2015/09/22 by Xi Kang, Peng Wang · 51 citations
Engineering · Physics and Astronomy · #Accretion (finance) #Anisotropy #Astronomy and Astrophysical Research #Cosmology #Dark matter #Dark matter halo #Galaxies: Formation, Evolution, Phenomena #Galaxy #Galaxy formation and evolution #Halo #Space Technology and Applications #astro-ph.CO
paper · pdf · doi:10.1088/0004-637x/813/1/6
published in The Astrophysical Journal 813(1), 6 (IOP Publishing) · 9 pages, 6 figures, accepted to ApJ
arxiv created 2015/09/22 · openalex publication_date 2015/10/21 · arxiv updated 2015/10/28 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
The distribution of galaxies displays anisotropy on different scales and it is often referred to as galaxy alignment. To understand the origin of galaxy alignments on small scales, one must investigate how galaxies were accreted in the early universe and quantify their primordial anisotropy at the time of accretion. In this paper we use N -body simulations to investigate the accretion of subhalos, focusing on their alignment with halo shape and the orientation of mass distribution on the large scale, defined using the Hessian matrix of the density field. The large/small ( e 1/ e 3) eigenvalues of the Hessian matrix define the fast/slow collapse direction of matter on the large scale. We find that: (1) the halo major axis is well aligned with the e 3 (slow collapse) direction, and it is stronger for massive halos; (2) subhalos are predominantly accreted along the major axis of the host halo, and the alignment increases with the host halo mass. Most importantly, this alignment is universal; (3) accretion of subhalos with respect to the e 3 direction is not universal. In massive halos, subhalos are accreted along the e 3 (even more strongly than the alignment with the halo major axis), but in low-mass halos subhalos are accreted perpendicular to e 3. The transitional mass is lower at high redshift. The last result well explains the puzzling correlation (both in recent observations and simulations) that massive galaxies/halos have their spin perpendicular to the filament, and the spin of low-mass galaxies/halos is slightly aligned with the filament, under the assumption that the orbital angular momentum of subhalos is converted to halo spin.