HST Grism Confirmation of Two $z \sim 2$ Structures from the Clusters
  Around Radio-Loud AGN (CARLA) Survey
  Using HST slitless grism data, we report the spectroscopic confirmation of
two distant structures at $z \sim 2$ associated with powerful high-redshift
radio-loud AGN. These rich structures, likely (forming) clusters, are among the
most distant currently known and were identified on the basis of Spitzer/IRAC
[3.6] - [4.5] color. We spectroscopically confirm 9 members in the field of MRC
2036-254, comprising eight star-forming galaxies and the targeted radio galaxy.
The median redshift is $z = 2.000$. We spectroscopically confirm 10 members in
the field of B3 0756+406, comprising eight star-forming galaxies and two AGN,
including the targeted radio-loud quasar. The median redshift is $z = 1.986$.
All confirmed members are within 500 kpc (1 arcmin) of the targeted AGN. We
derive median (mean) star-formation rates of $\sim 35~M_{\odot}\rm ~ yr^{-1}$
($\sim 50~M_{\odot}\rm ~ yr^{-1}$) for the confirmed star-forming members of
both structures based on their [OIII]$\lambda5007$ luminosities, and estimate
average galaxy stellar masses $\lesssim 1 \times 10^{11} ~M_{\odot}$ based on
mid-infrared fluxes and SED modeling. Most of our confirmed members are located
above the star-forming main-sequence towards starburst galaxies, consistent
with clusters at these early epochs being the sites of significant levels of
star formation. The structure around MRC 2036-254 shows an overdensity of
IRAC-selected candidate galaxy cluster members consistent with being quiescent
galaxies, while the structure around B3 0756+406 shows field values, albeit
with many lower limits to colors that could allow an overdensity of faint red
quiescent galaxies. The structure around MRC 2036-254 shows a red sequence of
passive galaxy candidates.
