Probing the Intrinsic Shape and Alignment of Dark Matter Haloes using SDSS Galaxy Groups

TL;DR

This study investigates the shapes and alignments of dark matter haloes in galaxy groups from SDSS data, revealing mass-dependent halo shapes and satellite galaxy alignments with central galaxies.

Contribution

It provides new insights into the three-dimensional shapes of dark matter haloes and their dependence on mass, as well as the alignment between central and satellite galaxies.

Findings

Halo shapes depend strongly on halo mass, with low-mass haloes nearly spherical and massive haloes prolate.

Average 3D axis ratio for certain haloes is about 1:0.46:0.46, with a projected axis ratio of ~0.77.

Satellite galaxies tend to align with the major axis of the central galaxy, influenced by halo mass and galaxy color.

Abstract

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 colors of their central galaxies. Our analysis further shows that the average three-dimensional axis ratio for haloes with $12<{\rm log}[M/(h^{-1}M_{\odot})]\leq15$ is about $1:0.46:0.46$, resulting in a projected axis ratio of $\sim 0.77$. 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 colors. (abridged)