Winds, Clumps, and Interacting Cosmic Rays in M82

TL;DR

This paper models cosmic ray evolution in M82, comparing predictions to observations to test the galaxy's calorimeter assumption and derive key physical parameters like magnetic field strength and wind speed.

Contribution

It introduces a detailed multiphase interstellar medium model for M82 and constrains cosmic ray parameters through spectral fitting, advancing understanding of cosmic ray behavior in starburst galaxies.

Findings

M82 has a magnetic field of about 250 μG.

The galaxy is a good electron calorimeter but not a perfect proton calorimeter.

Best-fit wind advection speed is between 300-700 km/s.

Abstract

We construct a family of models for the evolution of energetic particles in the starburst galaxy M82 and compare them to observations to test the calorimeter assumption that all cosmic ray energy is radiated in the starburst region. Assuming constant cosmic ray acceleration efficiency with Milky Way parameters, we calculate the cosmic-ray proton and primary and secondary electron/positron populations as a function of energy. Cosmic rays are injected with Galactic energy distributions and electron-to-proton ratio via type II supernovae at the observed rate of 0.07/yr. From the cosmic ray spectra, we predict the radio synchrotron and \gamma-ray spectra. To more accurately model the radio spectrum, we incorporate a multiphase interstellar medium in the starburst region of M82. Our model interstellar medium is highly fragmented with compact dense molecular clouds and dense photoionized gas, both embedded in a hot, low density medium in overall pressure equilibrium. The spectra predicted by this one-zone model are compared to the observed radio and \gamma-ray spectra of M82. Chi-squared tests are used with radio and \gamma-ray observations and a range of model predictions to find the best-fit parameters. The best-fit model yields constraints on key parameters in the starburst zone of M82, including a magnetic field strength of ~250 \mu G and a wind advection speed in the range of 300-700 km/s. We find that M82 is a good electron calorimeter but not an ideal cosmic-ray proton calorimeter and discuss the implications of our results for the astrophysics of the far infrared-radio correlation in starburst galaxies.