GRAVITATIONAL INTERACTIONS IN POOR GALAXY GROUPS

We report the results of the spatial analysis of deep ROSAT HRI observ ations, optical imaging and spectroscopy, and high-resolution VLA H I and continuum imaging of NGC 1961 and NGC 2276. These spirals were sel ected as showing some previous evidence for interaction with a surroun ding (hot) diffuse medium. Our results favor most aspects of these gal axies as being shaped by gravitational interactions with companions, r ather than the asymmetric pressure from motion through an external med ium. The old stars follow the asymmetric structures of young stars and ionized gas, which suggests a tidal origin for the lopsided appearanc e of these galaxies. In NGC 2276, the HI and star-forming regions are strongly concentrated along the western edge of the disk. In this case , the ROSAT HRI detects the brightest star-forming regions as well as the diffuse disk emission, the most distant galaxy with such a detecti on. An asymmetric ionization gradient in the H II regions suggests rad ial movement of gas, which might have occurred in either tidal or wind scenarios. The X-ray emission from NGC 1961 is dominated by a point s ource near the nucleus of the galaxy but extended emission is seen out to a radius of similar to 0.'8. Previous studies of the enrichment of the intragroup medium in the NGC 2300 group indicates that stripping may be important in this system, but the density of the IGM is much to o tenuous to effectively strip the gas from the galaxy. However, we pr opose that gravitational interactions in the group environment may enh ance stripping. During a gravitational encounter the disk of the spira l galaxy may be warped, making ram pressure stripping more efficient t han in a quiescent disk. (C) 1997 American Astronomical Society.