Triaxial haloes, intrinsic alignments and the dark matter power spectrum

We develop the halo model of large-scale structure to include triaxial dark matter haloes and their intrinsic alignments. As a direct application we derive general expressions for the twopoint correlation function and the power spectrum. We then focus on the power spectrum and numerically solve the general expressions for two different models of the triaxial profiles. The first is a toy model that allows us to isolate the dependence of clustering on halo shape alone and the second is the more realistic profile model of Jing & Suto. In both cases, we find that the effect of triaxiality is manifest as a suppression of power at the level of 5 per cent on scales k 1-10 h Mpc -1 , which in real space corresponds to the virial radii of clusters. When considered by mass, we find that for the first model the effects are again apparent as a suppression of power and that they are more significant for the high-mass haloes. For the Jing & Suto model, we find a suppression of power on large scales followed by a sharp amplification on small scales at the level of 10-15 per cent. Interestingly, when averaged over the entire mass function this amplification effect is suppressed. We also find for the one-halo term on scales k < 10 h Mpc -1 that the power is dominated by ellipsoidal haloes with semi-minor to semi-major axis ratios a/c < 0.7.