Concordance of X‐Ray Cluster Data with Big Bang Nucleosynthesis in Mixed Dark Matter Models

Abstract

If the hot, X-rayEemitting gas in rich clusters forms a fair sample of the universe as in cold dark matter (CDM) models and the universe is at the critical density then the data appear to imply a ) T 1, baryon fraction derived from X-ray cluster data), larger than that predicted by big bang ) b,X () b,X 4) b nucleosynthesis (BBN). While other systematic e†ects such as clumping can lower in this paper we ) b,X , use an elementary analysis to show that a simple admixture of hot dark matter (HDM; low-mass neutrinos) with CDM to yield mixed dark matter shifts down so that signi-cant overlap with ) b,X ) b from BBN can occur for km s~1 Mpc~1, even without invoking the possible aforementioned H 0 ( 73 e†ects. The overlap interval is slightly larger for lower mass neutrinos since fewer of them cluster on the scale of the hot X-ray gas. We illustrate this result quantitatively in terms of a simple isothermal model. More realistic velocity dispersion pro-les, with less centrally peaked density pro-les, imply that fewer neutrinos are trapped and thus further increase the interval of overlap. Subject headings: cosmology: theory E dark matter E galaxies: clusters: general E nuclear reactions, nucleosynthesis, abundances E X-rays: galaxies