Constraints on Ω from the IRAS redshift surveys

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 \ \ f (\) /b \ \^0. 6/b, where Omega 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, beta, and a small-scale velocity dispersion sigmaᵥ. Tests on N-body simulations show that this model recovers beta 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 beta=0. 52 +/- 0. 13 and beta=0. 54 +/- 0. 3, respectively. These results imply Omega approximately 0. 35 if galaxies trace mass, or a bias factor of about 2 if Omega=1.