Spectroscopy of an extreme [OIII] emitting active galactic nucleus at $z=3.212$: implications for the reionisation era

TL;DR

This study reports the discovery of a luminous AGN at z=3.21 with extreme nebular emission and a blue UV continuum, providing insights into AGN properties during the reionisation era and their role in cosmic reionisation.

Contribution

It presents the first detailed characterization of a high-redshift AGN with extreme emission lines and blue continuum, bridging the understanding of AGN features from intermediate to reionisation-era galaxies.

Findings

Detected a z=3.21 AGN with extreme [OIII]+Hβ emission (EW=1300 Å)

Found UV line ratios consistent with AGN photoionisation models

Observed broad and narrow components in optical lines indicating outflows

Abstract

Reionisation-era galaxies often display intense nebular emission lines, both in rest-frame optical ([OIII]+H$\beta$) and ultraviolet (UV; CIII], CIV). How such strong nebular emission is powered remains unclear, with both active galactic nuclei (AGN) and hot stars considered equally viable. The UV continuum slopes of these early systems tend to be very blue ($\beta<-2$), reflecting minimal dust obscuration, young ages, and low metallicities. This contrasts with narrow-lined AGN at $z\sim2-3$, whose UV slopes are significantly redder ($\beta>-1$) than typical star-forming systems in the reionisation era. To investigate the properties of AGN in the reionisation era, we have conducted a search for potential examples of rare analogues with blue continua at intermediate redshift ($z\sim2-3$). Our goals are to determine whether AGN with intense line emission and blue continua exist and thereby to establish the range of rest-frame UV and optical line ratios in this population. In this paper we report the detection of a X-ray luminous AGN at $z=3.21$ (UDS-24561) with extreme [OIII]+H$\beta$ line emission (EW $=1300$ \r{A}) and a blue UV continuum slope ($\beta=-2.34$). MMT/Binospec and Keck/MOSFIRE spectra indicate rest-frame UV line ratios consistent with AGN photoionisation models and rest-frame optical lines with both a narrow component (FWHM $=154$ km$/$s) and extended broad wings (FWHM $=977$ km$/$s), consistent with outflowing gas. We describe how such objects can be identified in future JWST emission line surveys in the reionisation era, thereby providing a valuable census of AGN activity at $z>6$ and understanding their contribution to cosmic reionisation.