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.
HI. 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
