3.7. 21 cm line observations of clusters
The 21 cm hyperfine line in hydrogen allows one to measure the mass of neutral hydrogen in galaxies or clusters of galaxies. There are an enormous number of 21 cm observations of galaxies, which would require an entire review to discuss. The main results for clusters are negative, and I shall briefly summarize these. The H I observations of clusters and superclusters have been reviewed recently by Chincarini (1984).
The 21 cm line observations have shown that the galaxies that make up clusters are deficient in neutral hydrogen gas. First, many observations have shown that elliptical and S0 galaxies in clusters have very little neutral hydrogen gas (Section 2.10.2; Davies and Lewis, 1973; Krumm and Salpeter, 1976; Bieging and Bierman, 1977). Similarly, spiral galaxies in clusters generally have less neutral hydrogen than spirals found in the field (Huchtmeier et al., 1976; Sullivan and Johnson, 1978; Helou et al., 1979; Krumm and Salpeter 1979; Chamaraux et al., 1980; Giovanelli et al., 1981; Sullivan et al., 1981), and the deficiency is stronger near the cluster center (Giovanelli et al., 1982; Giovanardi et al., 1983). As discussed in Section 2.10.2, these observations have been used to argue that stripping of gas from galaxies through ram pressure ablation or some other mechanism is an important process in clusters, and that S0 galaxies may be produced by this process.
Observations indicate that cD galaxies, including those accreting large amounts of intracluster gas (Sections 4.3.3 and 5.7.2), contain very little neutral hydrogen (less than 109 M; Burns et al., 1981a; Valentijn and Giovanelli, 1982). This indicates that the accreted gas is not all stored as neutral hydrogen gas.
Finally, 21 cm observations of clusters as a whole indicate that the missing mass component (Section 2.8) is not neutral hydrogen gas (Goldstein, 1966; Haynes et al., 1978; Peterson, 1978; Baan et al., 1978; Shostak et al., 1980).