X-ray evidence for multiphase hot gas with nearly solar Fe abundances in the brightest groups of galaxies

Abstract

We analyse the ASCA spectra accumulated within ∼100 kpc radii of 12 of the brightest groups of galaxies. Upon fitting isothermal models (IT) jointly to the ASCA SIS and GIS spectra we obtain fits for most groups that are of poor or at best marginal quality and give very subsolar metallicities similar to previous studies, 〈Z〉 = 0.29 ± 0.12 Z⊙. Two-temperature models (2T) provide significantly better fits for 11 out of the 12 groups, and in every case have metallicities that are substantially larger than obtained for the 1T models, 〈Z〉 = 0.75 ± 0.24 Z⊙. Though not very well constrained, for most of the groups absorption in excess of the Galactic value is indicated for the cooler temperature component of the 2T models. A simple multiphase cooling flow model gives results analogous to the 2T models including large metallicities, 〈Z〉 = 0.65 ± 0.17 Z⊙. The nearly solar Fe abundances and also solar α/Fe ratios indicated by the 2T and cooling flow models are consistent with models of the chemical enrichment of ellipticals, groups, and clusters which assume ratios of Type Ia to Type II supernovae and an initial mass function (IMF) similar to those of the Milky Way. Thus we have shown that the very subsolar Fe abundances and Si/Fe enhancements obtained from most previous studies within r ∼ 100 kpc of galaxy groups are an artefact of fitting isothermal models to the X-ray spectra, which also has been recently demonstrated for the brightest elliptical galaxies. Owing to the importance of these results for interpreting X-ray spectra, in an appendix we use simulated ASCA observations to examine in detail the 'Fe bias' and 'Si bias' associated with the spectral fitting of ellipticals, groups and clusters of galaxies.