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

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Abstract
We have determined the mass profiles of dark halos in 83 objects observed by ASCA. The point-spread function of the 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 Navarro, Frenk, & White model in (0.01-1.0)rvirial. We found a good correlation between the scale radius rs and the characteristic mass density δc, which indicates the self-similarity of dark halos. The spectrum index of primordial density fluctuation, P(k) ∝ kn, was determined from the slope of the rs-δc relation. For M200 = 1012-1015 M☉, 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 = zf. Assuming δc ∝ (1 + zf)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 + zf 15 and at 1 + zf 7-10, respectively.
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