Astro-particle Phenomena from Dark Matter and Cosmic Rays in MHD Galaxy Formation Simulations

Abstract

Over the last few decades, observations of diffuse gamma-ray emission in the Milky Way– in particular, the excess of GeV gamma-rays detected in the Milky Way’s galactic center, and the massive gamma-ray bubbles (the “Fermi bubbles”) centered about the Milky Way’s disk– have challenged astrophysical models. Nearly all past studies of galactic gamma-ray emission make simplifying assumptions about cosmic ray (CR) propagation that may not be valid (e.g., steady-state equilibrium), but recent numerical breakthroughs have enabled fully time-dependent dynamical evolution of CRs in magnetohydrodynamic (MHD) simulations with resolved, multi-phase small-scale structure in the interstellar medium (ISM), allowing self-consistent comparisons to the Milky Way observations. In this talk, I will present new work in which we model diffuse gamma-ray emission in simulations of Milky Way-mass galaxies with fully-resolved, multi-species CR spectra. We find that the gamma-ray spectrum in the galactic center can fluctuate by up to an order of magnitude on million-year timescales due to highly variable star formation and losses from variable structure in the turbulent ISM, with some fluctuations consistent with the Fermi-LAT galactic center excess. I will also show that Fermi bubble-like features arise from stellar feedback in these simulations. Finally, I will present the first results from a new suite of cosmological simulations in which a dark sector with an ultra-light mediator gives rise to a long-range (kiloparsec-scale) self-interaction. The addition of a long-range dark matter self-interaction has dramatic effects on the formation of galaxies and their host halos, and will be testable by current and upcoming astronomical surveys.