Diffuse Baryons in Groups and Clusters of Galaxies

Abstract

To predict the X-ray observables associated with the diffuse baryons in clusters of galaxies, we develop a new physical model for such a hot intracluster plasma. Our framework is provided by the hierarchical clustering cosmogony for the dark matter and by the standard FRW or Lemaître cosmologies constrained by cosmic ages. As to the plasma dynamics and thermodynamics we propose a semianalytical approach based on punctuated equilibria. This comprises the following blocks that we compute in detail: Monte Carlo "merging histories" to describe the dynamics of dark matter condensations on scales of order 1-10 Mpc and the associated evolution of the gravitational potential wells; the central hydrostatic disposition of the intracluster plasma (ICP), reset to a new equilibrium after each merging episode; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. Shocks of substantial strength are shown to prevail at the outskirts of rich clusters in a universe with decelerated expansion. From our model we predict the L-T relation, consistent with the data as for shape and scatter. This we combine with the mass distribution provided by the canonical hierarchical clustering; the initial perturbation spectra are dominated by cold dark matter but include enough baryons to account for the high abundance sampled by the X-ray clusters and are COBE normalized. Thus we predict the z-resolved luminosity functions, with the associated source counts and redshift distributions. We predict also the complementary contribution by the unresolved groups and clusters to the soft X-ray background. These results are compared to two recent surveys from ROSAT; one defines the local luminosity function over nearly three decades of L, and the other shows little or no evolution out to z ~ 0.8. Our results confirm that the critical cosmology coupled with standard cold dark matter (CDM) is ruled out by its overproduction of local clusters. On account of underproduction, instead, we rule out open cosmologies (the cheapest way to solve the baryonic crisis and to freeze evolution), except for a narrow range around Ω₀ = 0.5; even there, we find the consistency with the full database to be barely marginal. For the CDM cosmogony with Ω₀ = 0.3 but in flat geometry, we obtain acceptable fits. For the tilted CDM perturbation spectrum with high baryonic content in the critical universe, we obtain marginal consistency. The cosmogonical/cosmological sectors of the cluster history are independently testable by means of a lower bound to the evolved temperature distribution, as can be measured with BeppoSAX and XMM out to moderate z. Finally, we discuss the effective limitations of X-ray clusters and groups as cosmological signposts, and their brighter prospects toward the astrophysics of the ICP and the cosmogony of large, high-contrast structures.