Roche Lobe Overflow from Dwarf Stellar Systems

We examine the evolution of residual gas within tidally-limited dwarf galaxies and globular clusters. In systems where the gas sound speed exceeds about 10% of the central velocity dispersion, hydrostatic equilibrium causes the gas to have significant density at the tidal radius. In contrast to the stellar component, gas particles relax rapidly in response to mass loss. Gas lost through the Lagrangian points is quickly replenished, leading to continuous outflow, analogous to the Jeans’ escape of planetary atmospheres. In such systems, the gas may be lost on timescales as short as a few times the sound crossing time of the system. In colder systems, by contrast, the density contrast between the core and the tidal radius is much larger, greatly reducing the mass loss rate, and allowing the system to retain its gas for over a Hubble time. Using this criterion as a guide, we present the results of numerical simulations to show that either in the proximity of large galaxies or in clusters of galaxies, residual gas in typical dwarf spheroidal galaxies (dSphs) and globular clusters would be efficiently lost after it is ionized and heated, either by external or internal sources. The tidally removed gas shall follow an orbit close to that of the original host system, forming an extended stream of ionized, gaseous debris. Such tidal mass loss severely limits the ability of dSphs and globular clusters to continuously form stars. The ordinary gas content in many dwarf galaxies is fully ionized during high red-shift epochs. In extreme cases, therefore, star formation may be completely prevented by early segregation of dark and ordinary matter, leading to the formation of starless, dark-matter concentrations as the remnants of dwarf galaxy mergers in the CDM model of galaxy formation. In the field, where the dwarf galaxies are tidally isolated, and in the center of clusters of galaxies, where the tidal potential is compressive, ionized gas may be retained by dwarf galaxies, even though its sound speed may be comparable to or even exceed the velocity dispersion. In such situations, dwarf galaxies may retain their gas content until they merge with much larger galaxies. These processes may help to explain some observed differences among dwarf galaxy types, as well as observations of the haloes of massive galaxies. Subject headings: galaxies: dwarf — galaxies: evolution — clusters: globular — clusters: open — hydrodynamics — methods: numerical