Some galaxies show clear evidence for stripping, e.g. Observational evidence for this is marginal. If ram pressure or viscous stripping is effective at removing gas, then cluster spirals should have truncated discs deficient in H i. These occur even when the ram pressure is insufficient to strip the gas disc directly: turbulence in the gas flowing around the galaxy entrains the interstellar medium resulting in its depletion.
Related mechanisms to ram pressure are thermal evaporation of the interstellar medium ( Cowie & Songaila 1977) and viscous stripping of galaxy discs ( Nulsen 1982). Depending on the binding energy of the galaxy's own interstellar medium, the ICM will either be forced to flow around the galaxy or will blow through the galaxy, removing some or all of the diffuse interstellar medium. Although the ICM is tenuous, the rapid motion of the galaxy causes a large pressure front to build up in front of the galaxy. As a galaxy orbits through the cluster, it experiences a wind because of its motion relative to the diffuse gaseous intracluster medium (ICM). Ram-pressure stripping has been a long-standing possibility, dating from the analytic work of Gunn & Gott (1972), and a mechanism that has been cited in over 200 published abstracts. Several mechanisms have been proposed that may be capable of explaining the transformation of galaxy morphology in dense environments. 1997), or from a different level of infall between local and distant clusters ( Bower 1991 Kauffmann 1996). The differences in the fractions of blue, or actively star-forming, galaxies between local and distant clusters may result either from an increase in the general level of star formation activity at higher redshift (e.g. This leads to the natural conclusion that the primary effect of the cluster environment is to transform luminous spiral galaxies into S0 types through suppression of their star formation.Ī key ingredient in the explanation of the Butcher-Oemler effect is the rate at which ‘fresh’ galaxies are supplied from the field into the cluster environment. Comparison of the galaxy population of local and distant clusters provides the strongest evidence for this. Many authors have suggested that the predominance of early-type S0 galaxies in local clusters is the result of a mechanism that suppresses star formation in these environments, leading to a transformation of galaxy morphology. (1998) suggest that the predominant evolutionary effects are that the distant clusters have a substantial deficit of S0 systems compared with nearby systems, and at lower luminosities they contain primarily Sc-Sd spirals, compared with the large population of dwarf spheroidals in present-day clusters. The most recent advances have been made with the Hubble Space Telescope, which allows the morphology of the distant galaxies to be directly compared with the properties of their nearby counterparts. Subsequent work has established that many of the red galaxies in these clusters have spectral signatures of recent star formation ( Dressler & Gunn 1983 Couch & Sharples 1987 Poggianti et al. The pioneering work of Butcher & Oemler (1978, 1984) first demonstrated that distant clusters contained a far higher fraction of blue galaxies than their local counterparts. There is a long-standing debate concerning the effect of environment on galaxy morphology. Galaxies: clusters: general, galaxies: kinematics and dynamics 1 Introduction
We conclude that ram pressure alone is insufficient to account for the rapid and widespread truncation of star formation observed in cluster galaxies, or the morphological transformation of Sabs to S0s that is necessary to explain the Butcher-Oemler effect. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intracluster medium, will lose significantly less gas. The time-scale for this to occur is a fraction of a crossing time ∼10 7 yr. A spiral galaxy passing through the core of a rich cluster, such as Coma, will have its gaseous disc truncated to ∼4 kpc, thus losing ∼80 per cent of its diffuse gas mass. However, at small radii it is also important to consider the potential provided by the bulge component. We find that the analytic expectation of Gunn & Gott, relating the gravitational restoring force provided by the disc to the ram pressure force, provides a good approximation to the radius at which gas will be stripped from a galaxy. We use three-dimensional SPH/ N-body simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters.
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