X-Ray Emission from Early-Type Galaxies: A Complete Sample Observed by [ITAL]ROSAT[/ITAL]

Abstract

To test the cooling flow model of early-type galaxies, we obtained a complete magnitude-limited sample of 34 early-type galaxies, observed with the PSPC and the HRI on ROSAT. The X-ray-to-optical distribution of galaxies implies a lower envelope, which is consistent with the stellar emission inferred from Cen A. When this stellar component is removed, the gaseous emission is related to the optical luminosity by L_X ∝ L_B^m, where m = 3.0-3.5, which is significantly steeper than the standard theory (m = 1.7). The dispersion about the correlation is large, with a full range of 30-100 in L_X for fixed L_B. The X-ray temperature is related to the velocity dispersion temperature as T_X ∝ T_σⁿ, where n=1.43±0.21, although for several galaxies, T_X is about twice T_σ. The excessively hot galaxies are generally the most luminous and are associated with the richest environments. We suggest a model whereby the environment influences the X-ray behavior of these galaxies: early-type galaxies attempt to drive partial or total galactic winds, which can be stifled by the pressure of their environment. Stifled winds should lead to hotter and higher luminosity systems, which would occur most commonly in the richest environments, as observed.