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, 4472, 4636 and 5044, which are among the brightest elliptical galaxies in X-rays. 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 (which also include a component for discrete sources) are unable to fit the SIS data near 1 keV, although a marginal fit for NGC 4636 is obtained if the relative abundances of several elements with respect to Fe are allowed to depart substantially from their solar values. In addition, these isothermal models typically fail to produce the large equivalent widths of the Kα line blends of Si and S that are measured independently of the Fe L emission lines. Two-temperature models provide substantially better 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, 4472 and 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. From detailed comparison of broad-band fits to the ASCA data of these ellipticals using the MEKAL and Raymond-Smith plasma codes, we determine that the MEKAL plasma code (as expected) is significantly more accurate for the important energies ∼0.7–1.4 keV, and the small residuals in the Fe L region for the best-fitting multiphase models imply that remaining inaccuracies in the MEKAL code are insufficient to change qualitatively the results for data of the present quality. Using various approaches, we find that the temperature gradients inferred from the ROSAT PSPC data of these galaxies, especially for NGC 1399 and 5044, are inconsistent with the isothermal models obtained from fitting the ASCA data within a single aperture, but are very consistent with the multiphase models. Therefore, models that assume isothermal gas within r∼5 arcmin are inconsistent with the ASCA and ROSAT PSPC data of these elliptical galaxies. Simple two-temperature models and multiphase cooling flows provide much better 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.