A Multiphase Model for the Intracluster Medium

Constraints on the clustered mass density Ωm of the universe derived from the observed population mean intracluster gas fraction 〈fICM 〉 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 σ(r) = σ c (1 + r /r c ) . The model extends the work of Gunn & Thomas (1996) which assumed radially independent density fluctuations thoughout the ICM. Fixing the X–ray emission profile and emission weighted temperature, we explore two independently observable signatures of the model in the {σc, ǫ} 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 K cluster and demonstrate how the combination of SZ and X–ray spectroscopy can be used to identify a preferred location {σ̂c, ǫ̂} in the model plane. From these parameters, the correct value of 〈fICM 〉 in the multiphase model results, allowing an unbiased estimate of Ωm to be recovered. The consistency of recent determinations of the Hubble constant from SZ and X–ray observations with values determined by other methods suggests that biases in ICM gas fractions are small, ∼ < 20%.