arXiv subjects

Alankar Dutta

Publications and source records attributed to Alankar Dutta.

10 recordsLinked to original sources

arXiv2025

Multiphase gas in Circumgalactic cloud complexes: Insights from kiloparsec-scale Magnetohydrodynamic Turbulence Simulations

The circumgalactic medium (CGM) is the diffuse gas surrounding a galaxy's halo, and it plays a vital role in the galactic baryon cycle. However, its mass distribution across the virial phase and the cooler, denser atomic phase, remains uncertain. To investigate this, we perform high-resolution magnetohydrodynamic simulations of 0.125--8 kpc-scale representative patches of the CGM, with parameters informed by quasar absorption line observations. Our simulations resolve the cooling length (the minimum across all temperatures of $c_s t_{\rm cool}$, where $c_s$ is the sound speed and $t_{\rm cool}$ is the cooling time in isobaric conditions), allowing us to track the evolution of cold gas more accurately. We find that low-density CGM gas ($3\times10^{-4}$ cm$^{-3}$) cannot sustain cold gas below $10^4$ K for long, due to a large value of the ratio between the cooling to mixing time ($t_{\rm cool}/t_{\rm mix}$). In contrast, higher-density environments ($3\times10^{-3}~{\rm cm}^{-3}$) reach a turbulent multiphase steady state, with up to $50\%$ of the mass in the cold phase, occupying only about $1\%$ of the volume. To connect with large-volume cosmological simulations and small ${\rm pc}$-scale idealized simulations, we explore different box sizes (0.125--8 kpc) and identify a key scaling relation: simulations with similar $t_{\rm cool}/t_{\rm mix}$ exhibit comparable cold gas mass fractions and lifetimes. Importantly, we find that simply sub-sampling (reducing box-size) a small region from a large-volume simulation while maintaining a constant turbulent energy density injection rate from larger to smaller scales artificially shortens $t_\mathrm{mix}$, leading to inaccurate predictions for cold gas survival. This means that cold gas at small $\lesssim 10$ kpc scales arises in relatively dense, quiescent regions of the CGM rather than the turbulent ones undergoing cascade from large scales.

astro-ph.GA↗

arXiv2025

Woven by the Whirls: The growth and entrainment of cold clouds in turbulent hot winds

Galactic and intergalactic flows often exhibit relative motion between the cold dense gas and the hot diffuse medium. Such multiphase flows -- involving gas at different temperatures, densities, and ionization states -- for instance, galactic winds, are frequently turbulent. However, idealized simulations typically model the winds and driven turbulence separately, despite their intertwined roles in galaxy evolution. To address this, we investigate the survival of a dense cloud in a hot wind subject to continuous external turbulent forcing. We perform 3D hydrodynamic simulations across a range of turbulent Mach numbers in the hot phase $\mathcal{M}_{\rm turb}=v_{\rm turb}/c_{\rm s, wind}$ from 0.1 to 0.7 ($c_{\rm s, wind}$ and $v_{\rm turb}$ being the sound speed and the turbulent velocity in the hot phase, respectively). We find that in spite of the additional subsonic turbulence, cold clouds can survive if the cooling time of the mixed gas $t_{\rm cool, mix}$ is shorter than a modified destruction time $\tilde{t}_{\rm cc}$, i.e., $t_{\rm cool,mix}/\tilde{t}_{\rm cc}<1$ where $\tilde{t}_{\rm cc}=t_{\rm cc}/(1+\left(\mathcal{M}_{\rm turb}/\left(f_{\rm mix}\mathcal{M}_{\rm wind}\right)\right)^2)^{1/2}$, where $f_{\rm mix}\sim0.6$ is a fudge factor. Moreover, in the `survival regime', turbulence can enhance the growth of cold clouds by up to an order of magnitude because of more efficient stretching and an associated increase in the surface area. This increase in mass transfer between the phases leads to significantly faster entrainment of cold material in turbulent winds. In contrast to the narrow filamentary tails formed in laminar winds, turbulence stretches the cold gas orthogonally, dispersing it over a larger area and changing absorption line signatures.

