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.