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

Hardcastle, M. J.; Horton, M. A.; Williams, W. L.; Duncan, K. J.; Alegre, L.; Barkus, B.; Croston, J. H.; Dickinson, H.; Osinga, E.; Röttgering, H. J. A.; Sabater, J.; Shimwell, T. W.; Smith, D. J. B.; Best, P. N.; Botteon, A.; Brüggen, M.; Drabent, A.; de Gasperin, F.; Gürkan, G.; Hajduk, M.; Hale, C. L.; Hoeft, M.; Jamrozy, M.; Kunert-Bajraszewska, M.; Kondapally, R.; Magliocchetti, M.; Mahatma, V. H.; Mostert, R. I. J.; O’Sullivan, S. P.; Pajdosz-Śmierciak, U.; Petley, J.; Pierce, J. C. S.; Prandoni, I.; Schwarz, D. J.; Shulewski, A.; Siewert, T. M.; Stott, J. P.; Tang, H.; Vaccari, M.; Zheng, X.; Bailey, T.; Desbled, S.; Goyal, A.; Gonano, V.; Hanset, M.; Kurtz, W.; Lim, S. M.; Mielle, L.; Molloy, C. S.; Roth, R.; Terentev, I. A.; Torres, M.

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


M. J. Hardcastle

M. A. Horton

W. L. Williams

K. J. Duncan

L. Alegre

B. Barkus

J. H. Croston

H. Dickinson

E. Osinga

H. J. A. Röttgering

J. Sabater

T. W. Shimwell

D. J. B. Smith

P. N. Best

A. Botteon

M. Brüggen

A. Drabent

F. de Gasperin

G. Gürkan

M. Hajduk

C. L. Hale

M. Hoeft

M. Jamrozy

M. Kunert-Bajraszewska

R. Kondapally

M. Magliocchetti

V. H. Mahatma

R. I. J. Mostert

S. P. O’Sullivan

U. Pajdosz-Śmierciak

James Petley
PGR Student Doctor of Philosophy

J. C. S. Pierce

I. Prandoni

D. J. Schwarz

A. Shulewski

T. M. Siewert

J. P. Stott

H. Tang

M. Vaccari

X. Zheng

T. Bailey

S. Desbled

A. Goyal

V. Gonano

M. Hanset

W. Kurtz

S. M. Lim

L. Mielle

C. S. Molloy

R. Roth

I. A. Terentev

M. Torres


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.

Journal Article Type Article
Acceptance Date Aug 29, 2023
Online Publication Date Oct 18, 2023
Publication Date 2023-10
Deposit Date Jan 22, 2024
Publicly Available Date Jan 22, 2024
Journal Astronomy & Astrophysics
Print ISSN 0004-6361
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 678
Article Number A151
Keywords Space and Planetary Science; Astronomy and Astrophysics
Public URL


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