Determining the Amplitude of Mass Fluctuations in the Universe

Details Determining the Amplitude of Mass Fluctuations in the Universe Determining the Amplitude of Mass Fluctuations in the Universe

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...191.9505f&link_type=abstract

American Astronomical Society, 191st AAS Meeting, #95.05; Bulletin of the American Astronomical Society, Vol. 29, p.1364

Astronomy and Astrophysics

Astronomy

Scientific paper

We present a method for determining the rms mass fluctuations on 8 h(-1) Mpc scale, sigma_ {8}. The method utilizes the rate of evolution of the abundance of rich clusters of galaxies. Using the Press-Schechter approximation, we show that the cluster abundance evolution is a strong function of sigma_ {8}: dlog n/dz ~ -1/sigma_ {8}(2) ; low sigma_ {8} models evolve exponentially faster than high sigma_ {8} models, for a given mass cluster. For example, the number density of Coma-like clusters decreases by a factor of ~ 10(3) from z = 0 to z =~ 0.5 for sigma_ {8}=0.5 models, while the decrease is only a factor of ~ 5 for sigma_ {8} =~ 1. The strong exponential dependence on sigma_ {8} arises because clusters represent rarer density peaks in low sigma_ {8} models. We show that the evolution rate at z ≲ 1 is insensitive to the density parameter Omega or to the exact shape of the power spectrum. Cluster evolution therefore provides a powerful constraint on sigma_ {8}. Using available cluster data to z ~ 0.8, we find sigma_ {8} = 0.83 +/- 0.15. This amplitude implies a bias parameter b =~ sigma_ {8}(-1) = 1.2 +/- 0.2, i.e., a nearly unbiased universe with mass approximately tracing light on large scales.

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