Could the Compact Radio Sources in M82 be Cluster Wind–driven Bubbles?

Abstract

The compact non-thermal sources in M82 and other starburst galaxies are generally thought to be supernova remnants (SNRs). We consider an alternative hypothesis that most are wind driven bubbles (WDBs) associated with very young super star clusters (SSCs). In this scenario, the synchrotron emitting particles are produced at the site of the shock transition between the cluster wind and the hot bubble gas. The particles radiate in the strong magnetic field produced in the expanding shell of shocked ambient interstellar gas. One of the motivations for this hypothesis is the lack of observed time variability in most of the sources, implying ages greater than expected for SNRs, but comfortably within the range for WDBs. In addition, as SNRs, these sources are not effective in driving the starburst mass outflow associated with the nuclear region of M82, thus requiring a separate mechanism for coupling SN energy to this outflow. The WDB hypothesis is found to be feasible for underlying clusters in the mass range ~2x10^(4+/-1)Msun, and ambient gas densities in the range ~3x10^(3+/-1)cm^-3. The ages of the bubbles are between several x10^3 and several x10^4 years. Since the SNR picture cannont be ruled out, we provide suggestions for specific observational tests which could confirm or rule out the WDB hypothesis. Finally, we discuss the WDB hypothesis in the context of broader phenomena in M82, such as the rate of star formation and starburst outflows, and the possible interpretation of supershells in M82 as the products of multiple supernovae in young SSCs.