Probing Cosmic-ray Physics in Starburst Galaxies With Future High-Energy Missions

Cosmic ray feedback on a galaxy's interstellar medium (ISM) can substantially influence the galaxy's formation and evolution. Cosmic rays ionize and heat the ISM, regulating star formation. Cosmic-ray transport can also result in galactic winds that will drive material out of galaxy disks and enrich the intergalactic medium. In order to assess the influence of cosmic ray feedback (relative to other types of feedback) in driving the conditions for galaxy formation and evolution, we need to better understand the complex interplay among cosmic rays, galaxy magnetic fields, and the ISM (the CR-B-ISM relationship) in a variety of interstellar environments. The sole means for probing cosmic-ray physics in other galaxies is through the byproducts of their interactions with gas and radiation in their environments, including the broadband non-thermal diffuse spectrum observable from hard X-rays to TeV gamma rays. Observations of nearby galaxies by Fermi in the GeV band and Imaging Atmospheric Cherenkov Telescopes in the TeV band have resulted in the first detections of non-AGN galaxies outside of the Milky Way. Recent NuSTAR observations of starburst galaxies such as NGC 253 and M82 enable the most sensitive search to date for their diffuse inverse Compton emission in the hard X-ray band (10-30 keV). Such extremely deep observations are made with very long exposures and are currently only available for very nearby galaxies; however, they provide a glimpse of what we may expect from next-generation facilities. In this talk, I will discuss the latest results from high-energy observations of nearby star-forming galaxies, including the connection between the gamma-ray, radio, and far-infrared emission. I will discuss the implications of these results for these galaxies' cosmic ray populations. Finally, I will set the stage for next generation high-energy facilities, including (but not limited to) HEX-P, FORCE, AMEGO, CTA, and IceCube-Gen2.