X-Ray Evidence for Multiphase Hot Gas with Solar Abundances in the Brightest Elliptical Galaxies

Abstract

We examine whether isothermal models of the hot gas can successfully describe the ASCA and ROSAT spectra of NGC 1399, NGC 4472, NGC 4636, and NGC 5044. Broad-band spectral fitting of the ASCA SIS and GIS data accumulated within a radius of ~5 arcmin for each galaxy shows that isothermal models are unable to fit the SIS data near 1 keV. In addition, these isothermal models typically fail to produce the large equivalent widths of the K-alpha line blends of the H-like and He-like ions of Si and S which are measured independently of the Fe L emission lines. Two-temperature models provide excellent broad-band fits to both the SIS and GIS data of each galaxy with the relative abundances (except for NGC 4636) fixed at their solar values. A simple multiphase cooling flow model fits nearly as well as the two-temperature model for NGC 1399, NGC 4472, and NGC 5044. The multiphase models also predict more accurately the Si and S equivalent widths and the ratios of Si XIV/XIII and S XVI/XV than the isothermal models. Using various approaches we find that the temperature gradients inferred from the ROSAT PSPC data of these galaxies, especially for NGC 1399 and NGC 5044, are inconsistent with the isothermal models derived from fitting the ASCA data within a single-aperture but are very consistent with the multiphase models. Therefore, models which assume isothermal gas within r ~5 arcmin are inconsistent with both the ASCA and ROSAT PSPC data of these elliptical galaxies. Simple two-temperature models and multiphase cooling flows provide good descriptions of these data sets with Fe abundances of ~1-2 solar and (except for NGC 4636) relative abundances fixed at their solar values. We discuss the implications of these nearly solar abundances. (Abridged)