures have associated these filaments with astrophysical shocks and interactions with intracluster magneto-ionic media, more quantitative studies are needed to ascertain their precise nature and origin. Recent MeerKAT observations found peculiar filamentary structures (threaded radio structures) joining the lobes of a nearby FRII-like galaxy, ESO 137-006. Here we investigate the origin of these "synchrotron threads" to understand if they may be confined magnetically and could arise due to shocks associated with jet activity. Through simulation- and theory-based analysis, we find that the dynamical time (~70 Myr) associated with the shock front closely matches the estimated synchrotron age (~130 Myr) of the threads, thus making the shock origin hypothesis a favorable scenario for this particular filament.

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arXiv2024

Intergalactic-Absorption Confounding Circumgalactic Observations

The origin of warm ions in the circum-galactic medium (CGM) surrounding massive galaxies remains a mystery. In this paper, we argue that a significant fraction of the observed warm-ion columns may arise in the intergalactic medium (IGM) surrounding galactic halos. We use a simple spherical collapse model of the dark matter (DM) halos and their baryonic content to compute the evolving ion fractions within and outside virial halos. We show that the photoionized IGM may produce a thick blanket of warm ions around the CGM, thereby contaminating CGM observations. We find that the IGM contributes $> 75\%$ of the total \ion{O}{6} column densities in halos with virial masses exceeding a few times $10^{11}~M_\odot$, and that it may dominate the \ion{O}{6} absorption even for lower mass-halos, depending on the impact parameter. We compare our results with observations and find that our simplified model reproduces the overall \ion{O}{6} columns as well as their trend with the impact parameter and halo mass. We show that observed warm ion columns may be completely dominated by the IGM envelopes, consistent with CGM$^2$ data. We, therefore, suggest that theoretical interpretations of CGM-survey observations must consider the possible contribution of the surrounding IGM. Although our simplified model suggests that it may be possible to kinematically distinguish between CGM and IGM origins through the absorption line profiles, this distinction is likely unfeasible in realistic astrophysical halos, due to the complex velocity structure in the multi-phased CGM.

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arXiv2024

Feedback-Free Starbursts at Cosmic Dawn: Observable Predictions for JWST

We extend the analysis of a physical model within the standard cosmology that robustly predicts a high star-formation efficiency (SFE) in massive galaxies at cosmic dawn due to feedback-free starbursts (FFBs). It implies an excess of bright galaxies at z>~10 compared to the standard models based on the low SFE at later epochs, an excess indicated by JWST observations. Here we provide observable predictions based on the analytic FFB scenario. These can be compared with simulations and JWST observations. We approximate the SFE as a function of redshift and mass, assuming a maximum SFE of 0.2~1 in the FFB regime. From this, we derive the evolution of the galaxy mass and luminosity functions as well as the evolution of stellar and star-formation densities. We then predict the star-formation history (SFH), galaxy sizes, outflows, gas fractions, metallicities, and dust attenuation, all as functions of mass and redshift in the FFB regime. The major distinguishing feature is the occurrence of FFBs above a mass threshold that declines with redshift. The luminosities and star formation rates in bright galaxies are predicted to be in excess of extrapolations of standard empirical models and cosmological simulations, an excess that grows from z~9 to higher redshifts. The FFB phase of ~100 Myr is predicted to show a characteristic SFH that fluctuates on a timescale of ~10 Myr. The stellar systems are compact (Re~0.3 kpc at z~10 and declining with z). The galactic gas consists of a steady wind driven by supernovae from earlier generations, with high outflow velocities (FWHM~1400-6700km/s), low gas fractions (<0.1), low metallicities (<~0.1 solar), and low dust attenuation ($A_{UV}$~0.5 at z~10 and declining with z). We make tentative comparisons with current JWST observations for initial insights, anticipating more complete and reliable datasets for detailed quantitative comparisons in the future.

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arXiv2024

X-ray signatures of galactic outflows into the circumgalactic medium

We present a set of controlled hydrodynamical simulations to study the effects of strong galactic outflows on the density and temperature structures, and associated X-ray signatures, of extra-planar and circumgalactic gas. We consider three initial state models, isothermal, isentropic, and rotating cooling-flow, for the hot circumgalactic medium (CGM) into which the outflows are driven. The energy sources are either stellar winds and supernovae, or active galactic nuclei. We consider energy injection rates in the range $10^{40} < \dot{E}_{\rm inj} <10^{44.5}$ erg s$^{-1}$, and compute the time-dependent soft X-ray (0.5-2 keV) surface brightness. For $\dot{E}_{\rm inj} \gtrsim 10^{41} - 10^{42}$ erg s$^{-1}$, with the exact threshold depending on the initial CGM state, the X-ray response is dominated by dense hot gas in the forward shock that eventually fades into the CGM as a sound wave. The shock surrounds an inner hot bubble leading to a radial flattening of the X-ray surface brightness. For lower energy injection rates, the X-ray surface brightness of the initial CGM state is almost unaffected. We present analytic approximations for the outflow shock propagation and the associated X-ray emissions.

