Hyperluminous starburst gives up its secrets
  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.
