1-arcsecond imaging of the ELAIS-N1 field at 144MHz using the LoTSS survey with the international LOFAR telescope

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² University of Cambridge, Cavendish Asterophysics group, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Department of Physics, South Road, Durham DH1 3LE, UK
⁴ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁵ SKA Observatory, Jordrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
⁶ ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁷ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁸ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
⁹ Hamburg Observatory, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁰ GEPI & ORN, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France and Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa

^★ Corresponding author; hye@strw.leidenuniv.nl, hy297@cam.ac.uk

Received: 28 September 2023
Accepted: 23 September 2024

Abstract

We present the first wide area (2.5 × 2.5 deg²) LOFAR high band antenna image at a resolution of 1.2″ × 2″ with a median noise of ≈80 µJy beam⁻¹. It was made from an 8-hour International LOFAR Telescope (ILT) observation of the ELAIS-N1 field at frequencies ranging from 120 to 168 MHz with the most up-to-date ILT imaging methods. This intermediate resolution falls between the highest possible resolution (0.3″) achievable by using all ILT baselines and the standard 6-arcsec resolution in the LOFAR Two-meter Sky Survey (LoTSS) image products utilising the LOFAR Dutch baselines only. This is the first demonstration of the feasibility of imaging using the ILT at a resolution of ~1″, which provides unique information on source morphology at scales that fall below the surface brightness limits at higher resolutions. The total calibration and imaging computational time is approximately 52 000 core hours, which is nearly five times more than required to produce a 6″ resolution image. We also present a radio source catalogue containing 2263 sources detected over the 2.5 × 2.5 deg² image of the ELAIS-N1 field, with a peak intensity threshold of 5.5σ. The catalogue has been cross-matched with the LoTSS deep ELAIS-N1 field radio catalogue, and its flux density and positional accuracy have been investigated and corrected accordingly. We find that ~80% of sources that we expect to be detectable based on their peak brightness in the LoTSS 6″ resolution image are detected in this image, which is approximately a factor of two higher than for 0.3″ resolution imaging in the Lockman Hole, implying there is a wealth of information on these intermediate scales.