Anisotropic Haloes and Intrinsic Alignments

Abstract

Dark matter haloes are structures of gravitationally bound matter found at the nodes of the cosmic web. The halo model uses these as building blocks of the universe, and provides an empirical method for studying its large-scale structure. However, in the standard approach, haloes are incorrectly assumed to be spherical. In this work, we study the use of the anisotropic (triaxial) halo model (AHM) for new and existing observables. This model is physically more accurate, but also presents considerable numerical difficulties due to high-dimensional integrals that need to be computed. We first consider the matter distribution through its power spectrum, computing the integrals using Monte Carlo integration. Extending previous results to higher redshifts we can calculate angular power spectra, which we use to study the effect of modelling choices on the estimate of cosmological parameters, using a Fisher matrix analysis and a Markov Chain Monte Carlo method. We find significant biases for using both the isotropic (spherical) and anisotropic halo models, and conclude these should not be used for the matter power spectrum. Next, we extend the AHM to predictions of intrinsic alignments (IAs) of galaxies, in order to better model aspects of the small-scale environment of haloes like the anisotropic distribution of satellite galaxies. We derive analytic expressions for different correlation functions and power spectra. Due to the projection involved in observables of IA, our expressions are even more complex to compute than for matter correlations, and require further simplifications or a more efficient implementation before they can yield to computation. We suggest paths forward for these issues, and conclude with additional proposals for studying the AHM.