The Observed Mass Profiles of Dark Halos and the Formation Epoch of Galaxies

Abstract

We have determined the mass profiles of dark halos in 83 objects observed by ASCA. The point spread function of X-ray telescope was deconvoled by the Richardson-Lucy algorithm and the temperature profiles were calculated to obtain the mass profiles. The derived mass profiles are consistent with the NFW model in 0.01-1.0 r_virial. We found a good correlation between the scale radius r_s and the characteristic mass density \delta_c, which indicates the self-similarity of dark halos. The spectrum index of primordial density fluctuation, P(k) \propto k^n, was determined from the slope of r_s - delta_c relation. For M_200=10^12-10^15 M_solar, our analysis gives n=-1.2 +/- 0.3 with a confidence level of 90%. The mass density of dark halos is a good indicator of the mean mass density of the universe at the time when the halos were assembled, z=z_f. Assuming delta_c \propto (1+z_f)^3, we have determined the epoch when each dark halo was assembled. Our analysis indicates that the field elliptical galaxies and groups of galaxies formed approximately at 1+z_f ~ 15 and at 1+z_f ~ 7-10 respectively.