The LOFAR Two-metre Sky Survey

II. First data release^⋆,⋆⋆

¹ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: shimwell@astron.nl
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁴ Department of Physics & Electronics, Rhodes University, PO Box 94 Grahamstown 6140, South Africa
⁵ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁶ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ University of Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁸ SKA South Africa, 3rd Floor, The Park, Park Road, Pinelands 7405, South Africa
⁹ Ampyx Power B.V. Lulofsstraat 55–Unit 13, 2521 AL The Hague, The Netherlands
¹⁰ INAF–Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹¹ School of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹² RAL Space, The Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0NL, UK
¹³ LESIA, Observatoire de Paris, PSL, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹⁴ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹⁵ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁶ SURFsara, Science Park 140, 1098 XG Amsterdam, The Netherlands
¹⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁸ CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia
¹⁹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
²⁰ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²¹ Physics and Astronomy Department, University of the Western Cape, Bellville 7535, South Africa
²² Oxford e-Research Centre, University of Oxford, 7 Keble Road, Oxford OX1 3QG, UK
²³ Wolfson College, University of Oxford, Linton Road, Oxford OX2 6UD, UK
²⁴ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
²⁵ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
²⁶ Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 5AR, UK
²⁷ Joint Institute for VLBI ERIC, PO Box 2, 7990 AA, The Netherlands
²⁸ Dipartimento di Fisica e Astronomia, Università di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
²⁹ Center for Astronomy and Particle Theory, University of Nottingham, NG7 2RD Nottingham, UK
³⁰ Astronomical Institute, Ruhr-University Bochum, 44780 Bochum, Germany
³¹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
³² Mbarara University of Science & Technology, PO Box 1410 Mbarara, Uganda
³³ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
³⁴ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Blvd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
³⁵ Instituto de Astrofisíca de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain
³⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³⁷ Toruń Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziacka 5, 87-100 Toruń, Poland
³⁸ CSIRO Astronomy and Space Science, PO Box 76 Epping, NSW 1710, Australia
³⁹ Max-Planck Institute für Extraterrestrische Physik, Giessenbachstr. 1, 85741 Garching, Germany
⁴⁰ Excellence Cluster Universe, Boltzmann Strasse 2, 85748 Garching, Germany
⁴¹ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁴² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴³ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany

Received: 4 June 2018
Accepted: 12 September 2018

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S_144 MHz = 71 μJy beam⁻¹ and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.