How cosmic rays mediate the evolution of the interstellar medium

Abstract

We explore the impact of diffusive cosmic rays (CRs) on the evolution of the interstellar medium (ISM) under varying assumptions of supernova explosion environment. In practice, we systematically vary the relative fractions of supernovae (SN) occurring in star-forming high-density gas and those occurring in random locations decoupled from star-forming gas to account for SN from run-away stars or explosions in regions that have been cleared by prior SN, stellar winds, or radiation. We find that in the simple system of a periodic stratified gas layer the ISM structure will evolve to one of two solutions: a ‘peak driving’ state where warm gas is volume filling or a ‘thermal runaway’ state where hot gas is volume filling. CR pressure and transport are important factors that strongly influence the solution state the ISM reaches and have the ability to flip the ISM between solutions. Observable signatures such as gamma-ray emission and H I gas are explored. We find that gamma-ray luminosity from pion decay is largely consistent with observations for a range of model parameters. The thickness of the H I gas layer may be too compact, however, this may be due to a large cold neutral fraction of mid-plane gas. The volume fraction of hot gas evolves to stable states in both solutions, but neither settles to a Milky Way-like configuration, suggesting that additional physics omitted here (e.g. a cosmological circumgalactic medium, radiation transport, or spectrally resolved and spatially varying CR transport) may be required.