Galaxy Merger Fractions in Two Clusters at $z\sim2$ Using the Hubble
  Space Telescope
  We measure the fraction of galaxy-galaxy mergers in two clusters at $z\sim2$
using imaging and grism observations from the {\it Hubble Space Telescope}. The
two galaxy cluster candidates were originally identified as overdensities of
objects using deep mid-infrared imaging and observations from the {\it Spitzer
Space Telescope}, and were subsequently followed up with HST/WFC3 imaging and
grism observations. We identify galaxy-galaxy merger candidates using high
resolution imaging with the WFC3 in the F105W, F125W, and F160W bands. Coarse
redshifts for the same objects are obtained with grism observations in G102 for
the $z\sim1.6$ cluster (IRC0222A) and G141 for the $z\sim2$ cluster (IRC0222B).
Using visual classifications as well as a variety of selection techniques, we
measure merger fractions of $11_{-3.2}^{+8.2}$ in IRC0222A and
$18_{-4.5}^{+7.8}$ in IRC0222B. In comparison, we measure a merger fraction of
$5.0_{-0.8}^{+1.1}\%$ for field galaxies at $z\sim2$. Our study indicates that
the galaxy-galaxy merger fraction in clusters at $z\sim2$ is enhanced compared
the field population, but note that more cluster measurements at this epoch are
needed to confirm our findings.
