X-Ray Spectral Analysis of Elliptical Galaxies from ASCA: the Fe Abundance in a Multi-Phase Medium

Abstract

We present spectral analysis of ASCA data of 17 elliptical and 3 lenticular galaxies most of which have high L_x/L_B. Single-temperature models (MEKAL and Raymond-Smith) give unacceptable fits (chi^2_red > 1.5) in most cases and, in agreement with previous studies, give very sub-solar abundances, <Z> = 0.19 +/- 0.12 Z_sun (MEKAL). The spectra for approximately half the sample are better fit with a cooling-flow model which in three cases gives a substantially better fit. The abundances derived from the cooling-flow model are also significantly larger, <Z> = 0.6 +/- 0.5 Z_sun. We empirically tested the reliability of the plasma codes in the Fe-L region and found no evidence for serious problems with the determined temperatures and abundances. Two-temperature models give substantially better fits that are formally acceptable (chi^2_red ~ 1.0) in all but a few cases. The highest S/N galaxies (which also have highest L_x/L_B) have fitted temperatures < 2 keV for both components consistent with each being distinct phases of hot gas. The lowest S/N galaxies (which also have lowest L_x/L_B) generally have a hot component with temperature, T_H > ~5 keV, which is consistent with emission from discrete sources. (We discuss the origin of these two components from analysis of L_x-L_B and L_x/L_B.) The abundances of these two-temperature models are approximately solar, <Z> = 0.9 +/- 0.7 Z_sun (MEKAL), consistent with a recent multi-phase model for the evolution of hot gas in ellipticals. Finally, for several galaxies we find evidence for absorption in excess of the Galactic value and discuss its implications.