Red Supergiants in the Disk of M81: Tracing the Spatial Distribution of Star Formation 25 Myr in the PastThis publication makes use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center at the California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

Near-infrared images obtained with the CFHTIR imager on the Canada-France-Hawaii Telescope 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 that peaks near MK= -11.5; RSGs with this peak brightness are seen throughout all fields that were studied, indicating that star formation occurred 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. The number density of bright RSGs is similar in three of four fields, indicating that the bright RSGs tend to be well mixed with the older stellar populations that dominate the integrated light in the Kband. However, the density of bright RSGs in a northern disk field is ?2 times higher than average, suggesting that the star formation rate (SFR) in this part of the disk was higher than average 10-25 Myr in the past. The northern disk field contains areas of ongoing star formation, and it is suggested that this is a region of prolonged star-forming activity. The number density of RSGs that formed during the past 10-25 Myr at galactocentric distances between ?4 and 7 kpc is also comparable to that which formed between ?7 and 10 kpc. We conclude that star-forming activity in M81 during the past 10-25 Myr was (1) 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 M?yr?1, which is not greatly different from the present-day SFR estimated from H? and far-UV emission.