Understanding the shape of the galaxy two-point correlation function at z~1 in the COSMOS field

TL;DR

This study shows that the flat galaxy two-point correlation function at z~1 in the COSMOS field results from over-representation of high-density environments, and removing these regions aligns observations with LCDM predictions.

Contribution

It demonstrates that local environment density variations significantly influence the shape of the galaxy two-point correlation function at z~1, explaining previous anomalies.

Findings

Excluding high-density regions steepens the correlation function to match LCDM predictions.

Over-abundance of high-density environments causes the observed flat correlation function.

The effect is consistent with halo-environment correlations in hierarchical models.

Abstract

We investigate how the shape of the galaxy two-point correlation function as measured in the zCOSMOS survey depends on local environment, quantified in terms of the density contrast on scales of 5 Mpc/h. We show that the flat shape previously observed at redshifts between z=0.6 and z=1 can be explained by this volume being simply 10% over-abundant in high-density environments, with respect to a Universal density probability distribution function. When galaxies corresponding to the top 10% tail of the distribution are excluded, the measured w_p(r_p) steepens and becomes indistinguishable from LCDM predictions on all scales. This is the same effect recognised by Abbas & Sheth in the SDSS data at z~0 and explained as a natural consequence of halo-environment correlations in a hierarchical scenario. Galaxies living in high-density regions trace dark matter halos with typically higher masses, which are more correlated. If the density probability distribution function of the sample is particularly rich in high-density regions because of the variance introduced by its finite size, this produces a distorted two-point correlation function. We argue that this is the dominant effect responsible for the observed "peculiar" clustering in the COSMOS field.