Ultraviolet Signatures of Tidal Interaction in the Giant Spiral Galaxy M101

We present new evidence for the occurrence of tidal interactions in the disk of M101 during the last 10⁸-10⁹ yr. Recent imaging of the far-ultraviolet emission from M101 by the shuttle-borne Ultraviolet Imaging Telescope (UIT) reveals with unprecedented clarity a disk-wide pattern of multiple linear arm segments (``crooked arms''). The deep far-ultraviolet (FUV) image also shows a faint outer spiral arm with a (``curly tail'') feature that appears to loop around the supergiant H II region NGC 5471, linking this outlying starburst with the rest of the galaxy. These FUV-bright features most likely trace hot O- and B-type stars along with scattered light from associated nebular dust. Counterparts of the outermost ``crooked arms'' are evident in maps at visible wavelengths and in the 21 cm line of H I. The inner-disk FUV arms are most closely associated with Hα knots and the outer (downstream) sides of CO arms. Comparisons of the ``crooked arm'' and ``curly tail'' morphologies with dynamical simulations yield the greatest similitude, when the nonaxisymmetric forcing comes from a combination of external interactions with one or more companion galaxies and 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 M101 and a smaller galaxy.