The LOFAR Two-metre Sky Survey

III. First data release: Optical/infrared identifications and value-added catalogue^{⋆,⋆⋆,⋆⋆⋆}

¹ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield, AL10 9AB UK
e-mail: wwilliams@strw.leidenuniv.nl
² SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ UK
³ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁵ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA, 6102 Australia
⁷ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
⁸ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH UK
⁹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL UK
¹⁰ Toruń Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziacka 5, 87-100 Toruń, Poland
¹¹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹³ GEPI, Observatoire de Paris, CNRS, Universite Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁴ Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown, 6140 South Africa
¹⁵ Space Science & Technology Department, The Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0NL UK
¹⁶ University of Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁷ Thüringer Landessternwarte (TLS), Sternwarte 5, 07778 Tautenburg, Germany
¹⁸ Kapteyn Astronomical Institute, PO Box 800 9700 AV Groningen, The Netherlands
¹⁹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany

Received: 4 June 2018
Accepted: 9 November 2018

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168 MHz survey of the northern sky with diverse and ambitious science goals. Many of the scientific objectives of LoTSS rely upon, or are enhanced by, the association or separation of the sometimes incorrectly catalogued radio components into distinct radio sources and the identification and characterisation of the optical counterparts to these sources. We present the source associations and optical and/or IR identifications for sources in the first data release, which are made using a combination of statistical techniques and visual association and identification. We document in detail the colour- and magnitude-dependent likelihood ratio method used for statistical identification as well as the Zooniverse project, called LOFAR Galaxy Zoo, used for visual classification. We describe the process used to select which of these two different methods is most appropriate for each LoTSS source. The final LoTSS-DR1-IDs value-added catalogue presented contains 318 520 radio sources, of which 231 716 (73%) have optical and/or IR identifications in Pan-STARRS and WISE.