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D.M. Elmegreen (Vassar), M. Kaufman (Ohio State), B.G. Elmegreen (IBM), E. Brinks (U. Guanajuato MX), C. Struck (Iowa State), M. Thomasson (Onsala Obs.), M. Klaric (Columbia, SC)
IC 2163 has a central eye-shaped ("ocular") structure with two long tidal arms indicative of a very recent, close, tidal encounter. It is partially obscured by its spiral companion NGC 2207. Numerical models (Elmegreen et al. 1995, ApJ, 453,139) suggest that perigalacticon occurred 40 Myr ago. We use UBVI images taken with the WFPC2 camera of HST to study the star-forming regions and to make a direct measurement of extinction through the disk of NGC 2207. We find no evidence for super star clusters (SSCs) and no difference between the star-forming regions in the fast-streaming gas on the compressed rim of the ocular oval and those elsewhere in the system. The star-forming regions have luminosity functions typical of intermediate and late-type spiral galaxies, with ages of a few times 106 to a few times 107 years and masses of 103 M\odot to 106 M\odot. Either the 40 Myr since perigalacticon is too small for SSCs to have formed yet or the interaction was too weak to produce them.
For the part of NGC 2207 that overlaps IC 2163 along the line of sight, AV \approx1.1 mag for the dust lanes and AV \approx0.4 mag for the inter-lane disk of NGC 2207. Our values for the extinction of NGC 2207 in B, V, and I are closer to a Whitford reddening curve than Berlind et al. (1997, AJ, 114, 107) derived from ground-based measurements because of our higher resolution images. This result supports their interpretation that extinction becomes grey when patchy dust is unresolved.
Support for this work was provided by NASA through grant number GO-06483-95A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.