astro-ph.GA↗

arXiv2025

Fading in the Flow: Suppression of cold gas growth in expanding galactic outflows

Multiphase outflows, revealed by multi-wavelength observations, are crucial in redistributing gas and metals within and around galaxies. These outflows are often modelled theoretically using wind tunnel simulations of a cold ($\sim 10^4$ K) cloud interacting with a uniform hot ($\sim 10^6$ K) wind. However, real outflows expand downstream, a feature overlooked in most idealised simulations. We study how an expanding wind affects the survival, morphology, and dynamics of a cloud. We conduct idealised hydrodynamic simulations with optically thin radiative cooling of a cloud in an expanding wind, modelled using the adiabatic Chevalier & Clegg (1985) solution. We find that clouds remain locally isobaric with the wind, leading to a steep decline in their density contrast and eventual dissolution downstream. Compared to a plane-parallel wind, this suppresses cold gas mass growth because as clouds travel downstream, the surrounding mixed boundary layer becomes diffuse and less radiative. Our analytical scaling arguments show that cloud expansion and local pressure equilibrium are the key regulators of cold mass growth. Unlike traditional simulations, our model accounts for the differential expansion experienced by the long cometary tails of clouds in wind tunnels. This creates a strong head-to-tail emission gradient in the filamentary cold gas, which is more consistent with observations. We also demonstrate that the dynamics of individual clouds can substantially alter the radial properties of their host multiphase outflows.

astro-ph.GA↗

arXiv2024

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.

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arXiv2024

Ram pressure stripping in clusters: Gravity can bind the ISM but not the CGM

We explore the survival of a galaxy's circumgalactic medium (CGM) as it experiences ram pressure stripping (RPS) moving through the intracluster medium (ICM). For a satellite galaxy, the CGM is often assumed to be entirely stripped/evaporated, an assumption that may not always be justified. We carry out 3D-hydrodynamic simulations of the interstellar and circumgalactic media (ISM+CGM) of a galaxy like JO201 moving through the ICM. The CGM can survive long at cluster outskirts ($\gtrsim2 \rm \ Gyr$) but at smaller cluster-centric distances, 90\% of the CGM mass is lost within $\sim 500$ Myr. The gravitational restoring force on the CGM is mostly negligible and the CGM-ICM interaction is analogous to \textit{`cloud-wind interaction'}. The CGM stripping timescale does not depend on the ram pressure but on the CGM to ICM density contrast $χ$. Two distinct regimes emerge for CGM stripping: the $χ>1$ regime, which is the well-known \textit{`cloud crushing'} problem, and the $χ<1$ regime, which we refer to as the (relatively unexplored) \textit{`bubble drag'} problem. The first pericentric passage near the cluster core can rapidly -- over a crossing time $t_{\rm drag} \sim R/v_{\rm rel}$ -- strip the CGM in the \textit{bubble drag} regime. The ISM stripping criterion unlike the CGM criterion, still depends on the ram pressure $ρ_{\rm ICM} v_{\rm rel}^2$. The stripped tails of satellites contain contributions from both the disk and the CGM. The X-ray plume in M89 in the Virgo cluster and a lack of it in the nearby M90 might be attributed to their orbital histories. M90 has likely undergone stripping in the bubble drag regime due to a pericentric passage close to the cluster center.

