A hidden active galactic nucleus powering bright nebulae in a protocluster at z=4.5 revealed by JWST

Abstract

Galaxy protoclusters are sites of rapid growth, with a high density of massive galaxies driving elevated global rates of star formation and accretion onto supermassive black holes. Here, we present new JWST/NIRSpec IFU observations of the J1000+0234 group at $z=4.54$, a dense region of a protocluster hosting a massive, dusty star forming galaxy (DSFG) with a low-luminosity radio counterpart. The new data reveal two extended, high-equivalent-width (EW$_0> angstrom nebulae that appear either side of the DSFG along its minor axis (namely O3-N and O3-S). On one hand, the spectrum of O3-N shows a prominent, broad, and blueshifted component with a full width at half maximum (FWHM) of $ second $ , suggesting an outflow origin. On the other hand, O3-S stretches over parsec and has a velocity gradient that spans second but shows no evidence of a broad component. However, both sources seem to be powered at least partially by an active galactic nucleus (AGN), and so we classified them as extended emission-line regions (EELRs). The strongest evidence comes from the detection of the high-ionization Ne v line toward O3-N, which paired with the non-detection of hard X-rays implies an obscuring column density above the Compton-thick regime. The Ne\ v line is not detected in O3-S, but we measure a He II which is well above the expectation for star formation. Despite the remarkable alignment of O3-N and O3-S with the radio sources, we do not find evidence of shocks from a radio jet that could be powering the EELRs. We interpret this as O3-S being externally irradiated by the AGN, akin to the famous Hanny’s Voorwerp object in the local Universe. In addition, more classical line ratio diagnostics (e.g. vs N\ ii put the DSFG itself in the AGN region of the diagrams, and therefore suggest it to be the most probable AGN host. These results showcase the ability of JWST to unveil highly obscured AGN at high redshifts.