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The study of the gas content of galaxies in clusters affords us the opportunity to search for environmental effects on the star formation process in cluster members and by implication may lead to an estimate of the effectiveness of morphological alteration at the current epoch. Currently, HI observations exist for significant numbers of galaxies in about a dozen clusters. While the link between the HI content of a galaxy and its current evolutionary state can only be surmised in a statistical sense, the availability of HI observations for increasing numbers of galaxies in clusters of differing properties makes the comparative studies performed using the HI line a sensitive, though indirect, probe of the effect of environment on the evolution of spiral disks.

It has been over 15 years since the study of the HI content of galaxies in different environments was first addressed. Davies and Lewis (1973) conducted a survey of about 25 galaxies in the Virgo cluster and compared those observations with a similar sample of nearby galaxies. Davies and Lewis used both the hydrogen mass to luminosity ratio MHI / L and the HI surface density sigmaHI to conclude that the Virgo cluster spirals were HI poor with respect to their field sample. This result was quickly questioned by Bottinelli and Gouguenheim (1974) who pointed out that the Virgo spirals were typically more luminous than the field objects and hence the comparison was effected by bias. As the Virgo cluster HI sample has grown and extended to fainter objects, the initial conclusion of Davies and Lewis seems to have been established (Huchtmeier et al. 1976; Chamaraux et al. 1980; Giovanardi et al. 1983a; Haynes and Giovanelli 1986; Hoffman et al. 1988), although some doubts have been raised even fairly recently (Tully and Shaya 1984).

One of the difficulties over the years has been in the definition of a comparative HI content parameter and its possible dependence on intrinsic properties like luminosity and morphological type, both of which are likely to be represented differently in typical cluster and field samples. The definition and choice of the HI deficiency parameter is discussed in Haynes et al. (1984) and Giovanelli and Haynes (1988). For spirals, the disk size, not the luminosity, seems to provide the best normalization: the HI, as a disk constituent, is marked by a nearly constant globally averaged surface density sigmaHI. In contrast, the HI mass to luminosity ratio MHI / L is not purely a disk property, particularly for early type systems for which the gas-free bulge contributes significantly to the light. Thus MHI / L is suspected to be more type-dependent and susceptible to differences in the luminosity distribution of the cluster and comparison samples. In keeping with the general discussions given in the preceding references, I will hereafter use our standard definition of the comparative HI content of galaxies in terms of the HI deficiency parameter

Equation 1

defined as the difference, on a logarithmic scale, between the observed HI mass and that expected for a "normal" galaxy of similar linear size and optical morphology. A galaxy that is HI poor by a factor of ten thus has = +1.0. In order to set a standard of normalcy, Haynes and Giovanelli (1984) observed a subset of galaxies listed in the Catalog of Isolated Galaxies (Karachentseva 1973; CIG), allowing them to establish the relationships between HI mass and linear diameter for galaxies of morphological type Sab to Irr. Normal galaxies show a range in that is approximately gaussian with a dispersion of about 0.25. In this definition, galaxies are not considered "deficient" unless > +0.3 (deficient by a factor of two), or preferably, +0.48 (factor of three). It should be kept in mind that the establishment of a normal HI content for galaxies earlier than Sab is still in doubt, and indeed even new high sensitivity HI observations of nearby Sa's show that class to be much more heterogeneous in its properties than later type spirals (Magri and Haynes, in preparation).

In the years since the publication of the Virgo results, numerous studies have presented conclusions, some in agreement and some in disagreement, with the picture of HI depletion of cluster spirals. The accumulation of observations of galaxies in a variety of clusters themselves characterized by different morphologies, densities and X-ray luminosities has permitted an examination of the circumstances in which HI deficiency is observed. Furthermore, more recent work on the Virgo cluster has probed the molecular constituent as well as the atomic and has attempted to investigate the star formation rate at the current epoch. In the remainder of this paper, I will review the status of today's results concerning the observed gas deficiency in cluster spirals with particular regard to their implications for star formation and galaxy evolution.

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