A multiphase model for the intracluster medium

Abstract

Constraints on the clustered mass density Omega(m) of the Universe derived from the observed population mean intracluster gas fraction [f(ICM)] of X-ray clusters may be biased by reliance on a single-phase assumption for the thermodynamic structure of the intracluster medium (ICM). We propose a descriptive model for multiphase structure in which a spherically symmetric ICM contains isobaric density perturbations with a radially dependent variance sigma(2)(r) = sigma(c)(2)(1 + r(2)/r(c)(2))(-epsilon). The model extends the work of Gunn & Thomas, which assumed radially independent density fluctuations. The main limitation of the model is its lack of connection to ICM dynamics. Fixing the X-ray emission profile and emission-weighted temperature, we explore two independently observable signatures of the model in the {sigma(c), epsilon} space. For bremsstrahlung-dominated emission, the central Sunyaev-Zeldovich (SZ) decrement in the multiphase case is increased over the single-phase case, and multiphase X-ray spectra in the range 0.1-20 keV are flatter in the continuum and exhibit stronger low-energy emission lines than their single-phase counterpart. We quantify these effects for a fiducial 10(8) K cluster and demonstrate how the combination of SZ and X-ray spectroscopy can be used to identify a preferred location {<(sigma)over cap>(c), <(epsilon)over cap>} in the model plane. From these parameters, the correct value of [f(ICM)] in the multiphase model results, allowing an unbiased estimate of Omega(m) to be recovered. The degree of consistency of current determinations of the Hubble constant via SZ and X-ray observations with values determined by other methods implies that biases in ICM gas fractions are not large.