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 \pm 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 \pm 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 \pm 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 .