Witnessing the transformation of a quasar host galaxy at z=1.6

TL;DR

This study presents deep optical imaging and spectroscopy of a high-redshift radio galaxy, revealing a rare phase of galaxy evolution characterized by shock ionization and gas clearing in the host galaxy.

Contribution

It provides new observational evidence of a transitional phase in quasar host galaxy evolution at z=1.6, highlighting shock ionization and gas removal processes.

Findings

Detection of offset UV emission lines consistent with shock ionization.

Evidence of gas clearing in the galaxy's central region.

Observation of a rare evolutionary phase in quasar hosts.

Abstract

A significant minority of high redshift radio galaxy (HzRG) candidates show extremely red broad band colours and remain undetected in emission lines after optical `discovery' spectroscopy. In this paper we present deep GTC optical imaging and spectroscopy of one such radio galaxy, 5C 7.245, with the aim of better understanding the nature of these enigmatic objects. Our g-band image shows no significant emission coincident with the stellar emission of the host galaxy, but does reveal faint emission offset by ~3" (26 kpc) therefrom along a similar position angle to that of the radio jets, reminiscent of the `alignment effect' often seen in the optically luminous HzRGs. This offset g-band source is also detected in several UV emission lines, giving it a redshift of 1.609, with emission line flux ratios inconsistent with photoionization by young stars or an AGN, but consistent with ionization by fast shocks. Based on its unusual gas geometry, we argue that in 5C 7.245 we are witnessing a rare (or rarely observed) phase in the evolution of quasar hosts when stellar mass assembly, accretion onto the back hole, and powerful feedback activity has eradicated its cold gas from the central ~20 kpc, but is still in the process of cleansing cold gas from its extended halo.