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

TL;DR

This paper presents the optical identification and redshift estimation for over 4 million radio sources from the second LoTSS data release, significantly expanding the largest optically identified radio catalog to enable advanced galaxy studies.

Contribution

It introduces new methods combining likelihood-ratio matching, citizen science, and algorithmic approaches for optical ID of radio sources in a large survey.

Findings

85% of sources have optical/infrared IDs

58% have redshift estimates

Largest optically identified radio catalog to date

Abstract

The second data release of the LOFAR Two-Metre Sky Survey (LoTSS) covers 27% of the northern sky, with a total area of $\sim 5,700$ deg$^2$. 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.