Searching for obscured AGN in z ∼ 2 submillimetre galaxies

¹ Shanghai Astronomical Observatory, 80 Nandan Road, Xuhui District, Shanghai 200030, PR China
² Jodrell Bank Centre for Astrophysics (JBCA), Department of Physics & Astronomy, Alan Turing Building, The University of Manchester, Manchester M13 9PL, UK
e-mail: hongying.chen@postgrad.manchester.ac.uk
³ University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, PR China
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁶ Joint Institute for VLBI in Europe (JIVE), PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁷ Netherlands Foundation for Research in Astronomy (ASTRON), PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁸ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁹ Department of Physics, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0083, South Africa
¹⁰ South African Radio Astronomy Observatory, 3rd Floor, The Park, Park Road, Pinelands, Cape Town 7405, South Africa
¹¹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹² Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
¹³ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK

Received: 21 November 2019
Accepted: 10 April 2020

Abstract

Aims. Submillimetre-selected galaxies (SMGs) at high redshift (z ∼ 2) are potential host galaxies of active galactic nuclei (AGN). If the local Universe is a good guide, ∼50% of the obscured AGN amongst the SMG population could be missed even in the deepest X-ray surveys. Radio observations are insensitive to obscuration; therefore, very long baseline interferometry (VLBI) can be used as a tool to identify AGN in obscured systems. A well-established upper limit to the brightness temperature of 10⁵ K exists in star-forming systems, thus VLBI observations can distinguish AGN from star-forming systems via brightness temperature measurements.

Methods. We present 1.6 GHz European VLBI Network (EVN) observations of four SMGs (with measured redshifts) to search for evidence of compact radio components associated with AGN cores. For two of the sources, e-MERLIN images are also presented.

Results. Out of the four SMGs observed, we detect one source, J123555.14, that has an integrated EVN flux density of 201 ± 15.2 μJy, corresponding to a brightness temperature of 5.2 ± 0.7 × 10⁵ K. We therefore identify that the radio emission from J123555.14 is associated with an AGN. We do not detect compact radio emission from a possible AGN in the remaining sources (J123600.10, J131225.73, and J163650.43). In the case of J131225.73, this is particularly surprising, and the data suggest that this may be an extended, jet-dominated AGN that is resolved by VLBI. Since the morphology of the faint radio source population is still largely unknown at these scales, it is possible that with a ∼10 mas resolution, VLBI misses (or resolves) many radio AGN extended on kiloparsec scales.