# Hyperluminous starburst gives up its secrets

## TL;DR

This study investigates a hyperluminous starburst galaxy with an active galactic nucleus, revealing the presence of a supermassive black hole and exploring scenarios for its observed properties and structure.

## Contribution

The paper confirms a distant protocluster member and provides detailed analysis of the AGN in a HyLIRG, highlighting its characteristics and proposing models for its structure.

## Key findings

- Discovery of a supermassive black hole in the HyLIRG
- Evidence that AGN are common in HyLIRGs
- Multiple scenarios explaining the AGN's observed features

## Abstract

HATLAS J084933.4+021443 was identified as a dusty starburst via its rest-frame far-IR emission. Multi-frequency imaging and spectroscopy revealed a cluster of four dusty galaxies at $z = 2.41$, covering 80 kpc. Here, we use ALMA to confirm a more distant, fifth protocluster member, and present X-ray and rest-frame optical imaging spectroscopy of the brightest, an unlensed hyperluminous IR galaxy (HyLIRG). The data reveal broad H \alpha and bright [N II] lines, and bright X-ray emission, characteristics that betray a Type-1 active galactic nucleus (AGN), strengthening evidence that AGN are ubiquitous amongst HyLIRGs. The accreting black hole is super massive, $M_{\rm bh}\approx 2\times 10^9$ M$_\odot$, with little intrinsic absorption, $N_{\rm H}\approx 5\times 10^{21}$ cm$^{-2}$. The X-ray properties suggest the accretion luminosity rivals that of the starburst, yet it is not obvious where this emerges in its panchromatic spectral energy distribution (SED). We outline three scenarios that could give rise to the observed characteristics, and how we might distinguish between them. In the first, we see the AGN through the host galaxy because of the cavity it excavates. In the others, the AGN is not co-spatial with the starburst, having been ejected via asymmetric gravitational radiation, or having evolved towards the naked quasar phase in an unseen companion.