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 views 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 surprisingly litHe evidence for gas and star formation within the inner ring. The images clearly resolve a network of obscuring dust lanes in this region, including a prominent dust circle that defines the inner edge 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 corrected 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 by massive clouds or cloud complexes radially mixed in the disk as a result of the collision, (2) that they are formed from dense clouds raining down on the disk from a bridge which connects the Cartwheel to the intruder galaxy, and (3) the mirror-image theory that they are disk clouds traversing a tenuous high-velocity accretion stream. If any of these explanations is correct, then the inner ring of the Cartwheel appears to be a very good site for the study of supersonic collisions between massive clouds and large-scale density waves.