4.8. Cluster Spirals and Environment
Environment and interactions appear to play a significant role in the evolution of galaxies, particularly in dense environments. Interactions with satellites may be responsible for the significant fraction of lopsided spiral galaxies (Rudnick & Rix 1998). Disk asymmetry may affect the inferred spatial distribution of metals in the interstellar gas. For example, Kennicutt & Garnett (1996) noted an asymmetry in O/H between the NW and SE sides of M101, which may be related to the asymmetry in the structure of the disk. Zaritsky (1995) found a possible correlation between disk B - V and the slope of the O/H gradient such that bluer galaxies tended to have steeper gradients. He suggested that accretion of metal-poor, gas-rich dwarf galaxies in the outer disk could steepen abundance gradients and make the colors bluer through increased star formation. On the other hand, the trend may simply reflect the fact that spirals with steep metallicity gradients tend to be lower-luminosity late Hubble types with bluer colors on average.
Rich clusters offer a variety of galaxy-galaxy and galaxy-ICM interactions. The cluster environment certainly affects the morphology of galaxies (Dressler 1980). It is also known that spirals near the center of rich clusters, for example, the Virgo cluster, show evidence for stripping of H I, especially from the outer disks (Warmels 1988; Cayatte et al. 1994). Such stripping is inferred to result from interaction of the galaxy ISM with the hot X-ray intracluster gas. The degree of H I stripping correlates with projected distance from the cluster core, although in Virgo the molecular content appears to be not affected (Kenney & Young 1989).
If field galaxies evolve through continuing infall of gas (Gunn & Gott 1972), then the truncation of H I disks in cluster spirals should have an effect on the chemical evolution. Specifically, infall of metal-poor gas reduces the metallicity of the gas at a given gas fraction. Truncation of such infall should then cause the chemical evolution of cluster spirals to behave more like the simple closed box model, and thus should have higher metallicities than comparable field spirals. This idea has led to several studies of oxygen abundances in Virgo spirals. The largest study so far is that of Skillman et al. (1996), who obtained data for nine Virgo spirals covering the full range of H I deficiencies. The results indicate that cluster spirals with the largest H I deficiencies have higher O/H abundances than field spirals with comparable MB, rotation speeds, and Hubble types, while spirals with only modest or little H I stripping have abundances comparable to those of similar field spirals (Skillman et al. 1996, Henry et al. 1994, 1996). The samples studied so far have been small, and one must worry about possible systematic errors in abundance caused by the lack of measured electron temperatures. This is an area that could benefit from further study with larger galaxy samples and more secure abundance measurements.