The impact of halo shapes on the bispectrum in cosmology

Abstract

(Abridged) We use the triaxial halo model of Smith & Watts to investigate the impact of dark matter halo shapes on the bispectrum. Analytic expressions are derived and evaluated through numerical integration. Two models for the ellipsoidal halo profiles are considered: a toy model designed to isolate the effects of halo shape on the clustering alone; and the realistic model of Jing & Suto. For equilateral k-space triangles, we show that the predictions of the triaxial model are suppressed, relative to the spherical model, by roughly ~7% and ~4% for the two profiles respectively. The reduced bispectrum as a function of triangle configuration is found to be very sensitive to halo shapes on small scales. The generic features are that, relative to the spherical model, the signal is suppressed for k-space triangles that are close to equilateral and boosted for configurations that are colinear. This appears to be a unique signature of halo triaxiality. The galaxy bispectrum is also explored. Two models for the halo occupation distribution (HOD) are considered: the binomial distribution of Scoccimarro et al. and the Poisson satellite model of Kravtsov et al. Our predictions show that the galaxy bispectrum is also sensitive to halo shapes, although relative to the mass, the effects are reduced. The HOD of Kravtsov et al. is found to be more sensitive. This owes to the mean number of galaxies per halo being a steeper function of mass in this model, hence giving stronger weight to the high mass (more triaxial) haloes. Interestingly, the functional form is not strongly sensitive to the details of the HOD, but is mainly determined by the halo shape. However, a combination of measurements made on different scales and for different k-space triangles are sensitive to both halo shape and the HOD.