Mixed Dark Matter Model in Halos of Clusters

Abstract

We discuss the structure of clusters in a class of flat cosmological models with the fraction of mass Ω_{CDM} ~0.8 in cold dark matter, and the rest in hot dark matter in the form of massive neutrinos. We consider such Cold+Hot Dark Matter (CHDM) models with one, two or three massive neutrinos, with total mass ~4.6eV. Neutrinos of such low mass cannot constitute halos of galaxies and groups, but only of clusters of galaxies. The limit on the density of neutrinos in the central parts of galaxy clusters is estimated from the phase space den- sity constraints. The ratio of the neutrino density to that of CDM through the cluster is found analytically. It appears that the density of neutrinos is suppressed within the Abell radius. However, neutrinos contribute ~20% of the mass density to the cluster halo. Our numerical simulations match analytical results. The simulations indicate that the cluster halo dark matter density profile has the power-law slope ~-2.5 which is close to that in the model with cosmological constant. We also found that in the CHDM models the velocity dispersion is almost constant across the cluster. This is quite different from the model with cosmological constant or the open model where the velocity dispersion falls in the cluster outskirts. We discuss X-ray emission and weak gravitational lensing by clusters in the model. We input the found spherically symmetrical fit to the CHDM mass density profile and the X-ray surface brightness for the cluster A2256 into simple equation of hydrostatic equilibrium of the hot gas. X-ray temperature derived this way departures from both the data and actual prediction of the model, which give almost constant temperature. We found also that the problem of high baryonic fraction in clusters is not resolved in the CHDM models.

19 pages + 9 figures, self-expanding uuencoded compressed tar archive of postscript files. Complete paper is also available via anon ftp at ftp://ftp.cita.utoronto.ca/ftp/cita/pogosyan/cita-95-19/cita-95-19_full.uu

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