The LOFAR Two-Metre Sky Survey

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.

What Have We Learned about the Life Cycle of Radio Galaxies from New Radio Surveys

“…The expanding possibilities for obtaining optical identifications for the radio sources and characterise the properties of the host galaxy are a major, key addition. The optical identification of the entire LoTSS DR2 has been recently published [38] and provides a great database of crucial ancillary information. These data also improve our knowledge of the environment, which is needed for a complete understanding of the parameters impacting the evolution of the radio sources.…”

“…Furthermore, by combining some of the surveys, spectral index images can be obtained. • Availability of high-quality ancillary data (e.g., optical ID and redshift) for larger areas of the sky (e.g., [36][37][38]) as well as improved methods for deriving photometric redshifts for a large number of galaxies [37].…”