  We investigate the effects of halo ellipticity and alignment with
larger-scale structure on the galaxy correlation function. We base our analysis
on the galaxy formation models of Guo et al. (2011), run on the Millennium
Simulations. We quantify the importance of these properties of the galaxy
distribution 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. Sphericalizing the ellipsoidal
distributions of galaxies within haloes decreases the correlation function by
up to 20 per cent for r<1 Mpc/h. Similar results apply to power spectra and
redshift-space correlation functions. Models such as those based on the Halo
Occupation Distribution, which adopt a spherically averaged profile for the
galaxy distributions within haloes, will therefore significantly underestimate
the clustering on sub-Mpc scales.

Cosmology and Nongalactic Astrophysics