|Annu. Rev. Astron. Astrophys. 1984. 22:
Copyright © 1984 by . All rights reserved
4.3 The Virgo Cluster
Because of its proximity, the Virgo cluster has been the object of special attention. For comparison, note that the faintest galaxies observable in, say, the Hercules cluster would appear at an apparent magnitude of about 12 in Virgo. The measurements of H I content are complete, for the galaxies projected within a 5 or 6° radius from the center, to a magnitude of 13.5, and they are rapidly nearing completion to a magnitude of about 15.0. In addition to offering a much larger sample size, Virgo also lends itself to studies of the spatial variations of H I properties of early-type galaxies, which are very difficult to detect even outside clusters at larger distances because of their low H I content. Finally, and most importantly, the large angular sizes of the Virgo galaxies can be easily resolved not only by synthesis instruments but also by the Arecibo telescope, allowing the study of the variations in the extent of the gaseous disks, the outer parts of which may be most vulnerable to hostile environments.
The first reported result of H I deficiency in Virgo cluster galaxies 3 appeared in 1973 (28). Based on the observations of 25 galaxies, an average deficiency of Virgo galaxies with respect to field spirals of a factor of 1.6 was inferred. As discussed in Section 2.1, this result was subject to skeptical questioning for several years; the results of larger samples, however, did confirm the thrust of the initial finding of Davies & Lewis. Chamaraux et al. (22) extended the analysis to a sample of 56 Virgo galaxies and obtained an average deficiency of 2.2 ± 0.3 with respect to their comparison sample, a result confirmed by studies based on samples observed at Arecibo (40, 44). When H I deficiency is analyzed using the optical diameters as a diagnostic parameter, it is necessary to appraise the possible impact of systematic differences in the sizes of optical disks, within and outside of clusters (see Section 2.1). It appears that Virgo cluster spirals are on the average smaller than those in the field (83); this effect, which is not confirmed in other clusters such as Hercules, may produce underestimates by as much as a factor of 1.6 in the average H I deficiency of Virgo galaxies; thus the average H I deficiency may be as high as a factor of 4 (44). The sky distribution of H I-deficient galaxies appears to slightly favor the region of the cluster around M87 and M86, rather than the southern concentration around NGC 4472; a recent analysis based on a deep redshift survey of the Virgo cluster (68) suggests that the ``clump'' around M87 may constitute a separate dynamical unit with higher velocity dispersion and mass-to-light ratio than the cluster as a whole; by contrast, the clump around NGC 4472 has a lower velocity dispersion (489 vs. 761 km s-1 for the M87 clump) and much weaker X-ray emission.
The depressed H I content of galaxies projected within the cluster core is also detectable in Virgo if the analysis is restricted to early-type systems; although the scatter in the values of the H I content for these galaxies, which is larger than for later spiral systems, makes the determination of an H I deficiency uncertain, the differences in the detection rates inside and outside the core are quite significant. Figure 4 (after 41) illustrates this effect separately for S0 and S0a-Sa galaxies; the associated detection rates for S0 galaxies are 4/39 inside a 6° radius and 21/49 outside, while the corresponding figures for S0a-Sa galaxies are, respectively, 7/19 and 32/38 (according to 41).
H I sizes of spiral disks in Virgo have been measured both with the Arecibo antenna and synthesis telescopes. The single-dish measurements indicate that the extent of the disks is smaller within the cluster core than outside by about a factor of 1.5 (40, 44); the reduced size of H I disks is also well correlated with H I deficiency, suggesting that the mechanism(s) that causes the H I deficiency does so by depleting preferentially the outer regions of the disks. This result has been confirmed by VLA observations (112a), as is illustrated in Figure 5, although Westerbork results still cast doubt on the exact factor by which cluster H I disks appear to be shrunk (114).