Carnegie Mellon University

# Astro Lunch

## A Flexible Halo Model for the Intrinsic Alignment of Galaxies

Galaxies exhibit alignments with both close neighbours and the large scale structure.  The nature of intrinsic alignments is interesting for both testing galaxy formation models and in cosmological probes of dark energy.  Any intrinsic alignment between galaxies may be a significant source of systematic error in cosmological probes of dark energy which rely on weak lensing.  Because upcoming surveys rely on photometric redshifts, some lens and source galaxies may be physically close, and any intrinsic alignment between galaxies will masquerade as a lensing signal.  In addition, intrinsic alignments with large scale structure inject an unavoidable systematic that must be modeled out.  In this talk, I will present a flexible, empirical, halo based model for the intrinsic alignments of galaxies.  I examine the necessary components and phenomenology of such a model by examining the alignments between (sub-)haloes in a cosmological dark matter only simulation.  The primary assumption of this model is that galaxies exhibit an alignment with their host dark matter (sub-)halo.  By modelling the misalignment between galaxies' orientation and their host halo, we show that the (sub-)halo two-point position and shape correlation functions can be accurately reproduced down to $0.1~h^{-1}{\rm Mpc}$.  To model the small scale alignments of dark matter sub-haloes, a radial alignment model is sufficient when combined with a model for satellite anisotropy.