X-Ray Emission Line Ratios and Multiphase Gas in Elliptical Galaxies and Galaxy Clusters

Abstract

We examine the K shell emission lines produced by isothermal and simple multiphase models of the hot gas in elliptical galaxies and galaxy clusters to determine the most effective means for constraining the width of the differential emission measure (xi(T)) in these systems which we characterize by a dimensionless parameter, sigma_xi. Comparison of line ratios of two-temperature (sigma_xi << 1) and cooling flow (sigma_xi ~1) models is presented in detail. We find that a two-temperature model can approximate very accurately a cooling flow spectrum over 0.5-10 keV. We have re-analyzed the ASCA spectra of three of the brightest galaxy clusters to assess the evidence for multiphase gas in their cores: M87 (Virgo), the Centaurus cluster, and the Perseus cluster. K-alpha emission line blends of Si, S, Ar, Ca, and Fe are detected in each system as is significant Fe K-beta emission. The Fe K-beta/K-alpha ratios are consistent with optically thin plasma models and do not suggest resonance scattering in these systems. Consideration of both the ratios of H-like to He-like K-alpha lines and the local continuum temperatures clearly rules out isothermal gas in each case. To obtain more detailed constraints we fitted plasma models over 1.6-9 keV where the emission is dominated by these K shell lines and by continuum. In each case the ASCA spectra cannot determine whether the gas emits at only two temperatures or over a continuous range of temperatures as expected for a cooling flow. The metal abundances are near solar for all of the multiphase models. We discuss the implications of these results and examine the prospects for determining the temperature structure in these systems with upcoming X-ray missions.