Gravitational Interactions in Poor Galaxy Groups

We report the results of the spatial analysis of deep ROSAT HRI observations, optical imaging and spectroscopy, and high–resolution VLA H I and continuum imaging of NGC 1961 and NGC 2276. These spirals were selected as showing some previous evidence for interaction with a surrounding (hot) diffuse medium. Our results favor most aspects of these galaxies as being shaped by gravitational interactions with companions, rather than the asymmetric pressure from motion through an external medium. The old stars follow the asymmetric structures of young stars and ionized gas, which suggests a tidal origin for the lopsided appearance of these galaxies. In NGC 2276, the H I and star-forming regions are strongly concentrated along Present address: MIT Center for Space Research, Building 37-662B, Cambridge, MA 02139-4307 Visiting astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by AURA, Inc., under cooperative agreement with the National Science Foundation.