Optical Properties and Dynamics of Galaxies in the Hickson Compact Groups

We have obtained broad-band R images, Hα images, and Hα long- slit spectroscopy for a set of galaxies in 21 Hickson compact groups, to see how the galaxy properties in the dense galaxy environments compare with the properties of galaxies isolated in the field. Some of the galaxies are tidally interacting and distorted; some appear morphologically normal. Observed velocity patterns range from too peculiar to form rotation curves (one-third of spiral sample), to abnormal (one-third), to normal (one- third); peculiar rotation curves are only loosely coupled with peculiar morphology. For individual galaxies, values of integral M/L_B_ are low (median = 3.5 M_sun_/L_sun_), slightly lower than for field spirals of equivalent Hubble type. There is a surprisingly good correlation of absolute magnitude with (log) maximum rotation velocity, i.e., the Tully- Fisher relation, particularly for galaxies with normal rotation curves. Virtually all of the elliptical and S0 galaxies (10 of 12) show nuclear [NII] emission, and rotating ionized gas disks extend beyond the nucleus in about half of them. In Hickson 23C (NGC 1215; S0), the nuclear gas disk is rotating counter to the direction of rotation of the stellar disk and, like the other nuclear gas disks, is probably a recent acquisition. All of these observations support a model in which the compact group galaxies have only recently accumulated from the lumpy general galaxy distribution, and in which tidal interactions are frequent and ongoing. The computer simulations of Barnes (1989) indicate that the galaxies within such groups will merge within a few orbital periods to form a relatively normal elliptical.