The multi-phase nature of three intracluster media

Abstract

Among the models proposed to account for the new component of diffuse EUV and soft X-ray emission from clusters of galaxies (first discovered in Virgo) are two key contestants: the non-thermal scenario which postulates a population of relativistic electrons undergoing inverse-Compton (IC) interaction with the cosmic microwave background, and the original conjecture that the radiation is from a thermal warm gas at a temperature of $\sim$ 10$^{5-6}$ K. Currently a consensus set of limiting values on cosmological parameters favor the thermal gas interpretation. We also argued, based on pressure balance within the intracluster medium (ICM), that the non-thermal approach has formidable difficulties. Here we describe a spatial analysis of the soft X-ray excess emission of three clusters (Virgo, A2199, and Coma), using archival ROSAT/PSPC data, which reveals resolved features of cold intracluster clouds in absorption spreading over vast distances. Within the sample there is good indication that the soft excess radial trend (SERT, which qualitatively means a rising importance of the soft component with cluster radius) is due to a centrally peaked distribution of cold matter, with Coma having the least effect and no direct evidence for absorption. The data strongly suggest an intermixed ICM which contains gas masses at a wide range of temperatures, and the soft excess is due to a warm intermediate phase.