$HST$ Grism Confirmation of $16$ Structures at $1.4<z<2.8$ from the Clusters Around Radio-Loud AGN (CARLA) Survey

Abstract

We report spectroscopic results from our 40-orbit $HubbleSpaceTelescope$ slitless grism spectroscopy program observing the 20 densest CARLA candidate galaxy clusters at $1.4<z<2.8$. These candidate rich structures, among the richest and most distant known, were identified on the basis of $[3.6]-[4.5]$ color from a $408$hr multi-cycle $Spitzer$ program targeting $420$ distant radio-loud AGN. We report the spectroscopic confirmation of $16$ distant structures at $1.4<z<2.8$ associated with the targeted powerful high-redshift radio-loud AGN. We also report the serendipitous discovery and spectroscopic confirmation of seven additional structures at $0.87<z<2.12$ not associated with the targeted radio-loud AGN. We find that $10^{10}-10^{11}{M}_{\odot }$ member galaxies of our confirmed CARLA structures form significantly fewer stars than their field counterparts at all redshifts within $1.4\le z\le 2$. We also observe higher star-forming activity in the structure cores up to $z=2$, finding similar trends as cluster surveys at slightly lower redshifts ($1.0<z<1.5$). By design, our efficient strategy of obtaining just two grism orbits per field only obtains spectroscopic confirmation of emission-line galaxies. Deeper spectroscopy will be required to study the population of evolved, massive galaxies in these (forming) clusters. Lacking multi-band coverage of the fields, we adopt a very conservative approach of calling all confirmations "structures", although we note that a number of features are consistent with some of them being bona fide galaxy clusters. Together, this survey represents a unique and large homogenous sample of spectroscopically confirmed structures at high redshifts, potentially more than doubling the census of confirmed, massive clusters at $z>1.4$.

Accepted for publication in the Astrophysical Journal