HUBBLE-SPACE-TELESCOPE IMAGING OF DUST LANES AND COMETARY STRUCTURES IN THE INNER DISK OF THE CARTWHEEL RING GALAXY

We have obtained high-resolution B and I-band Hubble Space Telescope ( HST) images of the Cartwheel ring galaxy which provide unprecedented v iews of the structure and color of the two rings and spokes, Here, we focus on the imaging results for the central regions of the Cartwheel, its inner ring and spokes. Prior to these observations there was surp risingly little evidence for gas and star formation within the inner r ing. The images clearly resolve a network of obscuring dust lanes in t his region, including a prominent dust circle that defines the inner e dge of the inner ring. There is also evidence for luminous, kiloparsec -sized, cometary structures in the inner ring which are suggestive of massive dense clouds traveling supersonically through the ambient gas. These cometary structures are bounded by sharp dust lanes, and when c orrected for the light from the inner ring are quite blue (0.4<B-I<0.6 ). We suggest the following possible explanations for these structures : (1) that they are the supersonic orbit crossings of the inner ring b y massive clouds or cloud complexes radially mixed in the disk as a re sult of the collision, (2) that they are formed from dense clouds rain ing down on the disk from a bridge which connects the Cartwheel to the intruder galaxy, and (3) the minor-image theory that they are disk cl ouds traversing a tenuous high-velocity accretion stream. If any of th ese explanations is correct, then the inner ring of the Cartwheel appe ars to be a very good site for the study of supersonic collisions betw een massive clouds and large-scale density waves. (C) 1996 American As tronomical Society.