Estimating beta from redshift-space distortions in the 2dF galaxy survey

Given the failure of existing models for redshift-space distortions to prov ide a highly accurate measure of the beta-parameter, and the ability of for thcoming surveys to obtain data with very low random errors, it becomes nec essary to develop better models for these distortions. Here we review the f ailures of the commonly used velocity dispersion models and present an empi rical method for extracting beta from the quadrupole statistic that has lit tle systematic offset over a wide range of beta and cosmologies. This empir ical model is then applied to an ensemble of mock 2dF southern strip survey s, to illustrate the technique and see how accurately we can recover the tr ue value of beta. We compare this treatment with the error we expect to fin d caused only by the finite volume of the survey. We find that non-linear e ffects reduce the range of scales over which beta can be fitted, and introd uce covariances between nearby modes in addition to those introduced by the convolution with the survey window function. The result is that we are onl y able to constrain beta to a 1 sigma accuracy of 25 per cent (beta = 0.55 +/- 0.14 for the cosmological model considered). We explore one possible me ans of reducing this error, that of cluster collapse, and show that accurat e application of this method can greatly reduce the effect of non-lineariti es, improving the determination of beta. We conclude by demonstrating that, when the contaminating effects of clusters are dealt with, this simple ana lysis of the full 2dF survey yields beta=0.55 +/- 0.04. For this model, thi s represents a determination of beta to an accuracy of 8 per cent and hence an important constraint on the cosmological density parameter Omega(0).