A VIOLENT INTERACTION BETWEEN THE DWARF GALAXY UGC-7636 AND THE GIANTELLIPTIC GALAXY NGC-4472

We present new U, B, R, and H I imagery of the Virgo Cluster giant ell iptical galaxy NGC 4472 and its interacting dwarf companion galaxy UGC 7636. Using a composite image reconstruction technique, we show that a trail of debris similar to 5 arcmin in length and similar to 1 arcmi n in width (30x6 kpc for a Virgo cluster distance of 20 Mpc) is projec ted northward from the dwarf galaxy. A cloud of H I is projected along the northwest edge of the debris between the dwarf and gE. The dwarf' s nuclear morphology is irregular and bow-shaped on what appears to be its leading edge. Apart from a number of isolated blue regions, most of of the trailing debris is similar in color to the dwarf's nucleus. Only a modest enhancement of star formation appears to have been induc ed by the interaction. Although separated by 15 kpc, the H I and stell ar morphologies are remarkably similar. The stars and H I appear to ha ve been tidally distorted in situ, prior to the cloud's removal by ram pressure. If the H I has maintained its shape by magnetic support, a magnetic field strength an order of magnitude larger than the galaxy's is required. Ram pressure deceleration due to the cloud's motion thro ugh NGC 4472's x-ray-emitting interstellar medium should be sufficient for the cloud to become gravitationally bound to NGC 4472. The H I cl oud is not self-gravitating and may fragment and be destroyed in the i nteraction. UGC 7636 will probably be disrupted by NGC 4472's strong t idal forces; the stellar debris will disperse into the Virgo cluster o r become bound to NGC 4472's halo on eccentric orbits. The debris capt ured in the collision will have a negligible impact on NGC 4472's stel lar and gaseous content. On the other hand, if similar interactions ar e common in giant elliptical galaxies, they could alter or deplete sur rounding dwarf galaxy populations, fuel bursts of nuclear activity, an d perhaps provide a source of magnetic energy to their interstellar me dia.