Cosmological evolution of orientations of cluster-sized dark matter haloes and their central galaxies in the Horizon-AGN simulation

Abstract

It is known observationally that the major axes of galaxy clusters and their brightest cluster galaxies are roughly aligned with each other. To understand the origin of the alignment, we identify 40 cluster-sized dark matter (DM) haloes with masses higher than 5 × 10 13 M and their central galaxies (CGs) at z ≈ 0 in the Horizon-AGN cosmological hydrodynamical simulation. We trace the progenitors at 50 different epochs between 0 < z < 5. We then fit their shapes and orientations with a triaxial ellipsoid model. While the orientations of both DM haloes and CGs change significantly due to repeated mergers and mass accretions, their relative orientations are well aligned at each epoch even at high redshifts, z > 1. The alignment becomes tighter with cosmic time; the major axes of the CGs and their host DM haloes at present are aligned on average within ∼ 30 • in the three dimensional space and ∼ 20 • in the projected plane. The orientations of the major axes of DM haloes on average follow one of the eigen-vectors of the surrounding tidal field that corresponds to the slowest collapsing (or even stretching) mode, and the alignment with the tidal field also becomes tighter. This implies that the orientations of CGs and DM haloes at the present epoch are largely imprinted in the primordial density field of the Universe, whereas strong dynamical interactions such as mergers are important to explain their mutual alignment at each epoch.

Galaxy-group (halo) alignments from SDSS DR7 and the ELUCID simulation

“…Such kind of mass dependence of the alignment signal is also found in the hydrodynamical simulations (Velliscig et al 2015;Tenneti et al 2020). From the Horizon-AGN simulation, Okabe et al (2020) claimed that the major axes of central galaxies are aligned with their subhalo systems with the average angle ∼ 20 degrees in the projected plane for 40 cluster-sized halos with masses larger that 5×10 13 h −1 M⊙. As shown in Figure 10, the two-dimensional average alignment angle is θ = 17 of 13 ≤ log(M h / h −1 M⊙) ≤ 14, which are in agreement with the results of Okabe et al (2020).…”

“…Besides the few observational measurements using galaxy clusters, a vast majority of studies in literature rely on the cosmological hydrodynamical simulations to determine the alignment between galaxies and their host halos (Bett et al 2010;Okabe et al 2018Okabe et al , 2020Brainerd & Yamamoto 2019;Bhowmick et al 2020;Soussana et al 2020;Tang et al 2020;Tenneti et al 2020). There is general agreement in literature that the major axes of central galaxies are well aligned with their halos, and the alignment signals are stronger galaxies in more massive halos.…”

Evolution and role of mergers in the BCG–cluster alignment. A view from cosmological hydrosimulations

“…Once again, we find just a very weak, if not negligible, tendency for a better alignment with time. By converse, Okabe et al (2020) claim for a clear improvement of the alignment towards z = 0. However, we note that they study a significantly less massive set of clusters and do not remove subhaloes to describe the DM distribution.…”

“…Most previous analyses of the BCG-cluster alignment based on cosmological hydrodynamical simulations (e.g. Dong et al 2014;Tenneti et al 2015;Velliscig et al 2015;Okabe et al 2020;Zhang & Wang 2019) have been focused on smaller mass clusters ( a few 10 14 M ), selected in significantly smaller volumes than that of our parent simulation. Moreover, in this work, we are specifically interested in quantifying the role of major mergers (mass ratio >0.25) on the alignment.…”

Brightest Cluster Galaxies: the centre can(not?) hold

“…★ E-mail: rodepr@utu.fi Biernacka et al 2015). This is believed to be a relic of their formation history, as it is found even at high redshifts both in observations (Li et al 2013;West et al 2017) and cosmological hydrodynamical simulations (Okabe et al 2020b;Ragone-Figueroa et al 2020). The conventional explanation is that the BCG lies at the centre of the forming cluster halo and accretes galaxies along a preferential direction (collimated infall) coinciding with the major accretion filament feeding the cluster growth within the cosmic web (e.g., West 1994;Dubinski 1998 see also Donahue et al 2016;Okabe et al 2020b,a;Ragone-Figueroa et al 2020).…”