Gravitational Interactions in Poor Galaxy Groups

Abstract

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 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 detection. An asymmetric ionization gradient in the H II regions suggests radial 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 source near the nucleus of the galaxy but extended emission is seen out to a radius of ~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 too tenuous to effectively strip the gas from the galaxy. However, we propose that gravitational interactions in the group environment may enhance stripping. During a gravitational encounter the disk of the spiral galaxy may be warped, making ram pressure stripping more efficient than in a quiescent disk.