Constraints on Ω from the IRAS redshift surveys

Abstract

We measure the anisotropy of the redshift-space power spectrum in the 1.2-Jy and QDOT redshift surveys of IRAS-selected galaxies. On large scales, this anisotropy is caused by coherent peculiar motions, and gravitational instability theory predicts a distortion of the power spectrum that depends only on the ratio $$\beta\equiv f(\Omega)/b\approx\Omega^{0.6}/b$$, where Ω is the cosmological density parameter and b is the bias parameter. On small scales, the distortion is dominated by the random velocity dispersion in non-linear structures. We fit the observed anisotropy with an analytic model that incorporates two parameters, β and a small-scale velocity dispersion $$\sigma_\upsilon$$. Tests on N-body simulations show that this model recovers β quite accurately on the scales accessible to the existing IRAS redshift surveys. Applying our procedure to the 1.2-Jy and QDOT surveys, we find β = 0.52 ± 0.13 and β = 0.54±0.3, respectively. These results imply Ω ≈ 0.35 if galaxies trace mass, or a bias factor of about 2 if Ω = 1.