Large-scale fluctuations in the distribution of galaxies from the two-degree galaxy redshift survey

¹ Centro Studi e Ricerche Enrico Fermi, via Panisperna 89 A, Compendio del Viminale, 00184 Rome, Italy e-mail: Francesco.SylosLabini@roma1.infn.it
² Istituto dei Sistemi Complessi CNR, via dei Taurini 19, 00185 Rome, Italy
³ Institute of Astronomy, St.Petersburg State University, Staryj Peterhoff, 198504, St. Petersburg, Russia

Received: 11 July 2008
Accepted: 5 December 2008

Abstract

We study statistical properties of galaxy structures in several samples extracted from the two-degree galaxy redshift survey (2dFRGS). In particular, we measure conditional fluctuations by means of the scale-length method and determined their probability distribution. In this way we find that galaxy distribution in these samples is characterized by large amplitude fluctuations with a large spatial extension, whose size is only limited by the sample's boundaries. These fluctuations are quite typical and persistent in the sample's volumes, and they are detected in two independent regions in the northern and southern galactic caps. We discuss the relation of the scale-length method to several statistical quantities, such as counts of galaxies as a function of redshift and apparent magnitude. We confirm previous results, which have determined by magnitude and redshift counts that there are fluctuations of about 30% between the southern and the northern galactic caps and we relate explicitly these counts to structures in redshift space. We show that the estimation of fluctuation amplitude normalized to the sample density is biased by systematic effects, which we discuss in detail. We consider the type of fluctuations predicted by standard cosmological models of structure formation in the linear regime and, to study nonlinear clustering, we analyze several samples of mock-galaxy catalogs generated from the distribution of dark matter in cosmological N-body simulations. In this way we conclude that the galaxy fluctuations present in these samples are too large in amplitude and too extended in space to be compatible with the predictions of the standard models of structure formation.