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

TL;DR

This study demonstrates that LOFAR can effectively image high-redshift, luminous radio galaxies at low frequencies with high resolution, revealing their morphology and spectral properties, and highlighting its potential for studying early Universe galaxy evolution.

Contribution

First high-resolution low-frequency imaging of high-redshift galaxies with LOFAR, showing its capability to study galaxy and protocluster formation in the early Universe.

Findings

Steepest radio emission coincides with Lyα halos in high-z galaxies.

LOFAR's sensitivity and resolution enable detailed morphological studies at high redshift.

Low-frequency spectral index maps reveal insights into galaxy evolution.

Abstract

It has long been known that luminous, ultra-steep spectrum radio sources are preferentially associated with massive galaxies at high redshifts. Here we describe a pilot project directed at such objects, to demonstrate the feasibility and importance of using LOFAR to study the most distant forming massive galaxies and protoclusters. We have 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 ($z=3.8$), the "Anthill", B2 0902+34 ($z=3.4$), 4C 34.34 ($z=2.4$) and 4C 43.15 ($z=2.5$). Their low-frequency morphologies and the spatial distributions of their low-frequency spectral indices have been mapped, and compared with 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 Ly$\alpha$ emitting ionized gas halo. Our pilot project demonstrates that, because of 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.