Herschel-ATLAS: a binary HyLIRG pinpointing a cluster of starbursting proto-ellipticals

TL;DR

This paper reports the discovery of a cluster of four luminous, starbursting proto-elliptical galaxies at z=2.41, revealing extreme star formation and complex gas dynamics, likely evolving into a massive galaxy cluster.

Contribution

It presents the first detailed multi-galaxy analysis of a HyLIRG cluster at high redshift, highlighting complex gas kinematics and starburst activity in proto-ellipticals.

Findings

Discovered at least four luminous galaxies in a 100-kpc region at z=2.41.

Measured gas and star-formation surface densities with sub-arcsecond imaging.

Identified counter-rotating gas disks with high star-formation efficiency.

Abstract

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.