The LOFAR Two-metre Sky Survey: II. First data release

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data to 57 ◦ 00 00 ) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325694 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 144MHz = 71 µ Jybeam − 1 and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6 (cid:48)(cid:48) and the positional accuracy is within 0.2 (cid:48)(cid:48) . 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.