2008AJ....136.2502D

Deep images obtained with MegaCam and WIRCam on the Canada-France-Hawaii Telescope are used to probe the stellar content outside of the central star-forming regions of M82. Stars evolving on the asymptotic giant branch (AGB) are traced along the major axis out to projected distances of 12 kpc, which corresponds to 13 disk scale lengths. The numbers of red supergiants (RSGs) and AGB stars normalized to local surface brightness (the "specific frequency" or SF) is constant when R_GC> 4 kpc, indicating that RSGs and AGB stars are well mixed throughout the disk. Moreover, the SF of bright AGB stars in the outer disks of M82 and the Sc galaxy NGC 2403 are identical, suggesting that the specific star-formation rates (SFRs) in these galaxies during intermediate epochs were similar. This similarity in stellar content, coupled with the presence of an extended stellar disk, is consistent with M82 having been a late-type disk galaxy prior to interacting with M81. Still, there is a paucity of RSGs in the outer disk of M82 when compared with NGC 2403, indicating that the SFR in the outer regions of M82 during the past ∼0.1 Gyr has declined dramatically with respect to that in isolated late-type galaxies. The stellar content of the M82 disk plane is also investigated. A mixture of bright main-sequence stars, RSGs, and AGB stars is detected out to minor-axis distances of 7 kpc. These stars, which span a range of ages, are concentrated along the outflow. The brightest extraplanar AGB stars define a system with an exponential scale height of 1.8±0.2 kpc, as measured along the minor axis. It is suggested that the young- and intermediate-aged stars in the extraplanar regions formed in structures similar to M82 South, and that these were subsequently disrupted by the tidal action of M82.