Annu. Rev. Astron. Astrophys. 1984. 22: 445-70
Copyright © 1984 by . All rights reserved

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3.3 The Fate of Tidal Debris

By disturbing and perhaps removing a substantial portion of a galaxy's interstellar gas, a tidal encounter can wreak substantial havoc on a victim's evolution. Peculiar morphological characteristics likely resulting from the dramatic tidal events in which the galaxy is participating appear in numerous systems with H I streams, notably NGC 3077 and M82 (111), NGC 4747 (53, 116), and M51 (59, P. Appleton and R. Davies, private communication). In addition to the general tidal disruption, the gas can either be removed from the galaxy's disk or caused to fall toward the center of the galaxy by a loss of angular momentum. Mass transfer from one galaxy to another is likely if the orbital sense coincides with the spin orientation, prolonging the activity of the tidal acceleration. In the latter circumstance, gas that was raised to high-z extents will fall back to the nucleus, perhaps inducing a burst of star formation or other activity. Cottrell (26) suggests that the Irr II galaxies are the end products of collisions between galaxies in groups, and in fact, a significant fraction of the galaxies with H I streams are classified as such.

The potential of mass transfer is of particular importance for the observed H I distribution in early-type galaxies. In many of the elliptical and lenticular galaxies with detectable H I masses, there is a marked discrepancy between the gas distribution and dynamics and those of the stars (71, 85). The incompatibility of the angular momentum vectors between the stellar and gaseous components argues in favor of accretion of the gas from the outside (102). However, direct evidence for the accretion of substantial amounts of H I gas contributed by a late-type galaxy to an earlier-type companion is rare, e.g. NGC 678 / 680 (54), NGC 1510 / 1512 (52), and NGC 4026 (2). More often, there is no neighbor with sufficient proximity to explain the mass transfer. Moreover, there may be no relationship between the H I mass of ellipticals and their environment at all (31). At present, there is no satisfactory explanation for either the absence of significant interstellar gas in many early-type galaxies or the presence of H I in others.

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