Affiliation: Curtin Institute of Radio Astronomy

Contribution: Oral

Title: Powerful high-redshift radio galaxies with under-massive hosts: The radio-loud analogues to Little Red Dots

Abstract: High-redshift AGN are a key puzzle piece to understanding the full picture of galaxy evolution due to the co-evolution of the central black hole and their host galaxies. The radio-loud and obscured population of AGN at z>5 remain elusive (with fewer than five known), but provide significant insight into the complete AGN population and the different stages of their growth. The most massive and powerful of these radio-loud AGN are high-redshift radio galaxies, which have been routinely selected by the K-z relation when matching wide-area radio surveys against wide-area NIR surveys. The most efficient ways to obtain redshifts for these optical/IR-faint samples have been to follow-up with deep K-band observations to infer a redshift from scaling relations, or to follow-up with millimetre/submillimetre wide-bandwidth scans targeting multiple molecular CO lines which trace the dense ISM and CGM. Here, we present results from ALMA and HAWK-I observations of an extreme sample of powerful radio sources with K(AB)~24 which demonstrate that at least some radio galaxies expected to be at z>5 are found at 3<z<4. This result implies they have over-massive black holes compared to their hosts (i.e. radio-loud analogues to Little Red Dots) and we identify one of our targets as a dusty, active merger. Although we have not been able to confirm any z>5 radio galaxies with this search, we have instead discovered a number of sources belonging to an uniquely interesting, extreme phase of co-evolution between SMBHs and their host galaxies. Future deep wide-area NIR surveys from Euclid will enable effective candidate selection for additional extreme radio-IR sources to follow-up with ALMA. Depending on the extent of this population with over-massive black holes, we stand to gain insight into whether many of these black holes outgrew their host galaxies, at what stage this occurred and how the host might then “catch up”, pushing the object back towards the local M-σ relation.

This contribution can be found here (pdf).