The 2dF Galaxy Redshift Survey: stochastic relative biasing between galaxy populations
Abstract
It is well known that the clustering of galaxies depends on galaxy type. Such relative bias complicates the inference of cosmological parameters from galaxy redshift surveys, and is a challenge to theories of galaxy formation and evolution. In this paper we perform a joint countsincells analysis on galaxies in the 2dF Galaxy Redshift Survey, classified by both colour and spectral type, η, as early or latetype galaxies. We fit three different models of relative bias to the joint probability distribution of the cell counts, assuming Poisson sampling of the galaxy density field. We investigate the nonlinearity and stochasticity of the relative bias, with cubic cells of side 10 <=L<= 45 Mpc (h= 0.7). Exact linear bias is ruled out with high significance on all scales. Powerlaw bias gives a better fit, but likelihood ratios prefer a bivariate lognormal distribution, with a nonzero `stochasticity', i.e. scatter that may result from physical effects on galaxy formation other than those from the local density field. Using this model, we measure a correlation coefficient in logdensity space (r_{LN}) of 0.958 for cells of length L= 10 Mpc, increasing to 0.970 by L= 45 Mpc. This corresponds to a stochasticity of 0.44 +/ 0.02 and 0.27 +/ 0.05, respectively. For smaller cells, the Poissonsampled lognormal distribution presents an increasingly poor fit to the data, especially with regard to the fraction of completely empty cells. We compare these trends with the predictions of semianalytic galaxy formation models: these match the data well in terms of the overall level of stochasticity, variation with scale and the fraction of empty cells.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 January 2005
 DOI:
 10.1111/j.13652966.2004.08447.x
 arXiv:
 arXiv:astroph/0404275
 Bibcode:
 2005MNRAS.356..247W
 Keywords:

 methods: statistical;
 surveys;
 galaxies: distances and redshifts;
 galaxies: statistics;
 largescale structure of Universe;
 Astrophysics
 EPrint:
 24 pages, 13 figures, LaTex, minor changes to match version accepted to MNRAS. Version with high resolution and colour figures available from http://www.ast.cam.ac.uk/~vw/stocbias.html