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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.