Over the past fifteen years, observations of cluster spirals have been accumulated by a number of groups. In this presentation, I am not attempting a review of all findings, but rather an assimilation of the results, and hence apologize in advance to those whose work is not mentioned here explicitly.
Tremendous improvements in radio receiver and spectrometer technology have allowed the acquisition of HI spectra from a galaxy in a cluster like Hercules to require a factor of ten less time than such observations typically needed in the early 1970's. In 1973, the Arecibo antenna was in the midst of its resurfacing and the first real observations of extragalactic HI were made with it in 1975. Because of its great collecting area, Arecibo has been the instrument of choice for studies of HI in distant objects. In nearer clusters, improvements have increased the potential of studying clusters with smaller filled aperture telescopes so that several clusters outside of Arecibo's declination window have also been studied. In addition, aperture synthesis studies have allowed the detailed mapping of the HI distribution and velocity field in galaxies in the nearest clusters.
The design of experiments to look for HI deficiency in clusters has been dominated by technical constraints placed on scheduling, integration time and interference potential. Studies of redshifted HI are subject to man-made radio interference that can not only saturate receivers but also mimic weak HI emission. In the United States, the band from 1400 to 1427 MHz is protected for passive use with an extension to 1360 MHz in the form of a footnote to the regulations that urges non-interference. No protection is set below 1360 MHz, which corresponds to a redshift z > 0.04. Other countries have similar regulations. The interference situation in reality varies widely and can be devastating at certain times and frequencies.
Most of the clusters with z < 0.03 that are accessible to Arecibo have been studied, as well as several others that are of particular interest. The clusters for which HI content observations have been compiled are listed in Table 1. These clusters cover a range in redshift, in morphology, in galaxy density and in X-ray luminosity. Note that, because of the nature of a study of HI in galaxies, all of these clusters contain a significant population of spirals.
The need for a sample with which to compare the cluster galaxies has been fulfilled in two different ways. It is possible to compare the HI content of a core sample with a sample at larger distance from the cluster center in order to look for radius-dependent differences within a given cluster: the "IN versus OUT" approach. Because of the controversy over possible differences between the cluster and comparison samples, Haynes and Giovanelli (1984) established their definition of HI content with respect to a sample of isolated objects that are outlying members of the nearby superclusters (Haynes and Giovanelli 1983). The isolated galaxies have similar distributions of magnitudes, sizes and redshifts as the galaxies in most nearby clusters, so that selection biases based on sensitivity and resolution are likely to be minimized. As expected from morphological segregation, the isolated galaxy sample is heavily represented by late type spirals and irregulars.
a Redshifts and velocity dispersions are taken from Struble and Rood (1987) where available and are corrected with respect to the barycenter of the Local Group.
b The X-ray luminosity is given in units of 10-43 ergs s-1 and is derived within the 0.5 - 3.0 keV range over the inner 0.5 Mpc of the cluster.
c The deficient fraction d.f. is the fraction of observed cluster galaxies that are HI deficient by more than a factor of two.
d The listed reference presents a recent comprehensive summary of observations of this cluster. See references therein for details of other results.
e Mean dispersion for the cluster. The relaxed elliptical population shows a lower characteristic dispersion and the spirals a higher one.
The first studies of clusters more distant than Virgo produced some contradictory results in which some clusters appeared to show marked HI deficiency and others none at all (Giovanelli et al. 1981; Sullivan et al. 1981; Schommer et al. 1981; Chincarini et al. 1983). It took the accumulation of amounts of data by several groups to permit the understanding of the discrepancies. Indeed, not all clusters contain HI poor galaxies, but in some, almost all of the core spirals are severely deficient in their interstellar HI gas.
An examination of the results for nine clusters has been presented in Giovanelli and Haynes (1985; hereafter GH85). That analysis made use of most of the Arecibo observations made at that time, including those made by other groups, and included about 300 objects. The clusters themselves cover a range in distance, in degree of central concentration and morphological mix, and in X-ray luminosity. Of the nine clusters, six show significant HI deficiency, but to varying degrees, and three contain galaxies with "normal" HI content. The latter three clusters are all characterized by a loose, irregular structure, a low X-ray luminosity and a high spiral fraction, while the others are richer, more evolved objects. The zone of HI depletion within each deficient cluster is restricted to the inner Abell radius or less, except in the Coma cluster where HI poor spirals are observed out to 1.5 RA. Because a certain fraction of the galaxies at small radii are expected to be objects at large radii seen in projection, the results are consistent with gas removal in all spirals passing through the core.
With the accumulation of sufficient data, the case for gas sweeping in cluster core galaxies seems firmly established where the gas and galaxy density are high enough. Efforts to examine which of the possible gas removal mechanisms might be most effective in these rare environments require an examination of data at a variety of wavelengths and have been helped enormously by studies of the nearest cluster, Virgo. A review of the current state of relevant observations of galaxies in and around Virgo is thus presented in the next section.