The LOFAR Two-metre Sky Survey

IV. First Data Release: Photometric redshifts and rest-frame magnitudes^⋆,⋆⋆

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
e-mail: duncan@strw.leidenuniv.nl
² SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, EH9 3HJ Edinburgh, UK
³ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, OX1 3RH Oxford, UK
⁴ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, AL10 9AB Hatfield, UK
⁵ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ School of Physical Sciences, The Open University, Walton Hall, MK7 6AA Milton Keynes, UK
⁷ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁸ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁹ Space Science & Technology Department, The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0NL Oxfordshire, UK
¹⁰ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹¹ CSIRO Astronomy and Space Science, PO Box 1130, 6102 Bentley, WA, Australia
¹² Thüringer Landessternwarte (TLS), Sternwarte 5, 07778 Tautenburg, Germany
¹³ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany

Received: 4 June 2018
Accepted: 6 August 2018

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) is a sensitive, high-resolution 120–168 MHz survey of the Northern sky. The LoTSS First Data Release (DR1) presents 424 square degrees of radio continuum observations over the HETDEX Spring Field (10^h45^m00^s < right ascension <  15^h30^m00^s and 45°00′00″ < declination < 57°00′00″) with a median sensitivity of 71 μJy beam⁻¹ and a resolution of 6″. In this paper we present photometric redshifts (photo-z) for 94.4% of optical sources over this region that are detected in the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 3π steradian survey. Combining the Pan-STARRS optical data with mid-infrared photometry from the Wide-field Infrared Survey Explorer, we estimate photo-zs using a novel hybrid photometric redshift methodology optimised to produce the best possible performance for the diverse sample of radio continuum selected sources. For the radio-continuum detected population, we find an overall scatter in the photo-z of 3.9% and an outlier fraction (|z_phot−z_spec|/(1 + z_spec) > 0.15) of 7.9%. We also find that, at a given redshift, there is no strong trend in photo-z quality as a function of radio luminosity. However there are strong trends as a function of redshift for a given radio luminosity, a result of selection effects in the spectroscopic sample and/or intrinsic evolution within the radio source population. Additionally, for the sample of sources in the LoTSS First Data Release with optical counterparts, we present rest-frame optical and mid-infrared magnitudes based on template fits to the consensus photometric (or spectroscopic when available) redshift.