INTERSTELLAR GRAINS IN ELLIPTIC GALAXIES - GRAIN EVOLUTION

We consider the lifecycle of dust introduced into the hot interstellar medium in isolated elliptical galaxies. Dust grains are ejected into galactic-scale cooling flows in large ellipticals by normal mass loss from evolving red giants. Newly introduced dust rapidly enters the hot interstellar plasma and is sputtered away by thermal collisions with ions during the slow migration toward the galactic center in the cooling flow. Before the grains are completely sputtered away, however, they emit prodigious amounts of infrared radiation which may contribute to the large far infrared luminosities observed in ellipticals. The infrared emission depends critically on the sputtering rate. Since our understanding of both the plasma and radiation environments in ellipticals is quite good, these galaxies provide an excellent venue for studying the physical processes of dust grains and perhaps also their composition and size distribution. In older to study the global properties of grains in ellipticals we construct a new series of King-type galactic models which are consistent with the fundamental plane, galactic mass to light ratios and other relevant observational correlations. We describe a new ''continuity'' procedure to construct simple time-dependent gasdynamic models for cooling flows. Although grains can flow a considerable distance from their radius of origin in the hot interstellar medium of some galaxies before being sputtered away, we show that the grain size distribution at every radius is accurately determined by assuming in situ sputtering of dust grains, completely ignoring advection. This occurs since the stellar density profile is so steep that the majority of grains at any galactic radius is produced locally. Although thermal sputtering destroys the grains, we show that the dominant source of grain heating is absorption of starlight; grain heating by collisions with energetic thermal electrons or X-ray absorption are negligible. Previous studies have claimed that the loss of thermal energy from a hot, dusty plasma is dominated by grain heating via electron-grain collisions and subsequent IR radiation. However, we show that when self-consistent grain sputtering is included the dust-to-gas ratio is reduced and radiative cooling. not electron grain interactions dominates plasma cooling, even for the most massive ellipticals. This conclusion is insensitive to the grain size distribution assumed for the stellar ejecta.