Probing the intrinsic shape and alignment of dark matter haloes using SDSS galaxy groups

We study the three-dimensional and projected shapes of galaxy groups in the Sloan Digital Sky Survey Data Release 4, and examine the alignment between the orientation of the central galaxy and the spatial distribution of satellite galaxies. The projected ellipticity of a group is measured using the moments of the discrete distribution of its member galaxies. We infer the three-dimensional and projected axis ratios of their dark matter haloes by comparing the measured ellipticity distributions with those obtained from Monte Carlo simulations of projected, triaxial dark matter haloes with different axis ratios. We find that the halo shape has a strong dependence on the halo mass. While the haloes of low-mass groups are nearly spherical, those of massive groups tend to be prolate. For groups containing at least four members, the statistical distribution of their measured ellipticities does not have a strong dependence on the colours of their central galaxies. Our analysis further shows that the average three-dimensional axis ratio for haloes with 12 < log[M/(h(-1) M-circle dot)<= 15 is about 1 : 0.46 : 0.46, resulting in a projected axis ratio of similar to 0.77. Our results for the alignment between the orientation of the central galaxy of a group and the distribution of their satellite galaxies are in broad agreement with those obtained by Yang et al. The distribution of satellite galaxies preferentially aligns with the major axis of the central galaxy, with a clear dependence on both halo mass and galaxy colours. In particular, the alignment is stronger in more massive groups, and the strongest alignment is seen between red centrals and the distribution of red satellites. For groups with blue centrals, no significant alignment is detected. Finally, we examine how the observed alignment can be reproduced with the information about the halo axis ratios. The observed alignment signal can be reproduced if the angle between the major axis of the central galaxy and the projected major axis of the host halo has a Gaussian distribution with a mean of 0 degrees and a dispersion of similar to 23 degrees. This dispersion is larger for groups with blue centrals than those with red centrals.