The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1 - II. The ELAIS-N1 LOFAR deep field

J. Sabater , P.N. Best , C. Tasse (1) , M.J. Hardcastle , T.W. Shimwell , D. Nisbet , V. Jelic , J.R. Callingham , H.J.A. Röttgering , M. Bonato , M. Bondi , B. Ciardi , R.K. Cochrane , M.J. Jarvis , R. Kondapally , L.V.E. Koopmans , S.P. O'Sullivan , I. Prandoni , D.J. Schwarz , D.J.B. Smith , L. Wang , W.L. Williams , S. Zaroubi

1 GEPI - Galaxies, Etoiles, Physique, Instrumentation

The LOFAR Two-metre Sky Survey (LoTSS) will cover the full northern sky and, additionally, aims to observe the LoTSS deep fields to a noise level of ≲10 μJy beam−1 over several tens of square degrees in areas that have the most extensive ancillary data. This paper presents the ELAIS-N1 deep field, the deepest of the LoTSS deep fields to date. With an effective observing time of 163.7 h, it reaches a root mean square noise level of ≲20 μJy beam−1 in the central region (and below 30 μJy beam−1 over 10 square degrees). The resolution is ~6 arcsecs and 84 862 radio sources were detected in the full area (68 square degrees) with 74 127 sources in the highest quality area at less than 3 degrees from the pointing centre. The observation reaches a sky density of more than 5000 sources per square degree in the central region (~5 square degrees). We present the calibration procedure, which addresses the special configuration of some observations and the extended bandwidth covered (115–177 MHz; central frequency 146.2 MHz) compared to standard LoTSS. We also describe the methods used to calibrate the flux density scale using cross-matching with sources detected by other radio surveys in the literature. We find the flux density uncertainty related to the flux density scale to be ~6.5 per cent. By studying the variations of the flux density measurements between different epochs, we show that relative flux density calibration is reliable out to about a 3 degree radius, but that additional flux density uncertainty is present for all sources at about the 3 per cent level; this is likely to be associated with residual calibration errors, and is shown to be more significant in datasets with poorer ionosphere conditions. We also provide intra-band spectral indices, which can be useful to detect sources with unusual spectral properties. The final uncertainty in the flux densities is estimated to be ~10 per cent for ELAIS-N1.Key words: surveys / catalogs / radio continuum: general / radio continuum: galaxies★ Catalogs and images are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/648/A2★★ The data associated with this article are released at: https://lofar-surveys.org

Mots clés: surveys; catalogs; radio continuum: general; radio continuum: galaxies

Domaines: Astrophysique [astro-ph]

J. Sabater, P.N. Best, C. Tasse, M.J. Hardcastle, T.W. Shimwell, et al.. The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1 - II. The ELAIS-N1 LOFAR deep field. Astronomy and Astrophysics - A&A, 2021, 648, pp.A2. ⟨10.1051/0004-6361/202038828⟩. ⟨hal-03070704⟩