Interstellar events in elliptical galaxies

Abstract

view Abstract Citations (105) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Events in Elliptical Galaxies Mathews, William G. Abstract An understanding of the manner by which evolving stars in elliptical galaxies interact with the hot interstellar medium is required to clarify the nature of source terms used in gasdynamical models of cooling flows in elliptical galaxies. It is also important to establish if inhomogeneities in entropy that result from mass ejection from stars--as winds, planetary nebulae, or supernova explosions---can serve as sites for local or eventual star formation in the cooling flow or "if these inhomogeneities could be observed collectively or individually. The evolution of mass ejected from red giants depends in detail on the ionizing photon density in ellipticals. Most of the ionizing photons are produced by nuclei of planetary nebulae, the same component responsible for the rate of mass deposition from evolving stars. A significant additional contribution of ionizing photons may come from the cooling flow itself. If gas ejected from orbiting stars remains fully ionized, it proceeds through a definite evolutionary sequence: attainment of hydrostatic equilibrium with the ambient hot gas, disruption into many small clouds by Rayleigh- Taylor instabilities as orbital motion is dissipated into the hot gas, and conductive evaporation into the hot gas after coming to rest without colliding with other stellar ejecta. If some of the stellar ejecta should cool and become neutral, it is most unlikely that it could ever become self-gravitating and fall to merge with the hot gas or dissipate its orbital kinetic energy. Even if such orbiting, self-gravitating neutral objects existed, they would develop ionized halos when exposed to ambient ionizing photons. The gravitationally unbound ionized halos would be preferentially removed by drag forces, eventually destroying the entire cloud in <~ 10^8^ yr. Stellar ejecta, spread over many hundreds of parsecs in the wake of the orbiting parent star, must merge with the hot gas in at least ~ 10^6^ yr in order for the collective optical line emission not to exceed minimum line fluxes observed in elliptical galaxies. More observations of faint, extended line fluxes from ellipticals could reduce this merging time further. The efficient physical and thermal disruption of stellar ejecta in the cooling flow gas leaves no entropy fluctuations that could form into stars at large radius. But the metallicity of stellar ejecta always exceeds that of the local cooling flow; these small abundance fluctuations prepare the gas for eventual formation into objects of substellar mass through thermal instabilities. Type Ia supernovae produce long-lived inhomogeneities in the cooling flow environment. A series of computed model blast waves demonstrates that supernova blast waves dissipate their energy into superhot bubbles in ~ 3 x 10^4^ yr. These buoyant bubbles rise in the galactic potential, become Rayleigh-Taylor unstable, and dissipate their energy into the ambient interstellar gas in ~ 3 x 10^6^ yr. The collective emission of the system of hot supernova bubbles is significant and may dominate the thermal emission from elliptical galaxies at energies E >~ 10 keV, provided the collective hard X-ray emission from low-mass binaries is not too strong. The spatial and projected filling factors of the ensemble of bubbles are very small. It is therefore possible that individual bubbles could be observed in X-ray emission against the cooling flow background. A measure of the number of hot bubbles in large elliptical galaxies (~ 10^3^-10^4^) and knowledge of their thermal dissipation time provides a simple and direct means of determining the supernova rate. The ~ 1 M_sun_ of iron ejected into the cooling flow with each supernova event is trapped locally within the hot bubbles and may never successfully mix thoroughly with the ambient hot gas. Long-lived metal-rich inclusions in the cooling flow resulting from ancient supernovae could serve as sites of enhanced cooling and (low-mass) star formation near the centers of galactic cooling flows. Publication: The Astrophysical Journal Pub Date: May 1990 DOI: 10.1086/168708 Bibcode: 1990ApJ...354..468M Keywords: Elliptical Galaxies; Interstellar Gas; Star Formation; Planetary Nebulae; Red Giant Stars; Stellar Evolution; Stellar Mass Ejection; Supernova Remnants; Supernovae; Astrophysics; GALAXIES: INTERSTELLAR MATTER; HYDRODYNAMICS; STARS: EVOLUTION; STARS: MASS LOSS; STARS: SUPERNOVAE full text sources ADS |