Red Supergiants in the Disk of M81: Tracing the Spatial Distribution of Star Formation 25 Million Years in the Past

Abstract

Near-infrared images are used to investigate the brightest red stars in the disk of the nearby spiral galaxy M81. Red supergiants (RSGs) form a well-defined sequence on the color-magnitude diagrams (CMDs) that peaks near M_K = -11.5; RSGs with this peak brightness are seen throughout all fields that were studied, indicating that star formation occured over a large part of the M81 disk only ~ 10 Myr in the past. The number of RSGs per unit integrated K-band light is compared at various locations in the disk. We conclude that star-forming activity in M81 during the past 10 - 25 Myr (1) was distributed over a larger fraction of the disk than it is at the present day, and (2) was not restricted to a given radial interval, but was distributed in a manner that closely followed the stellar mass profile. Star counts indicate that the mean SFR of M81 between 10 and 25 Myr in the past was ~ 0.1 solar masses per year, which is not greatly different from the present day SFR estimated from Halpha and FUV emission.