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 HI 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
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
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.