Rajsekhar Mohapatra

Research

Postdoctoral research · 2022–ongoing

Type Ia supernovae in massive ellipticals

Local patch simulations

We modeled small portions of the interstellar medium (ISM) of an elliptical galaxy using AthenaK, and seeded type Ia supernovae (SNIa) that both heat the medium and drive turbulence in the ISM. We studied whether the SNIa can keep the ISM in a steady state, provided the heating balances the cooling. In our local patch simulations, we found the system always overheats once the cooling drops below the heating. I subsequently extended this work to global simulations of an isolated massive elliptical galaxy, described below.

Global simulations

Using 3D hydrodynamic simulations, we studied the central regions of massive galaxies, focusing on NGC 1399, including radiative cooling and mass and energy injection from AGB winds and SNIa, but excluding black hole feedback. Our simulations matched observed density and entropy profiles and revealed two steady-state profiles over 100 Myr: (i) a cooling flow if SNIa heating is less than cooling, and (ii) a slow subsonic outflow of AGB ejecta if SNIa heating is comparable to or exceeds cooling. This outflow, pressure-confined by the circumgalactic medium (CGM), adapts the ISM to CGM properties, influencing black hole accretion rates. We also showed that approximate methods of modeling heating due to type Ia supernovae, such as clustered supernovae and smooth heat distribution, fail to reproduce the observed density and entropy profiles of the ISM.

Stratified multiphase turbulence

We modeled a patch of the intracluster medium (ICM) using FLASH, driving turbulence and including radiative cooling. We derived a new condensation criterion for cold gas to condense out of the hot ICM, which takes into account the effects of both turbulent mixing and buoyant motions that prevent the formation of cold gas.

Publications

Turbulent radiative mixing layers

I study how hot and cold gas mix at their boundaries, creating intermediate-temperature material that can cool rapidly. With Lachlan Lancaster, Drummond Fielding and Greg Bryan, I have investigated how the structure of these interfaces controls their cooling. Our hydrodynamic studies show that apparently converged cooling rates can conceal compensating numerical effects. We also find that rapid cooling drives inflows that reduce the turbulent folding of the interface, changing how its cooling rate scales.

In my new submitted paper with Lachlan Lancaster, Drummond Fielding, Eliot Quataert and Greg Bryan, I investigate how magnetic fields change this picture. Using AthenaK simulations with over four billion cells, I find that even initially weak fields can strongly suppress mixing and cooling by reducing turbulent motions and the folding of gas interfaces. The strength of this suppression depends on magnetic geometry and remains resolution-dependent. These results motivate physically controlled calculations of transport across magnetized interfaces, with implications for cold-gas survival in galactic winds and cluster atmospheres. Watch the simulation movies →

Publications

Cold gas in circumgalactic cloud complexes

With Alankar Dutta and Prateek Sharma, I use high-resolution magnetohydrodynamic simulations to investigate cold gas embedded in turbulent galactic haloes. I study how cooling and mixing compete to determine the abundance and lifetime of cold structures, and how magnetic fields affect their evolution. Watch the simulation movies →

Publications

Turbulence driving mechanisms

In collaboration with Tejahni Desire and Chang-Goo Kim (Princeton), we contrast two ways of driving turbulence: idealised Fourier-space forcing and discrete, supernova-like point-source momentum injection, and examine how each imprints itself on turbulence statistics. We show that the turbulence driving parameter b, commonly used to infer the compressive or solenoidal nature of driving from observations, is degenerate with both the spatial locality and temporal correlation of the driving. This degeneracy complicates its use for distinguishing driving mechanisms in the ISM.

Publications

PhD thesis · 2018–2022

Multiphase turbulence

We performed local idealised simulations of gas in the intracluster medium using FLASH. We studied the effect of turbulence driving on gas properties such as morphology and density and temperature distribution functions. We also studied the different sources and sinks of enstrophy, i.e. solenoidal motions, as well as the kinematics of hot (~107 K) X-ray emitting and cold (~104 K) Hα emitting gas.

In an earlier study, we studied the effect of varying levels of turbulent feedback on these different statistical properties of gas. We also looked at the modes of perturbation of the gas, and the interactions and transitions between different temperature phases, the effect of magnetic fields, and two different idealised heating models. We find that a weakly turbulent feedback scenario best explains the observations of the multiphase ICM. The simulation movies are available as a YouTube playlist.

In a companion study, we studied the velocity structure functions (VSFs) in these high-resolution multiphase turbulence simulations, which are important in interpreting measurements of the kinematics of Hα filaments in cool core clusters. We looked at VSFs of hot and cold temperature phases separately, and studied the effect of our two idealised heating models (mass-weighted vs volume-weighted) and magnetic fields. We also discussed the effect of projection along the line of sight on the VSFs of hot and cold phase gas, and compared them with recent observations.

Publications

Stratified turbulence

We studied the effect of stratification on turbulence and its implications for the ICM. We derived a new scaling relation between the amplitude of density fluctuations, the degree of stratification (denoted by the Froude number), and the rms Mach number of the flow. We found the pressure fluctuations to be independent of the degree of stratification. In observations of galaxy clusters, the density and pressure variations show up as fluctuations in the X-ray surface brightness and the thermal Sunyaev–Zel'dovich effect, respectively. Simulation movies are available on the stratified turbulence movies page.

Publications

Bachelor thesis · 2017–2018

Turbulent heating in cool cluster cores

We used PLUTO to simulate gas in the intracluster medium. We studied the importance of turbulent heating in achieving thermal balance in cool cluster cores. Simulation movies are available on the bachelor thesis movies page and also as a YouTube playlist.

Publications