On the radial density profile of intracluster gas tracing the isothermal dark halo with a finite core

The cusped NFW universal density profile suggested by typical CDM models has been challenged in recent years by the discoveries of the soft cores with finite central density for a broad range of masses from dwarf galaxies to clusters of galaxies. It is thus desirable that a new, analytic model would instead become available for virialized dark halos. One promising candidate is probably the empirical density profile proposed by Burkert (1995), which resembles an isothermal profile with a constant core in the inner region and matches the NFW profile at large radii. Meanwhile, such a revised dark halo (RDH) profile has turned out a great success on galactic scales. This stimulates us to apply the RDH profile to more massive systems like clusters of galaxies. In this paper we have made an attempt to derive the radial density profile of intracluster gas from the RDH profile under the isothermal and hydrostatic equilibrium hypotheses, and compare it with those revealed by X-ray observations and inferred from the NFW profile. It is shown that the RDH predicted gas density can be well represented by the conventional β model with a typical β parameter of β ∼ 0.7–0.9. Alternatively, fitting the theoretically predicted X-ray surface brightness profile to an ensemble of 45 X-ray clusters observed by ROSAT, we find that the RDH and NFW profiles become to be almost indistinguishable from each other, and their characteristic density and scale length parameters are strongly correlated. Yet, unlike the NFW model, the RDH profile can allow us to work out straightforwardly the central dark matter density from X-ray measurements of the surface brightness and temperature of clusters. It appears that the resulted central densities of the 45 clusters have an average value of 〈ρ0〉 ≈ 0.01 M⊙ pc −3, in agreement with the result estimated on galactic scales, which reinforces the claim for the presence of the soft halo cores over the entire mass range. Subject headings: cosmology: theory — dark matter — galaxies: clusters: general — X-rays: galaxies