And the Rest: The Stellar Archeological Record of M82 Outside the Central Starburst

TL;DR

Deep imaging reveals the extended stellar populations of M82, showing its past as a late-type disk galaxy and evidence of recent star formation decline and tidal disruption effects.

Contribution

This study provides detailed analysis of M82's outer stellar content using deep imaging, highlighting its disk history and star formation changes post-interaction.

Findings

AGB star frequency in M82's outer disk matches NGC 2403

Outer disk shows reduced recent star formation compared to isolated galaxies

Detection of young stars in extraplanar regions suggests tidal disruption of star-forming structures

Abstract

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 specific frequency (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 (SFR) 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 red supergiants (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. A mixture of bright main sequence stars, RSGs, and AGB stars are detected out to minor axis distances of 7 kpc along the outflow. 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.