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arXiv2024

The Fermi/eROSITA Bubbles: A look into the nuclear outflow from the Milky Way

Galactic outflows are ubiquitous in galaxies containing active star formation or supermassive black hole activity. The presence of a large-scale outflow from the center of our own Galaxy was confirmed after the discovery of two large ($\sim 8-10$ kpc) γ-ray bubbles using the \textit{Fermi-LAT} telescope. These bubbles, known as the Fermi Bubbles, are highly symmetric about the Galactic disk as well as around the Galactic rotation axis and appear to emanate from the center of our Galaxy. The sharp edges of these bubbles suggest that they are related to the Galactic outflow. These bubbles are surrounded by two even bigger ($\sim 12-14$ kpc) X-ray structures, known as the eROSITA bubbles. Together, they represent the characteristics of an outflow from the Galaxy into the circumgalactic medium. Multi-wavelength observations such as in radio, microwave, and UV toward the Fermi Bubbles have provided us with much information in the last decade. However, the origin and the nature of these bubbles remain elusive. In this review, I summarize the observations related to the Fermi/eROSITA Bubbles at different scales and wavelengths, and give a brief overview of our current understanding of them.

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arXiv2024

Are Odd Radio Circles virial shocks around massive galaxies? Implications for cosmic-ray diffusion in the circumgalactic medium

Recently, a new population of circular radio ($\sim$GHz) objects have been discovered at high Galactic latitudes, called the Odd Radio Circles (ORCs). A fraction of the ORCs encircles massive galaxies in the sky with stellar mass $\sim 10^{11}\, M_\odot$ situated at $z=0.2$-$0.6$, suggesting a possible physical connection. In this work, we explore the possibility that these radio circles originate from the accretion shocks/virial shocks around massive ($\gtrsim10^{13}\, M_\odot$) dark matter halo at $z\sim0.5$. We found that the radio flux density of the emitting shell is marginally consistent with the ORCs. We also find that pure advection of electrons from the shock results in a radio-emitting shell that is considerably narrower than the observed one due to strong inverse-Compton cooling of electrons. Instead, we show that the diffusion of cosmic-ray (CR) electrons plays a significant role in increasing the width of the shell. We infer a diffusion coefficient, $D_{\rm cr} \sim 10^{30}\ {\rm cm^2\,s^{-1}}$, consistent with the values expected for low-density circumgalactic medium (CGM). If ORCs indeed trace virial shocks, then our derived CR diffusion coefficient represents one of the few estimations available for the low-density CGM. Finally, we show that the apparent discrepancy between ORC and halo number density can be mitigated by considering an incomplete halo virialization and the limited radiation efficiency of shocks. This study, therefore, opens up new avenues to probe such shocks and non-thermal particle acceleration within them. Furthermore, our results suggest that low-mass galaxies ($\lesssim 10^{13}\, M_\odot$) may not show ORCs due to their significantly lower radio surface brightness.

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arXiv2023

Dissipation of AGN jets in a clumpy interstellar medium

Accreting supermassive black holes (SMBHs) frequently power jets that interact with the interstellar/circumgalactic medium (ISM/CGM), regulating star-formation in the galaxy. Highly supersonic jets launched by active galactic nuclei (AGN) power a cocoon that confines them and shocks the ambient medium. We build upon the models of narrow conical jets interacting with a smooth ambient medium, to include the effect of dense clouds that are an essential ingredient of a multiphase ISM. The key physical ingredient of this model is that the clouds along the supersonic jet-beam strongly decelerate the jet-head, but the subsonic cocoon easily moves around the clouds without much resistance. We propose scalings for important physical quantities -- cocoon pressure, head & cocoon speed, and jet radius. We obtain, for the first time, the analytic condition on clumpiness of the ambient medium for the jet to dissipate within the cocoon and verify it with numerical simulations of conical jets interacting with a uniform ISM with embedded spherical clouds. A jet is defined to be dissipated when the cocoon speed exceeds the speed of the jet-head. We compare our models to more sophisticated numerical simulations, direct observations of jet-ISM interaction (e.g., quasar J1316+1753), and discuss implications for the Fermi/eROSITA bubbles. Our work also motivates effective subgrid models for AGN jet feedback in a clumpy ISM unresolved by the present generation of cosmological galaxy formation simulations.

