Hidden in plain sight: a massive, dusty starburst in a galaxy
  protocluster at z=5.7 in the COSMOS field
  We report the serendipitous discovery of a dusty, starbursting galaxy at
$z=5.667$ (hereafter called CRLE) in close physical association with the
"normal" main-sequence galaxy HZ10 at $z=5.654$. CRLE was identified by
detection of [CII], [NII] and CO(2-1) line emission, making it the highest
redshift, most luminous starburst in the COSMOS field. This massive, dusty
galaxy appears to be forming stars at a rate of at least 1500$\,M_\odot$
yr$^{-1}$ in a compact region only $\sim3$ kpc in diameter. The dynamical and
dust emission properties of CRLE suggest an ongoing merger driving the
starburst, in a potentially intermediate stage relative to other known dusty
galaxies at the same epoch. The ratio of [CII] to [NII] may suggest that an
important ($\sim15\%$) contribution to the [CII] emission comes from a diffuse
ionized gas component, which could be more extended than the dense,
starbursting gas. CRLE appears to be located in a significant galaxy
overdensity at the same redshift, potentially associated with a large-scale
cosmic structure recently identified in a Lyman Alpha Emitter survey. This
overdensity suggests that CRLE and HZ10 reside in a protocluster environment,
offering the tantalizing opportunity to study the effect of a massive starburst
on protocluster star formation. Our findings support the interpretation that a
significant fraction of the earliest galaxy formation may occur from the inside
out, within the central regions of the most massive halos, while rapidly
evolving into the massive galaxy clusters observed in the local Universe.
