The effects of halo alignment and shape on the clustering of galaxies

TL;DR

This study examines how halo shape and alignment influence galaxy clustering, revealing that deviations from spherical models and halo assembly bias significantly affect clustering measurements on small scales.

Contribution

It demonstrates the impact of halo shape and alignment on galaxy clustering, highlighting the limitations of spherical models and the importance of assembly bias effects.

Findings

Disrupting halo alignment reduces clustering by ~2% at 1.8 Mpc/h.

Sphericalizing haloes decreases small-scale clustering by up to 20%.

Halo assembly bias affects galaxy clustering and can be observed in group catalogues.

Abstract

We investigate the effects of halo shape and its alignment with larger scale structure on the galaxy correlation function. We base our analysis on the galaxy formation models of Guo et al., run on the Millennium Simulations. We quantify the importance of these effects by randomizing the angular positions of satellite galaxies within haloes, either coherently or individually, while keeping the distance to their respective central galaxies fixed. We find that the effect of disrupting the alignment with larger scale structure is a ~2 per cent decrease in the galaxy correlation function around r=1.8 Mpc/h. We find that sphericalizing the ellipsoidal distributions of galaxies within haloes decreases the correlation function by up to 20 per cent for r<1 Mpc/h and increases it slightly at somewhat larger radii. Similar results apply to power spectra and redshift-space correlation functions. Models based on the Halo Occupation Distribution, which place galaxies spherically within haloes according to a mean radial profile, will therefore significantly underestimate the clustering on sub-Mpc scales. In addition, we find that halo assembly bias, in particular the dependence of clustering on halo shape, propagates to the clustering of galaxies. We predict that this aspect of assembly bias should be observable through the use of extensive group catalogues.