Approaching a homogeneous galaxy distribution: results from the Stromlo-APM redshift survey

Abstract

Recent results from a number of redshift surveys suggest that the Universe is well described by an inhomogeneous, fractal distribution on the largest scales probed. This distribution has been found to have fractal dimension, D, approximately equal to 2.1, in contrast to a homogeneous distribution in which the dimension should approach the value 3 as the scale is increased. In this paper we demonstrate that estimates of D, based on the conditional density of galaxies, are prone to bias from several sources. These biases generally result in a smaller measured fractal dimension than the true dimension of the sample. We illustrate this behaviour in application to the Stromlo-APM redshift survey, showing that this dataset in fact provides evidence for fractal dimension increasing with survey depth. On the largest scale probed, r approximately 60 h^{-1}Mpc, we find evidence for a distribution with dimension D = 2.76 +/- 0.10. A comparison between this sample and mock Stromlo-APM catalogues taken from Nbody simulations (which assume a CDM cosmology) reveals a striking similarity in the behaviour of the fractal dimension. Thus we find no evidence for inhomogeneity in excess of that expected from conventional cosmological theory. We consider biases affecting future large surveys and demonstrate, using mock SDSS catalogues, that this survey will be able to measure the fractal dimension on scales at which we expect to see full turn-over to homogeneity, in an accurate and unbiased way.