Rapidly growing black holes and host galaxies in the distant Universe from the Herschel Radio Galaxy Evolution Project ^⋆,⋆⋆

¹ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching bei München, Germany
² Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
³ CSIRO Astronomy & Space Science, PO Box 76, Epping, NSW 1710, Australia
⁴ Department of Earth and Space Science, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: guidro@chalmers.se
⁵ Kapteyn Astronomical Institute, Univ. of Groningen, Netherlands
⁶ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Catlica de Chile, 306, Casilla 306 Santiago 22, Chile
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁸ Instituto de Física y Astronomía. Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
⁹ INAF – Osservatorio di Roma, via Frascati 33, 00040 Monteporzio, Italy
¹⁰ Astronomisches Institut, Ruhr-Universität Bochum, Universitätstr. 150, 44801 Bochum, Germany
¹¹ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
¹² Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
¹³ Institut d’Astrophysique Spatiale, CNRS, Université Paris-Sud, 91405 Orsay, France
¹⁴ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 169-221, Pasadena, CA 91109, USA

Received: 20 December 2013
Accepted: 28 March 2014

Abstract

We present results from a comprehensive survey of 70 radio galaxies at redshifts 1 <z< 5.2 using the PACS and SPIRE instruments on board the Herschel Space Observatory. Combined with existing mid-IR photometry from the Spitzer Space Telescope, published 870 μm photometry, and new observations obtained with LABOCA on the APEX telescope, the spectral energy distributions (SEDs) of galaxies in our sample are continuously covered across 3.6–870 μm. The total 8–1000 μm restframe infrared luminosities of these radio galaxies are such that almost all of them are either ultra-(L_tot^IR 10¹² L_⊙) or hyper-luminous (L_tot^IR 10¹³ L_⊙) infrared galaxies. We fit the infrared SEDs with a set of empirical templates which represent dust heated by a variety of starbursts (SB) and by an active galactic nucleus (AGN). We find that the SEDs of radio galaxies require the dust to be heated by both AGN and SB, but the luminosities of these two components are not strongly correlated. Assuming empirical relations and simple physical assumptions, we calculate the star formation rate (SFR), the black hole mass accretion rate (Ṁ_BH), and the black hole mass (M_BH) for each radio galaxy. We find that the host galaxies and their black holes are growing extremely rapidly, having SFR ≈ 100–5000 M_⊙ yr^-1 and Ṁ_BH ≈ 1–100 M_⊙ yr^-1. The mean specific SFRs (sSFR) of radio galaxies at z> 2.5 are higher than the sSFR of typical star forming galaxies over the same redshift range, but are similar or perhaps lower than the galaxy population for radio galaxies at z< 2.5. By comparing the sSFR and the specific Ṁ_BH (sṀ_BH), we conclude that black holes in radio loud AGN are already, or soon will be, overly massive compared to their host galaxies in terms of expectations from the local M_BH–M_Gal relation. In order to catch up with the black hole, the galaxies require about an order of magnitude more time to grow in mass at the observed SFRs compared to the time the black hole is actively accreting. However, during the current cycle of activity, we argue that this catching up is likely to be difficult because of the short gas depletion times. Finally, we speculate on how the host galaxies might grow sufficiently in stellar mass to ultimately fall onto the local M_BH–M_Gal relation.