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 ( 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 \sim 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 K - band .
However , the density of bright RSGs in a northern disk field is \sim 2 \times higher than average , suggesting that the SFR in this part of the disk was higher than average 10 - 25 Myr in the past .
The northern disk field contains areas of on-going star formation , and it is suggested that it 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 \sim 4 and 7 kpc is also comparable with that which formed between \sim 7 and 10 kpc .
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 \sim 0.1 M _ { \odot } year ^ { -1 } , which is not greatly different from the present day SFR estimated from H \alpha and FUV emission .