LOFAR properties of SILVERRUSH Lyα emitter candidates in the ELAIS-N1 field

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: gloudemans@strw.leidenuniv.nl
² SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
³ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁴ INAF–Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁵ Italian ALMA Regional Centre, Via Gobetti 101, 40129 Bologna, Italy
⁶ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warszawa, Poland
⁸ Aix-Marseille Univ. CNRS, CNES, LAM, Marseille, France
⁹ Astronomy Centre, Department of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁰ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹¹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
¹³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁴ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa

Received: 1 July 2020
Accepted: 16 October 2020

Abstract

Lyman alpha emitters (LAEs) in the Epoch of Reionization (EoR) offer valuable probes of both early galaxy evolution and the process of reionization itself; however, the exact evolution of their abundance and the nature of their emission remain open questions. We combine samples of 229 and 349 LAE candidates at z = 5.7 and z = 6.6, respectively, from the SILVERRUSH narrowband survey with deep Low Frequency Array (LOFAR) radio continuum observations in the European Large Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field to search for radio galaxies in the EoR and study the low-frequency radio properties of z ≳ 5.7 LAE emitters. Our LOFAR observations reach an unprecedented noise level of ~20 μJy beam⁻¹ at 150 MHz, and we detect five candidate LAEs at >5σ significance. Based on detailed spectral energy distribution modelling of independent multi-wavelength observations in the field, we conclude that these sources are likely [OII] emitters at z = 1.47, yielding no reliable z ≳ 5.7 radio galaxy candidates. We examine the 111 z = 5.7 and z = 6.6 LAE candidates from our panchromatic photometry catalogue that are undetected by LOFAR, finding contamination rates of 81–92% for the z = 5.7 and z = 6.6 subset of the LAE candidate samples. This subset of the full sample is biased towards brighter magnitudes and redder near-infrared colours. The contamination rates of the full sample will therefore likely be lower than the reported values. Contamination of these optically bright LAE samples by likely [OII] emitters is lowered significantly through constraints on the near-infrared colours, highlighting the need for infrared observations to robustly identify bright LAEs in narrowband surveys. Finally, the stacking of radio continuum observations for the robust LAE samples yields 2σ upper limits on radio luminosity of 8.2 × 10²³ and 8.7 × 10²³ W Hz⁻¹ at z = 5.7 and 6.6, respectively, corresponding to limits on their median star-formation rates of <53 and <56 M_⊙ yr⁻¹.