Precise weak lensing constraints from deep high-resolution
  $K_\mathrm{s}$ images: VLT/HAWK-I analysis of the super-massive galaxy
  cluster RCS2$J$232727.7$-$020437 at $z=0.70$
  We demonstrate that deep good-seeing VLT/HAWK-I $K_\mathrm{s}$ images
complemented with $g$+$z$-band photometry can yield a sensitivity for weak
lensing studies of massive galaxy clusters at redshifts \mbox{$0.7\lesssim z
\lesssim 1.1$}, which is almost identical to the sensitivity of HST/ACS mosaics
of single-orbit depth. Key reasons for this good performance are the excellent
image quality frequently achievable for $K_\mathrm{s}$ imaging from the ground,
a highly effective photometric selection of background galaxies, and a galaxy
ellipticity dispersion that is noticeably lower than for optically observed
high-redshift galaxy samples. Incorporating results from the 3D-HST and
UltraVISTA surveys we also obtained a more accurate calibration of the source
redshift distribution than previously achieved for similar optical weak lensing
data sets. Here we studied the extremely massive galaxy cluster
RCS2$J$232727.7$-$020437 (\mbox{$z=0.699$}), combining deep VLT/\mbox{HAWK-I}
$K_\mathrm{s}$ images (point spread function with a 0\farcs35 full width at
half maximum) with LBT/LBC photometry. The resulting weak lensing mass
reconstruction suggests that the cluster consists of a single overdensity,
which is detected with a peak significance of $10.1\sigma$. We constrained the
cluster mass to \mbox{$M_\mathrm{200c}/(10^{15} \mathrm{M}_\odot)
=2.06^{+0.28}_{-0.26}(\mathrm{stat.})\pm 0.12 (\mathrm{sys.})$} assuming a
spherical Navarro, Frenk \& White model and simulation-based priors on the
concentration, making it one of the most massive galaxy clusters known in the
\mbox{$z\gtrsim 0.7$} Universe. We also cross-checked the HAWK-I measurements
through an analysis of overlapping HST/ACS images, yielding fully consistent
estimates of the lensing signal.
