My primary research focus is on the physics of relativistic cosmic rays, their transport and how they affect galaxies and galaxy clusters. While cosmic rays comprise a small fraction of the total number of particles in galaxies, they are energetically important and can significantly affect galaxy evolution by driving galactic winds and heating diffuse gas. The impact that cosmic rays have is a strong function of how they are transported in the galaxy, which remains uncertain. As a result, how cosmic rays affect galaxies is one of the biggest unsolved puzzles in galaxy evolution. In my research, I use a combination of analytical and numerical work to study the physics of cosmic ray transport and how plausible cosmic ray transport models affect galactic halos and galaxy clusters.
Below is a brief summary of finished and almost finished research projects I have been working on.
Cosmic ray (CR) transport in galaxies remains uncertain. However, we can use detailed measurements of CR spectra in the Milky Way to put constraints on the transport. In this work, we tried to reconcile CR transport theory with local CR data. We considered two leading theories of CR transport: scattering of CRs by a turbulent cascade of MHD fast modes, which results in diffusion, and scattering of CRs by self-excited waves, which results in streaming. We demonstrated that CR transport is likely multi-phase and that a multi-phase combination of the two transport mechanisms can in principle explain the data, although significant fine tuning of ISM conditions is required. We also discuss significant uncertainties in the physics of the MHD fast mode cascade, which are usually not taken into account in the CR literature. In particular, we argue that the weak cascade of fast modes is suppressed by wave steepening even in subsonic turbulence. This raises the possibility that fast modes are not important for scattering CRs. These issues suggest that there likely is a significant gap in our understanding of MHD turbulence and how it affects CR transport. The paper can be found here.
Active Galactic Nuclei (AGN) and the gigantic radio bubbles inflated by their jets are believed to provide the energy required to keep intra-cluster medium (ICM) gas in approximate thermal balance despite significant cooling losses. However, how this energy is transported and thermalized throughout the ICM remains an open question. It is plausible that the buoyantly rising radio bubbles launch weak shocks/sound waves, excite turbulence and/or inject CRs into the ICM, which heat the gas. These feedback channels all have some observational support, but they are usually considered separately in theoretical models. Remarkably, we showed that the presence of streaming CRs in the ICM excites macroscopic sound waves, suggesting that CRs may be partially responsible for the presence of waves, shocks and turbulence in cluster cores (the paper can be found here). CR streaming in the ICM also drives a new type of buoyancy instability, by rendering a pressure-balanced mode highly compressible at short wavelengths (Kempski, Quataert & Squire, in prep). Preliminary numerical work on the saturation of the CR-driven acoustic instability suggests that it evolves into a series of weak shocks (Kempski et al. in prep.), which is broadly consistent with the acoustic disturbances observed in the core of the Perseus cluster.
By exciting Alfven waves which are subsequently damped, cosmic rays can heat the thermal gas. This heating may play an important role counteracting cooling and maintaining thermal balance in galaxy halos and clusters. In this work, we demonstrated the importance of CR heating in galactic halos and looked at how it affects local thermal instability. Thermal instability is a commonly invoked mechanism for the observed presence of cold gas in the circum-galactic medium (CGM) of galaxies and the cores of galaxy clusters. Interestingly, we showed that CR heating does not significantly affect thermal instability growth rates, even for energetically important CRs. Instead, the heating turns pure growth into oscillatory growth (overstability), but without affecting the growth rate itself. CRs do affect thermal instability growth rates for large CR pressures, although not via heating, but by providing nonthermal pressure support which allows the gas to cool isochorically rather than isobarically, thus reducing the density contrast between the cool and hot phases. Our paper can be found here. The surprisingly small impact of CR heating on the growth of thermal instability was later confirmed numerically by Butsky et al. (2020).
In radiatively inefficient accretion flows onto supermassive black holes (SMBHs), which includes the SMBHs targeted by the Event Horizon Telescope, a large fraction of gravitational energy is converted into thermal energy. This results in a very hot and dilute plasma, in which the collisional mean free path can exceed the system size by orders of magnitude. Simulations with low-collisionality physics are therefore needed to understand the accretion flow in such systems. In this work, we performed local shearing-box simulations using the weakly-collisional plasma regime (so called Braginskii MHD) to study the turbulence driven by the magneto-rotational instability (MRI) in low-collisionality systems and the resulting angular-momentum transport. Remarkably, while large anisotropic-pressure (anisotropic viscous) forces in Braginskii MHD significantly change the structure of MRI-driven turbulence and are an important dissipation channel, there is no significant effect on the overall angular-momentum transport. The paper can be found here.