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

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.

Discovery of 24 radio-bright quasars at 4.9 ≤ z ≤ 6.6 using low-frequency radio observations

“…We note that populations of fainter quasars have been found at z > 5.7 in optical surveys such as the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs; e.g. Matsuoka et al 2016Matsuoka et al , 2022, but these generally remain non-detected by LOFAR (Gloudemans et al 2021a).…”

Radio and far-IR emission associated with a massive star-forming galaxy candidate atz≃ 6.8: a radio-loud AGN in the reionization era?

“…Several other UV-bright galaxies at 𝑧 ∼ 7 − 9 have shown evidence of significant AGN activity via detections of NV𝜆1240 emission (Tilvi et al 2016;Laporte et al 2017;Mainali et al 2018;Endsley et al 2021b), perhaps indicating that AGN make a non-negligible contribution to the ionizing photon budget at early times (Madau & Haardt 2015). Notably, radio AGN appear to be very rare among narrow-band selected Ly𝛼 emitters at 𝑧 5.7 and 𝑧 6.6 (Gloudemans et al 2021), consistent with a scenario in which such bright Ly𝛼 emitters more often trace young unobscured starbursts rather than the most massive galaxies harboring luminous AGN. Wider-area mutli-wavelength coverage will be required to better determine the general properties of massive reionization-era galaxies, including those which are too reddened to be selected in the rest-UV.…”

Distinguishing AGN from starbursts as the origin of double-peaked Lyman-alpha emitters in the reionization era

“…Radio observations of LAEs at z ∼ 6 with the LOFAR survey (Gloudemans et al 2021) have provided upper limits of 10 23.90 and 10 23.94 W Hz −1 at z ∼ 5.7 and z ∼ 6.6, respectively. In converting the radio upper limits for COLA-1 (Jorryt Matthee & João Calhau, priv.…”