ULTRAVIOLET SIGNATURES OF TIDAL INTERACTION IN THE GIANT SPIRAL GALAXY M101

We present new evidence for the occurrence of tidal interactions in th e disk of M101 during the last 10(8)-10(9) yr. Recent imaging of the f ar-ultraviolet emission from M101 by the shuttle-borne Ultraviolet Ima ging Telescope (UIT) reveals with unprecedented clarity a disk-wide pa ttern 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. Counter parts 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 Ha knots and the outer (downstream) side s of CO arms. Comparisons of the ''crooked arm'' and ''curly tail'' mo rphologies with dynamical simulations yield the greatest similitude, w hen the nonaxisymmetric forcing comes from a combination of external i nteractions with one or more companion galaxies and internal perturbat ions from massive objects orbiting within the disk. We speculate that NGC 5471 represents one of these ''massive disturbers'' within the dis k, whose formation followed from a tidal interaction between M101 and a smaller galaxy.