The LOFAR Two-Metre Sky Survey

VI. Optical identifications for the second data release^★

¹ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
e-mail: m.j.hardcastle@herts.ac.uk
² Cavendish Astrophysics, University of Cambridge, Cavendish Laboratory, JJ Thomson Avenue Cambridge CB3 0HE, UK
³ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁸ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Krakow, Poland
⁹ Universidad Nacional Autonoma de Mexico (UNAM), Avenida Insurgentes Sur 3000, Mexico City, Mexico
¹⁰ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹¹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
¹² INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy
¹³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
¹⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziadzka 5, 87-100 Toruń, Poland
¹⁵ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, ul.Oczapowskiego 2, 10-719 Olsztyn, Poland
¹⁶ INAF-Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁷ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029, Hamburg, Germany
¹⁸ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
¹⁹ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
²⁰ CSIRO Space and Astronomy, ATNF, PO Box 1130, Bentley, WA 6102, Australia
²¹ Departamento de Fisica de la Tierra y Astrofisica, Universidad Complutense de Madrid, 28040 Madrid, Spain
²² Department of Astronomy, Tsinghua University, Beijing 100084, PR China
²³ Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, 7701 Rondebosch, Cape Town, South Africa
²⁴ Inter-University Institute for Data Intensive Astronomy, Department of Physics and Astronomy, University of the Western Cape, 7535 Bellville, Cape Town, South Africa
²⁵ Citizen scientist

Received: 1 July 2023
Accepted: 29 August 2023

Abstract

The second data release of the LOFAR Two-Metre Sky Survey (LoTSS) covers 27% of the northern sky, with a total area of ~5700 deg¹. The high angular resolution of LOFAR with Dutch baselines (6 arcsec) allows us to carry out optical identifications of a large fraction of the detected radio sources without further radio followup; however, the process is made more challenging by the many extended radio sources found in LOFAR images as a result of its excellent sensitivity to extended structure. In this paper we present source associations and identifications for sources in the second data release based on optical and near-infrared data, using a combination of a likelihood-ratio cross-match method developed for our first data release, our citizen science project Radio Galaxy Zoo: LOFAR, and new approaches to algorithmic optical identification, together with extensive visual inspection by astronomers. We also present spectroscopic or photometric redshifts for a large fraction of the optical identifications. In total 4 116 934 radio sources lie in the area with good optical data, of which 85% have an optical or infrared identification and 58% have a good redshift estimate. We demonstrate the quality of the dataset by comparing it with earlier optically identified radio surveys. This is by far the largest ever optically identified radio catalogue, and will permit robust statistical studies of star-forming and radio-loud active galaxies.