12.6.2. Gas Deficiency in Cluster Spirals
Although tidal encounters may not appear as dramatic as in low-velocity dispersion groups, the cores of rich clusters are nonetheless very harsh environments for the fragile galaxian disks which pass through them. Not only is the density of galaxies much higher than in loose groups, but also cluster cores are often pervaded by a hot (T 108 K) intracluster medium responsible for the observed extended X-ray emission. A variety of mechanisms have been proposed which could sweep the interstellar gas from the disk of a spiral moving through the cluster: these include galaxy-galaxy collisions, tidal interactions, ram pressure sweeping by the intra-cluster medium, and evaporation.
Evidence that the circumstance of cluster residence affects galaxy evolution is offered by a variety of observations. The intracluster gas is believed to exert sufficient pressure to bend radio source jets into a variety of head-tail morphologies. Galaxies in clusters have been shown to be much less likely than are field galaxies to exhibit the optical emission lines associated with active star formation. The segregation of early-type galaxies into cluster cores, in contrast with the more widespread distribution of spirals throughout low-density regions, has led to speculation about the removal of gas from galaxies in clusters. Yet the bulk of the evidence, particularly concerning lenticulars, has led to the conclusion that much of the observed morphological segregation was introduced early in the galaxy formation era.
As a tracer of potential star formation, the neutral hydrogen content of galaxies serves as a probe of the efficiency of proposed gas removal mechanisms. In 1973, Davies and Lewis concluded that galaxies in the Virgo cluster possessed a lower HI surface density than did their counterparts in the field. Although that initial study was limited because of selection effects, particularly the Malmquist (luminosity) bias, its conclusions have been borne out by more recent observations. At present, data suitable for analyses of the comparative HI content of spirals now exist for more than ten clusters and for the field. The clusters studied generally contain a sizable population of spirals and are at redshifts less than z = 0.04. Some of the clusters have a high galaxian density, conducive to galaxy-galaxy interaction mechanisms, while others are characterized by high cluster X-ray luminosity, implying the presence of a healthy intracluster medium which is producing thermal bremsstrahlung emission. Thus, not only can we test the reality of the HI deficiency in cluster spirals, but also we can investigate the nature of the gas removal process.
As the nearest rich cluster and center of the Local Supercluster, the Virgo cluster provides a laboratory for the investigation of environmental influences on the HI distribution and content of its member spirals. Virgo is dynamically still evolving. Its ellipticals and spirals form separate populations, and it appears that while the ellipticals form a relaxed, collapsed cluster core, the spirals are still infalling. The spirals in the Virgo core are large and near enough that the HI distribution can be mapped in some and the HI content can be measured in many. Numerous authors have carried out such observations of Virgo spirals using Arecibo, Naneay, Effelsberg, Westerbork, and the VLA and consistently find that spiral galaxies in the Virgo core are HI poor with respect to field galaxies. A substantial fraction of galaxies covering a wide range in luminosity and morphological type exhibit HI deficiency by factors which exceed ten. Furthermore, the Virgo core spirals have shrunken HI disks; in many, the HI disk is actually smaller than the optical extent. The latter observation, implying that the outer portions of the HI disk have been swept, is a forceful argument in support of external gas removal mechanisms. A number of Virgo spirals have now been mapped in the CO 2.6-mm line. CO emission traces the distribution of the dense molecular material in spiral disks. Kenney and Young (1986) find that the molecular disks, which are more centrally confined than the HI, show no trace of perturbation, reinforcing the finding based on HI data that sweeping affects primarily the outer gas layers.
The two Abell clusters A2151 and A2147 in the Hercules supercluster provide the opportunity to compare predictions of galaxy-galaxy interaction sweeping models with those of galaxy-intracluster gas ones. Lying at the same distance, the two clusters are quite different in their morphology. A2151 is a denser, and yet more loosely organized, cluster and is associated with a relatively weak X-ray source, while A2147 appears azimuthally symmetric and exhibits a higher X-ray luminosity. The degree of HI deficiency is more pronounced in A2147. Further comparison of the HI deficiencies observed in nine clusters by Giovanelli and Haynes (1985a) shows that the degree of HI deficiency seen in a cluster correlates with the presence of a hot intracluster medium, as implied by cluster X-ray emission. The exact nature of the gas removal process, either ram pressure sweeping by the intracluster gas as the galaxy moves through the cluster, evaporation, or a more complicated combination of the two, is yet elusive. However, it seems that spiral disks which pass through the hostile cluster core lose as much as 90% of their initial HI mass but retain most of their denser molecular clouds. The HI-poor galaxies furthermore show evidence for reduced star formation rates; that is, they are redder than field galaxies of similar morphology. At comparable galaxian densities, the fraction of galaxies which are classified as lenticular is significantly higher, while that of spirals is correspondingly lower, in clusters with high-X-ray luminosity; this suggests that a causal relationship might exist between morphological type and current cluster conditions. While not all cluster S0's need be swept spirals, the observed HI deficiency in, cluster spirals does argue that some reinforcement of the initial morphological segregation occurs in environments hostile to diffuse interstellar gas.