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