Abstract
Abstract We report spectroscopic results from our 40-orbit Hubble Space Telescope slitless grism spectroscopy program observing the 20 densest Clusters Around Radio-Loud AGN (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 1010–1011 M ⊙ member galaxies of our confirmed CARLA structures form significantly fewer stars than their field counterparts at all redshifts within 1.4 ≤ z ≤ 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.
Highlights
The last decade has seen an explosion in our understanding of the early stages of galaxy cluster formation, largely built upon samples of distant galaxy clusters identified from the Infrared Array Camera (IRAC; Fazio et al 2004) on the Spitzer Space Telescope
We report on a comprehensive, 40-orbit Hubble program using the unique near-infrared grism capabilities of the Wide Field Camera 3 (WFC3) to attempt confirmation of the 20 richest z > 1.4 galaxy cluster candidates identified from our 408 hr Warm Spitzer survey of 420 radio-loud AGN across the full sky, Clusters Around Radio-Loud AGN (CARLA; Wylezalek et al 2013, 2014)
The first column corresponds to sources detected in our Hubble Space Telescope (HST)/F140W imaging, the second column to secure sources detected in our Spitzer/IRAC imaging, and the third column to sources passing our Spitzer/IRAC color-selection criterion. e For unconfirmed structures, we show in parenthesis mean and median redshifts and source numbers based on the few confirmed sources at the radio-loud AGN (RLAGN) redshifts
Summary
The last decade has seen an explosion in our understanding of the early stages of galaxy cluster formation, largely built upon samples of distant galaxy clusters identified from the Infrared Array Camera (IRAC; Fazio et al 2004) on the Spitzer Space Telescope. Near-infrared grism spectroscopy using the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST) provides impressive sensitivity for studying galaxy clusters at high redshift. The majority of the most distant clusters currently known were selected on the basis of Spitzer mid-infrared data and confirmed with Hubble grism spectroscopy. This includes clusters at z = 1.75 (Stanford et al 2012), z = 1.80 (Newman et al 2014), z = 1.89 (Zeimann et al 2012), and our own recent confirmation of one evolved cluster and one younger forming structure at z = 2.00 and z = 1.99, respectively (Noirot et al 2016; hereafter N16).
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