The RAdio Galaxy Environment Reference Survey

Coordinators: Thomas R. Greve (UK), Chian-Chou Chen (TW), Scott Chapman (CA), Zhi-Yu Zhang (CN), Tadayuki Kodama (JP), Hyunjin Shim (KR) Wiphu Rujopakarn (TH)

High redshift radio galaxies (HzRGs) are powerful radio-loud active galactic nuclei (AGN) at z > 1 with a rest frame radio luminosites > 1027 W Hz−1. More than 20 years of painstaiking multi-wavelength observational efforts have produced a compelling picture in which HzRGs are extremely massive (M⋆ > 1010.5 M⊙ ) galaxies – with signatures of both AGN and extreme starbursts as well as evolved stellar populations – residing at the centers of rare galaxy overdensities (proto-clusters). Nonetheless, there is much we do not know about these systems. The growth and evolution of the aforementioned galaxy overdensities with redshift is to this day still not characterised. Nor is the effect the AGN activity has on the growth of the central overdensities. Most overdensity measurements have been done in UV/optical narrow-band filters, e.g., tracing low-mass Lyα or Hα emitters with unobscured star formation. However, more than 40% of the integrated cosmic star formation has been obscured by dust (e.g., Casey et al. 2014), and a full picture of the stellar mass build-up in clusters, therefore, require sub-millimeter observations.

RAGERS, the RAdio Galaxy Environment Reference Survey, is a JCMT Large Program, which has been awarded 168 hours of SCUBA-2 time (Band 1 and 2) to map the overdensities of dusty galaxies within the Mpc-scale environments of a sample 33 powerful radio galaxies uniformly distributed across the redshift range 1 < z < 3.5 and mass range M⋆ ≥ 1010.7 M⊙ . The survey will also observe an equally sized control sample of radio-quiet galaxies with stellar mass and redshift distributions matching that of the radio-loud sample. The target is to observed all fields in Daisy mapping mode down to a central r.m.s. of 0.7mJy/beam. The RAGERS survey will provide a defining sub-millimeter survey of dust-obscured star formation and of the prevalence of dusty galaxies in the most highly biased high-density regions in the Universe, where theory predicts that the formation and evolution of galaxies occur at an accelerated rate compared to regions of lower density. A unique aspect of RAGERS is that both radio-loud (RL) and radio-quiet (RQ) samples are targetted. This means we can study to the effects of powerful AGN feedback on the buildup of the stellar mass in massive galaxies. Effective AGN feedback would imply that radio-loud galaxies are expected to sit in more overdense sub-millimeter environments, and thus more massive dark matter halos, than their radio-quiet counterparts. The RAGERS survey will be the first sub-millimeter survey able to investigate the existence of systematic differences in the overdensities of dusty galaxies around radio-loud vs radio- quiet galaxies. Finally, the survey will also explor evolutionary effects between the sub-millimeter overdensities and intrinsic properties of the RL-sample galaxies, such as radio luminosity, and radio emission extent.

Right: The M⋆ − z plane for the 33 RL-galaxies (large pink squares) and 33 RQ-galaxies (yellow squares) targeted by RAGERS. Also shown are the control sample (green circles) from which our RQ-sample was seleted, and the overall UKIDSS UDS sample (grey circles). Right: The L500MHz − z plane of our RL-sample (blue squares) and of our RQ-sample (grey region). The radio galaxies in the RL-sample cover uniformly the L500MHz − z plane, which allows us to study the environments around them as a function of redshift and radio luminosity. The RQ galaxies have radio luminosities at least two orders of magnitude lower than the radio galaxies.

Coordinators: Thomas Greve, UCL, UK, Chian-Chou Chen, ASIAA, Taiwan, Zhiyu Zhang, Nanjing University, China, Tadayuki Kodama, Tohoku University, Japan, Hyunjin Shim, Kyungpook National University, Korea, Scott Chapman, Dalhousie University, Canada.

– JCMT program code M20AL015

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