Herschel-ATLAS: A binary HyLIRG pinpointing a cluster of starbursting protoellipticals
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Panchromatic observations of the best candidate hyperluminous infrared galaxies from the widest Herschel extragalactic imaging survey have led to the discovery of at least four intrinsically luminous z = 2.41 galaxies across an ≈100 kpc region—a cluster of starbursting protoellipticals. Via subarcsecond interferometric imaging we have measured accurate gas and star formation surface densities. The two brightest galaxies span ∼3 kpc FWHM in submillimeter/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 Mdyn of several ×1011M , 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 5×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 that this system is the progenitor of a B(inary)-type ≈1014.6-M cluster.