The Extended Optical Disk of M101

Abstract: We have used deep, wide-field optical imaging to study the faint outskirts of the luminous spiral galaxy M101 (NGC 5457), as well as its surrounding environment. Over six square degrees, our imaging has a limiting surface brightness of mu_B ~ 29.5 mag/arcsec^2, and has revealed the stellar structure of M101's disk out to nearly 25 arcminutes (50 kpc), three times our measured R25 isophotal size of the optical disk. At these radii, the well-known asymmetry of the inner disk slews 180 degrees, resulting in an asymmetric plume of light at large radius which follows the very extended HI disk to the northeast of M101. This plume has very blue colors (B-V ~ 0.2), suggesting it is the somewhat more evolved (few hundred Myr to ~ 1 Gyr) counterpart of the young far ultraviolet emitting population traced by GALEX imaging. We also detect another, redder spur of extended light to the east of the disk, and both structures are reminiscent of features produced during fly-by galaxy interactions. However, we see no evidence of very extended tidal tails around M101 or any of its companions which might be expected from a recent encounter with a massive companion. We consider the properties of M101's outer disk in light of possible past interactions with the nearby companion galaxies NGC 5477 and NGC 5474. The detection of optical starlight at such large radii gives us the ability to study star formation histories and stellar populations in outer disks over a longer timescales than those traced by the UV or Halpha emitting populations. Our data suggest ongoing buildup of the M101's outer disk due to encounters in the group environment triggering extended star formation and tidal heating of existing disk populations.