HST Grism Confirmation of Two z ~ 2 Structures from the Clusters around Radio-loud AGN (CARLA) Survey

Using Hubble Space Telescope slitless grism data, we report the spectroscopic confirmation of two distant structures at z ~ 2 associated with powerful high-redshift radio-loud active galactic nuclei (AGNs). These rich structures, likely (forming) clusters, are among the most distant structures currently known, and were identified on the basis of Spitzer/IRAC [3.6]–[4.5] color. We spectroscopically confirm nine 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 8 star-forming galaxies and 2 AGNs, 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 AGNs. We derive median (mean) star-formation rates of ~ 35 M_⊙ yr^(-1) (~ 50 M_⊙ yr^(-1)) for the confirmed star-forming members of both structures based on their [O iii]λ 5007 luminosities, and estimate average galaxy stellar masses ≾ 1 x 10^(11) M_⊙ based on mid-infrared fluxes and spectral energy distribution modeling. Most of our confirmed members are located above the star-forming main sequence toward 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.