Annu. Rev. Astron. Astrophys. 1984. 22: 445-70 Copyright © 1984 by Annual Reviews. All rights reserved

4.4 Gas Removal Mechanisms

In 1951, Spitzer & Baade (103) proposed that the morphological type segregation seen in clusters could be the result of environment-related mechanisms; collisions between galaxies, made frequent by the high densities and high velocity dispersions in cluster cores, would lead to removal of gas from spiral disks. As discussed in Section 3.1, tidal disruption is highest when the relative velocity of the two galaxies is smaller, a circumstance that makes galaxies in clusters less vulnerable to this proccss than those in small groups. For rapid encounters to be effective in producing stripping, impact parameters must be small, i.e. disks must collide. Chamaraux et al. (22) estimated that the probability that a bright spiral in Virgo undergoes such an encounter in a Hubble time is only about 0.03. In a cluster like Coma, the higher value of the velocity dispersion, which would increase the chance of encounters, is balanced by the reduced spiral fraction; thus there is no reason to expect an enhanced rate of spiral-spiral collisions, at least at the present time. Several authors have considered the integrated effect of encounters that occur when the constraints in the impact parameter are relaxed (34, 86); serious disruption occurs only in the outer regions of disks (r > 15 kpc), and stripping of large fractions of the interstellar gas is improbable.

The intracluster medium (ICM) observed in the X-ray domain in clusters (36) may play a more important role for galactic structure. The ICM is tightly coupled with the galaxian component of the cluster; it may accrete onto massive galaxies or remove the interstellar gas of galaxies that venture into cluster cores, either by means of the ram pressure generated by their motion relative to the ICM or by evaporation induced by heat conduction into the cooler interstellar gas. In the first instance, gas removal occurs if the ram pressure overcomes the restoring gravitational force per unit area that acts on the disk gas, i.e. if _icm v² > 2G _* (r) _g (r), where _icm is the ICM density, v the component of the velocity of the galaxy (with respect to the ICM) perpendicular to the disk of the galaxy, G the gravitational constant, and _* (r) and _g (r) respectively the surface density of collapsed matter and that of gas in the disk, at a distance r from the galactic center (51). Numerical simulations (48, 75, 97) have shown that ram pressure can be an effective mechanism of gas removal; however, these results should be considered with caution, as they are based on simplified models that neglect the effect of viscosity, cooling instabilities, and thermal conduction. Hydrodynamic calculations that include these effects suggest a more complex picture (97), with the possibility of star formation induced near the center of the galaxy, a prediction that may find observational support in UGC 6697 (11). Clearly, the degree of clumpiness of the interstellar medium will have a major effect on the outcome of the interaction (101). The effects of heat conduction and consequent evaporation of the interstellar gas have been investigated, most notably by Cowie & Songaila (27). The evaporative mass loss rate is a sensitive function of the ICM temperature and, somewhat less sensitive, of the ICM density. For a spiral disk of radius 15 kpc and thickness 200 pc, the evaporative mass loss rate goes as T₈^2.5 provided that n₃ > 3T₈², where n₃ and T₈ are the ICM density and temperature in units of 10^-3 cm^-3 and 10⁸ K, respectively (except when radiation from the interface becomes important, for n₃ > 10^2.2 T₈², in which case material condenses onto the galaxy). For n₃ < 3 T₈², heat flux saturation occurs, and the mass loss rate is proportional to (n₃ T₈²)^0.6 [see Figure 1 in (27)]. Evaporation could also be an effective means of gas removal from galaxies; a major uncertainty lies with the topology of magnetic fields, which may inhibit conduction.

Certainly the gas content of a galaxy does not remain unaltered in the absence of an active environment. Star formation occurs, and gas is restored to the disk via stellar mass loss; and supernovae may power galactic winds (77), a process that has been invoked particularly for elliptical galaxies and that appears to be effective as a gas removal mechanism only at early stages of galaxy evolution (48). The rates of these processes are uncertain; to what extent they are affected by an active environment is even more difficult to say. While the evidence for their relevance grows, the interpretation of the details remains highly speculative.