Study of the Environmental influences on spiral galaxies in clusters

Abstract

A study is made of some of the environmental factors that affect the dynamics and content of both the gas and stars in cluster spiral galaxies. Two issues form the focus of this thesis: (1) the H?I?deficiency of cluster spirals, and (2) the possibility that the internal dynamics of spiral galaxies is affected by the mean tidal field of the cluster as a whole. An analysis of existing data on spiral galaxies in the Virgo cluster shows that a small fraction of the galaxies have a significant atomic hydrogen (H?I) deficiency in their inner disks in addition to being globally deficient in H?I. It is shown that head?on collisions between spiral galaxies can preferentially strip H?I from the inner disks of the colliding galaxies while leaving their molecular components unaffected. The frequency of collisions in the Virgo cluster is sufficiently large to account for the fraction of galaxies that are deficient in their inner disks. An analysis of H?I data on spiral galaxies in four clusters shows that the fraction of H?I?deficient galaxies is strongly correlated with their optical sizes. Larger galaxies are more deficient than smaller galaxies, both in the fraction of galaxies affected and the amount of gas lost. A study of various gas?stripping mechanisms believed to operate in clusters—such as head?on collisions between galaxies, ram?pressure stripping by the intracluster medium, and thermal evaporation of interstellar H?I by the intracluster medium—showed that none of these mechanisms could account for the observed dependence of H?I?deficiency on galaxy size. It was therefore proposed that early tidal interactions between galaxies, which occurred in groups and sub?clusters that later merged to form clusters, were responsible for stripping gas from these galaxies. A numerical study of the effects of the mean cluster tidal field on a disk galaxy travelling through the cluster was carried out using a restricted three?body framework. For the adopted model, the tidal field of the extended mass distribution of the cluster is purely compressive within the core, unlike the disruptive tidal fields outside more compact mass distributions. An important effect of the tidal field is to increase the planar velocity dispersions of both gas clouds and stars in the disk galaxy, with the low?velocity?dispersion gas being more strongly perturbed. In most cases, the vertical motion of particles is relatively unaffected. The resulting anisotropy in the stellar velocity dispersion satisfies the criterion for the disk to become unstable to the fire?hose instability, which would eventually lead to an increase in disk thickness. Disks parallel to their orbital plane in the cluster develop strong two?armed spiral features. Disks perpendicular or inclined to their orbital plane undergo an oval compression in the plane of the disk. These non?axisymmetric tidal perturbations can cause outward transport of angular momentum. This suggests that gas can fall inward toward the galaxy center, and various triggering processes can produce a burst of star formation in these disks. A third effect of the tidal interaction-possibly the only directly observable one-is that the rotation curves of the disk galaxies begin to decline in the outer regions. This decline can be explained as arising from the increased planar velocity dispersion of stars and is due to the asymmetric?drift phenomenon. The main conclusion of this thesis is that tidal effects on spiral galaxies-whether arising from early interactions between individual galaxies or from long?lived interactions with the mean tidal field of the cluster-can significantly alter their evolutionary paths.