Massive galaxies on the road to quenching: ALMA observations of powerful high redshift radio galaxies

¹ European Southern Observatory, Karl-Schwarzchild-Str. 2, 85748 Garching, Germany
² Sorbonne Université, CNRS UMR 7095, Institut d’Astrophysique de Paris, 98bis bvd Arago, 75014 Paris, France
e-mail: theresa.falkendal@iap.fr
³ International Center for Radio Astronomy Research, Curtin University, Perth WA, 6845 Australia
⁴ National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA, 22903 USA
⁵ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 133-0033 Japan
⁶ National Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-0015 Japan
⁷ Institut d’Astrophysique Spatiale, CNRS, Université Paris Sud, 91405 Orsay, France
⁸ Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13013 Marseille, France
⁹ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham, DH1 3LE UK
¹⁰ Department of Physics and Astronomy, Johns Hopkins University, Bloomberg center, 3400 N. Charles St, Baltimore, MD, 21218 USA

Received: 18 December 2017
Accepted: 10 July 2018

Abstract

We present 0.″3 (band 6) and 1.″5 (band 3) ALMA observations of the (sub)millimeter dust continuum emission for 25 radio galaxies at 1 <  z <  5.2. Our survey reaches a rms flux density of ∼50 μJy in band 6 (200–250 GHz) and ∼20 μJy in band 3 (100–130 GHz). This is an order of magnitude deeper than single-dish 850 μm observations, and reaches fluxes where synchrotron and thermal dust emission are expected to be of the same order of magnitude. Combining our sensitive ALMA observations with low-resolution radio data from ATCA, higher resolution VLA data, and infrared photometry from Herschel and Spitzer, we have disentangled the synchrotron and thermal dust emission. We determine the star-formation rates and AGN infrared luminosities using our newly developed Multi-resolution and multi-object/origin spectral energy distribution fitting code (MR-MOOSE). We find that synchrotron emission contributes substantially at λ ∼ 1 mm. Through our sensitive flux limits and accounting for a contribution from synchrotron emission in the mm, we revise downward the median star-formation rate by a factor of seven compared to previous estimates based solely on Herschel and Spitzer data. The hosts of these radio-loud AGN appear predominantly below the main sequence of star-forming galaxies, indicating that the star formation in many of the host galaxies has been quenched. Future growth of the host galaxies without substantial black hole mass growth will be needed to bring these objects on the local relation between the supermassive black holes and their host galaxies. Given the mismatch in the timescales of any star formation that took place in the host galaxies and lifetime of the AGN, we hypothesize that a key role is played by star formation in depleting the gas before the action of the powerful radio jets quickly drives out the remaining gas. This positive feedback loop of efficient star formation rapidly consuming the gas coupled to the action of the radio jets in removing the residual gas is how massive galaxies are rapidly quenched.