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arXiv2022

Misaligned jets from Sgr A$^*$ and the origin of Fermi/eROSITA bubbles

One of the leading explanations for the origin of Fermi Bubbles is a past jet activity in the Galactic center supermassive black hole Sgr A$^*$. The claimed jets are often assumed to be perpendicular to the Galactic plane. Motivated by the orientation of pc-scale nuclear stellar disk and gas streams, and a low inclination of the accretion disk around Sgr A$^*$ inferred by the Event Horizon Telescope, we perform hydrodynamical simulations of nuclear jets significantly tilted relative to the Galactic rotation axis. The observed axisymmetry and hemisymmetry (north-south symmetry) of Fermi/eROSITA bubbles (FEBs) due to quasi-steady jets in Sgr A$^*$ can be produced if the jet had a super-Eddington power ($\gtrsim 5\times 10^{44}$ erg s$^{-1}$) for a short time (jet active period $\lesssim 6$ kyr) for a reasonable jet opening angle ($\lesssim 10^\circ$). Such powerful explosions are, however, incompatible with the observed O VIII/O VII line ratio towards the bubbles, even after considering electron-proton temperature non-equilibrium. We argue that the only remaining options for producing FEBs are i) a low-luminosity ($\approx 10^{40.5-41}$ erg s$^{-1}$)) magnetically dominated jet or accretion wind from the Sgr A$^*$, and ii) a SNe or TDE driven wind of a similar luminosity from the Galactic center.

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arXiv2022

Self-ionizing galactic winds

We present hydrodynamical simulations of galactic winds from star-forming galaxies including non-equilibrium ionization and frequency-dependent radiative transfer, processes that have remained largely unaccounted for in galactic wind studies. We consider radiation from massive stars, the metagalactic UV/X-ray background, and the self-radiation of the supernovae heated gas. We compare our results to classical galactic wind solutions and show the importance of our newly included physical processes toward observations of ions such as O III, O VI, O VII and O VIII plus the observable soft X-ray spectra. Non-equilibrium ionization is reflected in over-ionized gas compared to equilibrium solutions, leading to much enhanced column densities of highly ionized species. The wind produces excess soft X-ray ($E\gtrsim 100$ eV) radiation that is several orders of magnitude higher compared to the metagalactic background. This radiation ionizes the higher ions (such as O VII) somewhat, but affects the lower ions (such as O III) significantly. We predict that the observable X-ray spectra should contain the signatures of such non-equilibrium effects, especially in X-ray lines such as O VII and O VIII. Simple estimates suggest that both the temperature and density of the winds may be overestimated by factors of a few to almost 2 orders of magnitude using simple equilibrium models. We conclude that both the non-equilibrium ionization and the radiation from the wind itself need to be considered for proper modeling of the optical/UV/X-ray emitting plasma in galactic winds.

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arXiv2021

Fermi bubbles: the collimated outburst needed to explain forward-shock edges

The bipolar, nonthermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modeling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at $z\simeq100$ pc heights in our model) with a half-opening angle $θ\simeq4^\circ$, a normalized velocity $β_{-2}\equiv v/(0.01c)\gtrsim 0.4$, and an energy $E\gtrsim2β_{-2}^2\times 10^{55}$ erg, launched $\mathbb{T}\simeq 3.3β_{-2}^{-1}$ Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller $E/(β_{-2}θ)^{2}$, and a comparable $\mathbb{T}$, and follow a ballistic stage that must reach a height $z_{s}\gtrsim 5$ kpc.

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arXiv2019

The Global Star-Formation Law by Supernova Feedback

We address a simple model where the Kennicutt-Schmidt (KS) relation between the macroscopic densities of star-formation rate (SFR, $ρ_{\rm sfr}$) and gas ($n$) in galactic discs emerges from self-regulation of the SFR via supernova feedback. It arises from the physics of supernova bubbles, insensitive to the microscopic SFR recipe and not explicitly dependent on gravity. The key is that the filling factor of SFR-suppressed supernova bubbles self-regulates to a constant, $f\sim 0.5$. Expressing the bubble fading radius and time in terms of $n$, the filling factor is $f \propto S\,n^{-s}$ with $s\sim 1.5$, where $S$ is the supernova rate density. A constant $f$ thus refers to $ρ_{\rm sfr} \propto n^{1.5}$, with a density-independent SFR efficiency per free-fall time $\sim 0.01$. The self-regulation to $f \sim 0.5$ and the convergence to a KS relation independent of the local SFR recipe are demonstrated in cosmological and isolated-galaxy simulations using different codes and recipes. In parallel, the spherical analysis of bubble evolution is generalized to clustered supernovae, analytically and via simulations, yielding $s \simeq 1.5 \pm 0.5$. An analysis of photo-ionized bubbles about pre-supernova stars yields a range of KS slopes but the KS relation is dominated by the supernova bubbles. Superbubble blowouts may lead to an alternative self-regulation by outflows and recycling. While the model is over-simplified, its simplicity and validity in the simulations may argue that it captures the origin of the KS relation.

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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

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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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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