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

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Morphological classification is a fundamental tool for the identification of activity in galaxies. Even the most casual perusal of the Arp (4) atlas will provide convincing evidence that not all galaxies live quiescent existences. Furthermore, the conclusion immediately arises that it is the presence of neighboring galaxies that somehow leads to the morphological peculiarities.

At least as early as 1931, Hubble & Humason (67) noticed that the distribution of morphological types among galaxies differs between the central parts of the rich clusters and the field; such morphological segregation has been quantified more recently by several authors (21, 32, 82). A basic question raised by these studies concerns the formation of S0 galaxies: Did their formation as a separate Hubble class occur at early epochs, or are they instead the remnants of recently stripped spirals? The understanding of galaxy morphology is related to the very general problem of galaxy formation and evolution and how a galaxy, during its birth and evolution, is affected by its surroundings.

Since its detection in 1951, the 21-cm line of neutral hydrogen has offered clues to the nature of quite diverse galaxian attributes, from the obvious interstellar gas content to the more subtle mass-to-light ratios and the determination of the Hubble constant. As a tracer of potential star formation, the neutral hydrogen content is also an indirect probe of the evolutionary processes at work within a galaxy today. Here, we discuss one aspect of ``galaxian sociology'': how a galaxy's interstellar gas, as revealed by its neutral hydrogen, is influenced by its environment during its evolutionary history.

The observed morphological segregation implies that somehow the interstellar gas is removed from the galaxies in clusters, thereby reducing the star formation rate so that the galaxies appear as early-type. An alternative possibility is that galaxies of different morphological types are created as such in regions of differing mass density, with ellipticals and S0s preferentially formed in higher density regions, so that the differentiation among types is forced in early stages of evolution. While not attempting to rule out this scenario, we present the current evidence that testifies to the viability of at least some contribution of ongoing gas removal mechanisms, the result of interactions between galaxies and their neighbors as well as with the intergalactic medium within which they are immersed.

Evidence that galaxies are aware of their environment comes from several sources, such as the head-tail radio source morphology, the increased frequency of the [O II] line in cluster galaxies, and the large H I appendages seen extending from galaxies in pairs and small groups. There are a variety of mechanisms that may contribute to the depletion of the interstellar H I gas within a galaxy. Perhaps the most popular external models proposed are galaxy-galaxy collisions, tidal interactions, ram pressure sweeping, and evaporation. Internal ongoing mechanisms may include galactic winds or simply star formation with insufficient gas replenishment. The neutral hydrogen content and its distribution can be used not only to compare galaxies within differing intergalactic environments but also to estimate the relative efficiency of the several proposed gas removal mechanisms. In Section 2, we briefly review the current state of 21-cm line measures of the H I content of galaxies and the expected distribution of the neutral gas. Section 3 examines the environment of small aggregates of galaxies in which tidal encounters among neighbors can remove a substantial portion of a galaxy's interstellar gas, dramatically altering its appearance and subsequent evolution. In Section 4, the evidence for H I deficiency in clusters is presented, along with a discussion of the contributions of the various gas-sweeping mechanisms.

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