Galaxy Biasing: Nonlinear, Stochastic and Measurable

Abstract

I describe a general formalism for galaxy biasing (Dekel & Lahav 1998) and its application to measurements of beta (=Omega^0.6/b), e.g. via direct comparisons of light and mass and via redshift distortions. The linear and deterministic relation g=b*d between the density fluctuation fields of galaxies g and mass d is replaced by the conditional distribution P(g|d) of these as random fields, smoothed at a given scale and at a given time. The mean biasing and its nonlinearity are characterized by the conditional mean =b(d)*d and the local scatter by the conditional variance s_b^2(d). This scatter arises from hidden effects on galaxy formation and from shot noise. For applications involving second-order local moments, the biasing is defined by three natural parameters: the slope b_h of the regression of g on d (replacing b), a nonlinearity parameter b_t, and a scatter parameter s_b. The ratio of variances b_v^2 and the correlation coefficient r mix these parameters. The nonlinearity and scatter lead to underestimates of order b_t^2/b_h^2 and s_b^2/b_h^2 in the different estimators of beta, which may partly explain the range of estimates. Local stochasticity affects the redshift-distortion analysis only by limiting the useful range of scales. In this range, for linear stochastic biasing, the analysis reduces to Kaiser's formula for b_h (not b_v) independent of the scatter. The distortion analysis is affected by nonlinearity but in a weak way. Estimates of the nontrivial features of the biasing scheme are made based on simulations and toy models, and a new method for measuring them via distribution functions is proposed.