Institute for Advanced Study Informal Astrophysics Seminar

ABSTRACT: The cosmic rays (CRs) in star-forming galaxies dominate their emission at gamma-ray and radio wavelengths. The observed linear correlation between the nonthermal radio emission and the thermal infrared emission of galaxies, the far infrared (FIR)-radio correlation (FRC), links together the CR electron population, star-formation rate, and magnetic field strength of galaxies. Furthermore, gamma-ray data links the CR proton population, the star-formation rate, and gas density. We construct one-zone steady-state models of cosmic ray (CR) spectra in star-forming galaxies ranging from normal galaxies to the densest starbursts, calculating both the radio and gamma-ray emission. We then calculate the broadband emission of primary and secondary CR protons, electrons, and positrons. We find the FRC is caused by conspiracies of several factors for galaxies across the range of the correlation, including CR escape from galaxies, UV opacity, non-synchrotron cooling, and secondary electrons and positrons generated by CR protons. The conspiracies have great implications for the evolution of the FRC at high z, actually preserving it to higher redshift than previously thought but allowing variations in the FIR-radio ratio with different galaxy properties. I describe how the recent gamma-ray observations of M82 and NGC 253 compare with our models. These starbursts are somewhat less gamma-ray bright than we expect, but still indicate substantial pionic losses for CR protons and non-synchrotron cooling for CR electrons and positrons, supporting the conspiracy. Finally, I will describe our more recent work on the highest energy CR electrons in starbursts and the gamma-rays they produce. Starburst galaxies ought to be opaque to 30 TeV gamma-rays through pair production; in the strong magnetic fields of starbursts, these created electrons and positrons radiate synchrotron X-rays. We find that these synchrotron X-rays could make up ~10% of the diffuse hard X-ray emission from M82-like starbursts and even more in the brightest starbursts like Arp 220.