Ultraviolet Signatures of Tidal Interaction in the Giant Spiral Galaxy M101

We present new evidence for tidal interactions having occurred in the disk ofM101 in the last 10^8 - 10^9 years. Recent imaging of the far-ultravioletemission from M101 by the Shuttle-borne Ultraviolet Imaging Telescope (UIT)reveals with unprecedented clarity a disk-wide pattern of multiple linear armsegments (``crooked arms''). The deep FUV image also shows a faint outer spiralarm with a (``curly tail'') feature that appears to loop around the supergiantHII region NGC 5471 - linking this outlying starburst with the rest of thegalaxy. These FUV-bright features most likely trace hot O & B-type stars alongwith scattered light from associated nebular dust. Counterparts of theoutermost ``crooked arms'' are evident in maps at visible wavelengths and inthe 21-cm line of HI. The inner-disk FUV arms are most closely associated withH$\alpha$ knots and the outer (downstream) sides of CO arms. Comparisons of the``crooked arm'' and ``curly tail'' morphologies with dynamical simulationsyield the greatest similitude, when the non- axisymmetric forcing comes from acombination of ``external interactions'' with one or more companion galaxiesand ``internal perturbations'' from massive objects orbiting within the disk.We speculate that NGC 5471 represents one of these ``massive disturbers''within the disk, whose formation followed from a tidal interaction between M101and a smaller galaxy.Comment: Paper format (latex); length of paper (8); 4 gif figure files; uses aas2pp4.sty AASTeX macro file; to be published in Part I of the Astrophysical Journa