Shapes and alignments of dark matter haloes and their brightest cluster galaxies in 39 strong lensing clusters

Abstract

We study shapes and alignments of 45 dark matter (DM) haloes and their brightest cluster galaxies (BCGs) using a sample of 39 massive clusters from Hubble Frontier Field (HFF), Cluster Lensing And Supernova survey with Hubble (CLASH), and Reionization Lensing Cluster Survey (RELICS). We measure shapes of the DM haloes by strong gravitational lensing, whereas BCG shapes are derived from their light profiles in Hubble Space Telescope images. Our measurements from a large sample of massive clusters presented here provide new constraints on dark matter and cluster astrophysics. We find that DM haloes are on average highly elongated with the mean ellipticity of 0.482 ± 0.028, and position angles of major axes of DM haloes and their BCGs tend to be aligned well with the mean value of alignment angles of 22.2±3.9 deg. We find that DM haloes in our sample are on average more elongated than their BCGs with the mean difference of their ellipticities of 0.11 ± 0.03. In contrast, the Horizon-AGN cosmological hydrodynamical simulation predicts on average similar ellipticities between DM haloes and their central galaxies. While such a difference between the observations and the simulation may well be explained by the difference of their halo mass scales, other possibilities include the bias inherent to strong lensing measurements, limited knowledge of baryon physics, or a limitation of cold dark matter.

Reconciling galaxy cluster shapes, measured by theorists versus observers

“…We find for both the observed and simulated clusters all probes are aligned with the dark matter with a mis-alignment angle of ∼ 20 degrees (depending on the cluster-centric radius). This is again consistent with Okabe et al (2020) who found a mean mis-alignment angle of θ = 22.2 ± 3.9 degrees. However, we do find that although the simulations are consistent with the observations, they do suggest a higher variance and random mis-alignments.…”

“…Interestingly we find the X-ray and weak lensing estimates are both more spherical than the strong lensing, whilst the cluster members BCG and the moment of inertia are much more elliptical. This is slightly inconsistent with Okabe et al (2020) who found that the probes all tended to be less elliptical than the strong lensing estimate.…”

“…As such, it raises the question of the comparabil-ity of the full sample of currently observed clusters with simulations. A recent study compared the complete sample of CLASH, HFF and RELICS with simulations from the Horizon-AGN suite of simulations (Okabe et al 2020). An interesting study that found the observed clusters appeared to have strong lensing regions significantly more elliptical than the BCG.…”

Measuring the surface mass density ellipticity of redMaPPer galaxy clusters using weak-lensing

“…Also, the observed tendency between the low-and high-redshift cluster samples is obtained regardless which weight is considered in order to compute the cluster orientation angle. Furthermore, the average projected ellipticity estimated for the high-redshift sample ( z = 0.35), is in agreement with the analysis of Okabe et al (2020) (see Fig. 9 of their paper) based on the cosmological hydrodynamical simulation Horizon-AGN (Dubois et al 2014).…”

“…This effect can be well modelled to obtain the projected ellipticity for individual targets as long as massive galaxy clusters or deep data are considered (e.g. Oguri et al 2010;Harvey et al 2019;Okabe et al 2020). Nevertheless, when lower mass halos are analysed, constraining the lensing signal is a hard task.…”