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

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The 21-cm line of neutral hydrogen has been used since its discovery in 1951 as a probe of the interstellar matter within our own and other galaxies and, both directly and indirectly, of the intergalactic gas that surrounds the galaxy distribution. Investigations of the impact of the environment on the H I content of galaxies have shown that under the right conditions of intergalactic density, external influences upon galaxy evolution can be dramatic.

In groups, where relative velocities are small, tidal damage is frequent, and because the outer regions of the disks are most susceptible, disruptive encounters may be best delineated by the H I distribution. The formation of bridges and tails provides spectacular displays in which the appendages extend several galaxy diameters. H I masses in excess of 108 Msun may be freed from a parent galaxy; by falling back onto the disk of a parent or neighbor, such gas may then fuel bursts of activity. Still, in a number of the systems in which gas is detected far outside the disk, explanations in terms of recent tidal events are inadequate; are such gas clouds the remnants of the galaxy formation era or of not too recent disruptive encounters? While rare, some galaxies are endowed with extraordinarily extended H I envelopes; are these at the extreme of a normal distribution, or do they belong to a special category? And if the external gas reservoirs required by the continuous accretion models of slow spiral evolution (50) are only swept away by tides after encounters, why do the disks of isolated galaxies not all have extended gas distributions?

In clusters, the tidal damage is reduced by the much higher values of the relative velocities during close encounters. However, stripping of the gaseous component of spirals occurs in clusters, with consequences far more drastic than those of the tidal encounters in groups. Stripping appears to be associated primarily with the presence of a pervasive ICM, which acts on the interstellar gas via ram pressure or conductive processes. But is such galaxy-intergalactic medium interaction too efficient? If so, then why do we see spirals in clusters at all?

The analysis of H I deficiency in cluster galaxies is a delicate business demanding both optical and 21-cm data of high quality. The number of clusters studied in the 21-cm line is still relatively small, and the identification of statistical trends only tentative. Aperture synthesis observations with long integration times will be necessary in order to extend the sample of observed clusters beyond the redshift limits currently accessible to filled-aperture instruments. Much work remains to be done in the Virgo cluster. Further investigation is necessary to confirm the correlation between H I deficiency, the velocity relative to the cluster as a whole, at the density of the intergalactic gas. Observations of integrated CO will provide an indication of the fate of the clumpier molecular gas.

Perhaps the most perplexing issues relate to the ultimate fate of the gas-swept spirals: What do they look like after a few billion years? Can we somehow distinguish between those lenticulars whose type was determined at the time of cluster collapse and those perhaps similar in appearance that originated from recently stripped spirals? In other words, can we tell the genetic attributes from the sociological?

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