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 } \sim 29.5 mag arcsec ^ { -2 } , and has revealed the stellar structure of M101 ’ s disk out to nearly 25′ ( 50 kpc ) , three times our measured R _ { 25 } 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 \sim 0.2 ) , suggesting it is the somewhat more evolved ( few hundred Myr to \sim 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 H \alpha 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 .