THE EXTENDED OPTICAL DISK OF M101

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 6 deg super(2), our imaging has a limiting surface brightness of [mu] sub(B) ~ 29.5 mag arcsec super(-2), and has revealed the stellar structure of M101's disk out to nearly 25' (50 kpc), 3 times our measured R sub(25) isophotal size of the optical disk. 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. 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. Our data suggest an ongoing buildup of M101's outer disk due to encounters in the group environment triggering extended star formation and tidal heating of existing disk populations.