Projected alignment of non-sphericities of stellar, gas, and dark matter distributions in galaxy clusters: analysis of the Horizon-AGN simulation

Abstract

While various observations measured ellipticities of galaxy clusters and alignments between orientations of the brightest cluster galaxies and their host clusters, there are only a handful of numerical simulations that implement realistic baryon physics to allow direct comparisons with those observations. Here we investigate ellipticities of galaxy clusters and alignments between various components of them and the central galaxies in the state-of-the-art cosmological hydrodynamical simulation Horizon-AGN, which contains dark matter, stellar, and gas components in a large simulation box of (100h −1 Mpc) 3 with high spatial resolution (∼ 1 kpc). We estimate ellipticities of total matter, dark matter, stellar, gas surface mass density distributions, X-ray surface brightness, and the Compton y-parameter of the Sunyaev-Zel'dovich effect, as well as alignments between these components and the central galaxies for 120 projected images of galaxy clusters with masses M 200 > 5 × 10 13 M . Our results indicate that the distributions of these components are well aligned with the major-axes of the central galaxies, with the root mean square value of differences of their position angles of ∼ 20 • , which vary little from inner to the outer regions. We also estimate alignments of these various components with total matter distributions, and find tighter alignments than those for central galaxies with the root mean square value of ∼ 15 • . We compare our results with previous observations of ellipticities and position angle alignments and find reasonable agreements. The comprehensive analysis presented in this paper provides useful prior information for analyzing stacked lensing signals as well as designing future observations to study ellipticities and alignments of galaxy clusters.

Cluster–galaxy weak lensing

“…Umetsu et al (2018) obtained stacked constraints on the projected axis ratio of q ¼ 0:67 AE 0:10 (or 1 À q ¼ 0:33 AE 0:10), which is fully consistent with the median axis ratio q ¼ 0:67 AE 0:07 of this sample obtained from their two-dimensional shear and magnification analysis of the 20 individual clusters. Their results suggest that the total matter distribution is closely aligned with the X-ray brightness distribution (with a median misalignment angle of jDPAj ¼ 21 AE 7 ) as expected from cosmological hydrodynamical simulations (see Okabe et al 2018). Clampitt and Jain (2016).…”

“…More massive halos are less spherical and more prolate, as they tend to form later. The projected matter distributions around clusters are thus expected to be anisotropic, with typical axis ratios of q $ 0:6 (e.g., Okabe et al 2018). The projected axis ratio of cluster halos varies slowly with cluster-centric distance (e.g., Okabe et al 2018).…”

The Influence of the Mass Distribution of Stellar Objects on Their Gravitational Fields

“…2008;Pereira et al 2008;Bett et al 2010;Paz et al 2011;Kimm et al 2011;Varela et al 2012). Recently, the results of Okabe et al (2018); Codis et al (2018) based on Horizon-AGN simulation show a similar dependence. This allows to conclude that the analysis of angular momentum of luminous matter gives also information about the angular momentum of the total structure (i.e.…”

Shape and connectivity of groups and clusters: Effect of the dynamical state and accretion history

“…Nevertheless, it is important to note that the observational results based on both X-ray and Sunyaev-Zeldovich effect (SZ) only probe the intracluster medium of groups and clusters and mainly focus on the cluster core (<R 500 ). Moreover, given the collisional nature of the gaseous component of clusters, the gas mass distribution (and hence its X-ray surface brightness and the Compton y-parameter) is significantly more circular than the stellar and dark matter counterparts (as predicted by Okabe et al 2018;Harvey et al 2021, using hydrodynamical simulations). Moreover, simulations show that the shapes of different cluster-component distributions (stars, gas, and dark matter) qualitatively follow similar trends with halo mass, radius, and redshift (Velliscig et al 2015;Okabe et al 2018).…”