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

International audienceUsing 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× {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