CARLA: Star Formation Activity in Confirmed Clusters and Proto-Clusters at 1.4 < z < 2.8

Gael Noirot (Caltech)
April 12, 2018 - 12:15pm


Radio-loud AGN (RLAGN) tend to reside in the most massive dark matter halos, and have a
long history of being used to efficiently identify rich, high-redshift structures (i.e., clusters and
proto-clusters). Our team led the “Clusters Around Radio-Loud AGN” (CARLA) survey, which
was a 400-hr Spitzer program surveying 420 RLAGN (radio-loud quasars and high-redshift radio galaxies) at 1.3 < z < 3.2 across the full sky. CARLA identified 200 cluster candidates as
significant overdensities of red color-selected Spitzer/IRAC galaxies around the RLAGN, which
samples the previously largely unexplored transition epoch between proto-cluster and cluster
environments. I will introduce this high-redshift cluster survey and present results from our
follow-up 40-orbit Hubble Space Telescope program of the 20 densest CARLA cluster
candidates. We spectroscopically confirm 16 of the 20 distant structures associated with the
RLAGN, out to z=2.8 (Noirot et al. 2016, 2018). For the first time at these redshifts, we
statistically investigate the star-formation content of a large sample of galaxies in dense
structures. I will show that 1.4 < z < 2.0 massive member galaxies form fewer stars than their
field counterparts, consistent with an accelerated evolution in dense environments already at
these epochs. I will also show that at 1.4 < z < 2.0, CARLA cores are the main sites of star
formation, consistent with the onset of the reversal in the star formation rate - density relation at
z ~ 1.4. This survey represents a unique and large homogenous sample of spectroscopically confirmed structures at high redshifts, ideal to further study galaxy cluster physics, star-
formation, quenching mechanisms, and AGN feedback.