VLBI imaging of high-redshift galaxies and protoclusters at low radio frequencies with the International LOFAR Telescope

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
e-mail: c.m.cordun@umail.leidenuniv.nl
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, DH1 3LE UK
⁴ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham, DH1 3LE UK

Received: 4 March 2023
Accepted: 30 May 2023

Abstract

It is generally known that luminous, ultra-steep spectrum radio sources are preferentially associated with massive galaxies at high redshifts. In this paper, we describe a pilot project directed at such objects to demonstrate the feasibility and importance of using the LOw-Frequency ARray (LOFAR) data to study the most distant massive galaxies undergoing formation and protoclusters. We successfully imaged four high-redshift (z > 2), high-luminosity radio galaxies with sub-arcsecond resolution, at 144 MHz, using the International LOFAR Telescope (ILT). Our targets were 4C 41.17, which we dubbed “the Anthill” galaxy (z = 3.8), as well as B2 0902+34 (z = 3.4), 4C 34.34 (z = 2.4), and 4C 43.15 (z = 2.5). We mapped their low-frequency morphologies and the spatial distributions of their low-frequency spectral indices and then compared these results with the available optical, infrared, and X-ray images. Both for the Anthill at z = 3.8 and B2 0902+34 at z = 3.4, the location of the steepest radio emission coincides with the Lyα–emitting ionized gas halo. Our pilot project demonstrates that thanks to its outstanding sensitivity and high angular resolution at low frequencies, the ILT is a unique facility for studying the co-evolution and interaction of massive galaxies, galaxy clusters, and supermassive black holes in the early Universe.