  Panchromatic observations of the best candidate HyLIRG from the widest
Herschel extragalactic imaging survey have led to the discovery of at least
four intrinsically luminous z=2.41 galaxies across a ~100-kpc region - a
cluster of starbursting proto-ellipticals. Via sub-arcsecond interferometric
imaging we have measured accurate gas and star-formation surface densities. The
two brightest galaxies span ~3 kpc FWHM in submm/radio continuum and CO J=4-3,
and double that in CO J=1-0. The broad CO line is due partly to the multitude
of constituent galaxies and partly to large rotational velocities in two
counter-rotating gas disks -- a scenario predicted to lead to the most intense
starbursts, which will therefore come in pairs. The disks have M(dyn) of
several x 10^11 Msun, and gas fractions of ~40%. Velocity dispersions are
modest so the disks are unstable, potentially on scales commensurate with their
radii: these galaxies are undergoing extreme bursts of star formation, not
confined to their nuclei, at close to the Eddington limit. Their specific
star-formation rates place them ~>5x above the main sequence, which supposedly
comprises large gas disks like these. Their high star-formation efficiencies
are difficult to reconcile with a simple volumetric star-formation law. N-body
and dark matter simulations suggest this system is the progenitor of a
B(inary)-type ~10^14.6-Msun cluster.
