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Published October 20, 2016 | Published
Journal Article Open

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.

Additional Information

© 2016. The American Astronomical Society. Received 2016 January 31; revised 2016 July 20; accepted 2016 July 29; published 2016 October 14. We thank our anonymous referee for comments and suggestions that improved the quality of this paper. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This work is also based in part on observations made with the 200 inch Hale Telescope, Palomar Observatory, operated by the California Institute of Technology. D.W. acknowledges support by Akbari-Mack Postdoctoral Fellowship. S.M. acknowledges financial support from the Institut Universitaire de France (IUF), of which she is senior member. E.A.C. acknowledges the support of the STFC. N.A.H. acknowledges support from STFC through an Ernest Rutherford Fellowship. Facilities: HST (WFC3, STScI), Spitzer (IRAC, JPL/Caltech), Palomar (DBSP, Caltech).

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