Topology of non-linear structure in the 2dF Galaxy Redshift Survey

TL;DR

This study analyzes the non-linear evolution of cosmic structures in the 2dF Galaxy Redshift Survey using topology measures, revealing scale-dependent effects of gravitational evolution and galaxy bias that challenge existing analytic models.

Contribution

It provides the first detailed comparison of observed topological genus statistics with analytic predictions in the weakly non-linear regime, highlighting discrepancies and scale-dependent effects.

Findings

Detected significant asymmetry in genus statistics between high- and low-density regions.

Observed non-linear effects evolve with scale, affecting the topology of galaxy distributions.

Existing analytic models do not fully reproduce the measurements, especially in the non-linear regime.

Abstract

We study the evolution of non-linear structure as a function of scale in samples from the 2dF Galaxy Redshift Survey, constituting over 221 000 galaxies at a median redshift of z=0.11. The two flux-limited galaxy samples, located near the southern galactic pole and the galactic equator, are smoothed with Gaussian filters of width ranging from 5 to 8 Mpc/h to produce a continuous galaxy density field. The topological genus statistic is used to measure the relative abundance of overdense clusters to void regions at each scale; these results are compared to the predictions of analytic theory, in the form of the genus statistic for i) the linear regime case of a Gaussian random field; and ii) a first-order perturbative expansion of the weakly non-linear evolved field. The measurements demonstrate a statistically significant detection of an asymmetry in the genus statistic between regions corresponding to low- and high-density volumes of the universe. We attribute the asymmetry to the non-linear effects of gravitational evolution and biased galaxy formation, and demonstrate that these effects evolve as a function of scale. We find that neither analytic prescription satisfactorily reproduces the measurements, though the weakly non-linear theory yields substantially better results in some cases, and we discuss the potential explanations for this result.