The LOFAR Two-Metre Sky Survey: VI. Optical identifications for the second data release

Hardcastle, M.J. and Horton, M.A. and Williams, W.L. and Duncan, K.J. and Alegre, L. and Barkus, B. and Croston, J.H. and Dickinson, H. and Osinga, E. and Röttgering, H.J.A. and Sabater, J. and Shimwell, T.W. and Smith, D.J.B. and Best, P.N. and Botteon, A. and Brüggen, M. and Drabent, A. and De Gasperin, F. and Gürkan, G. and Hajduk, M. and Hale, C.L. and Hoeft, M. and Jamrozy, M. and Kunert-Bajraszewska, M. and Kondapally, R. and Magliocchetti, M. and Mahatma, V.H. and Mostert, R.I.J. and O'Sullivan, S.P. and Pajdosz-Smierciak, U. and Petley, J. and Pierce, J.C.S. and Prandoni, I. and Schwarz, D.J. and Shulewski, A. and Siewert, T.M. and Stott, J.P. and Tang, H. and Vaccari, M. and Zheng, X. and Bailey, T. and Desbled, S. and Goyal, A. and Gonano, V. and Hanset, M. and Kurtz, W. and Lim, S.M. and Mielle, L. and Molloy, C.S. and Roth, R. and Terentev, I.A. and Torres, M. (2023) The LOFAR Two-Metre Sky Survey: VI. Optical identifications for the second data release. Astronomy and Astrophysics, 678: A151. ISSN 1432-0746

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.