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↗

arXiv2021

Cooling flows around cold clouds in the circumgalactic medium: steady-state models & comparison with TNG50

Cold, non-self-gravitating clumps occur in various astrophysical systems, ranging from the interstellar and circumgalactic medium (CGM), to AGN outflows and solar coronal loops. Cold gas has diverse origins such as turbulent mixing or precipitation from hotter phases. We obtain the analytic solution for a steady pressure-driven 1-D cooling flow around cold, local over-densities, irrespective of their origin. Our solutions describe the slow and steady radiative cooling-driven gas inflow in the saturated regime of nonlinear thermal instability in clouds, sheets and filaments. Such a cooling flow develops when the gas around small clumps undergoes radiative cooling. These small-scale, cold `seeds' are embedded in a large volume-filling hot CGM maintained by feedback. We use a simple two-fluid treatment to include magnetic fields as an additional polytropic fluid. To test the limits of applicability of these analytic solutions, we compare with the gas structure found in and around small-scale cold clouds in the CGM of massive halos in the TNG50 cosmological MHD simulation from the IllustrisTNG suite. Despite qualitative resemblance of the gas structure, we find deviations from steady state profiles generated by our model. Complex geometries and turbulence all add complexity beyond our analytic solutions. We derive an exact relation between the mass cooling rate ($\dot{\rm M}_{\rm cool}$) and the radiative cooling rate ($\dot{\rm E}_{\rm cool}$) for a steady cooling flow. A comparison with the TNG50 clouds shows that this cooling flow relation only applies in a narrow temperature range around $\rm \sim 10^{4.5}$ K where the isobaric cooling time is the shortest. In general, turbulence and mixing, instead of radiative cooling, may dominate the transition of gas between different temperature phases.

astro-ph.GA↗

arXiv2020

Growth and structure of multiphase gas in the cloud-crushing problem with cooling

We revisit the problem of the growth of dense/cold gas in the cloud-crushing setup with radiative cooling. The relative motion between the dense cloud and the diffuse medium produces a turbulent boundary layer of mixed gas with a short cooling time. This mixed gas may explain the ubiquity of the range of absorption/emission lines observed in various sources such as the circumgalactic medium and galactic/stellar/AGN outflows. Recently Gronke & Oh showed that the efficient radiative cooling of the mixed gas can lead to the continuous growth of the dense cloud. They presented a threshold cloud size for the growth of dense gas which was contradicted by the more recent works of Li et al. & Sparre et al. These thresholds are qualitatively different as the former is based on the cooling time of the mixed gas whereas the latter is based on the cooling time of the hot gas. Our simulations agree with the threshold based on the cooling time of the mixed gas. We argue that the radiative cloud-crushing simulations should be run long enough to allow for the late-time growth of the dense gas due to cooling of the mixed gas but not so long that the background gas cools catastrophically. Moreover, the simulation domain should be large enough that the mixed gas is not lost through the boundaries. While the mixing layer is roughly isobaric, the emissivity of the gas at different temperatures is fundamentally different from an isobaric single-phase steady cooling flow.

astro-ph.GA↗

arXiv2020

On modelling CC85 wind in an expanding local box

The Chevalier & Clegg (1985) spherical wind is widely used to model galactic outflows. Efforts are underway to understand the production of multiphase gas in such outflows. Since the important scales for the multiphase gas are very small compared to the size of the wind, it is very useful to have a local model of the cold gas interacting with a volume-filling expanding wind. In this note we explicitly derive the governing hydrodynamic equations in such a local box for the first time. We highlight that the expansion in a wind occurs only in the transverse direction, and this results in anisotropic source terms in the momentum equation. Our equations will be very useful in realistic modelling of the small scale structure of multiphase galactic outflows. [original note does not include an abstract]

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arXiv2018

Mean Occupation Function of High Redshift Quasars from the Planck Cluster Catalog

We characterise the distribution of quasars within dark matter halos using a direct measurement technique for the first time at redshifts as high as $z \sim 1$. Using the Planck Sunyaev-Zeldovich (SZ) catalogue for galaxy groups and the Sloan Digital Sky Survey (SDSS) DR12 quasar dataset, we assign host clusters/groups to the quasars and make a measurement of the mean number of quasars within dark matter halos as a function of halo mass. We find that a simple power-law fit of $\log\left = (2.11 \pm 0.01) \log (M) -(32.77 \pm 0.11)$ can be used to model the quasar fraction in dark matter halos. This suggests that the quasar fraction increases monotonically as a function of halo mass even to redshifts as high as $z\sim 1$.

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