The discovery of the most UV-Lya luminous star-forming galaxy: a young, dust- and metal-poor starburst with QSO-like luminosities

Abstract

We report the discovery of BOSS-EUVLG1 at z = 2.469, by far the most luminous, almost un-obscured star-forming galaxy known at any redshift. First classified as a QSO within the Baryon Oscillation Spectroscopic Survey, follow-up observations with the Gran Telescopio Canarias reveal that its large luminosity, M UV −24.40 and log(L Lyα /erg s −1 ) 44.0, is due to an intense burst of star-formation, and not to an AGN or gravitational lensing. BOSS-EUVLG1 is a compact (r eff 1.2 kpc), young (4-5 Myr) starburst with a stellar mass log(M * /M )=10.0±0.1 and a prodigious star formation rate of 1000 M yr −1 . However, it is metal-and dust-poor (12+log(O/H)=8.13±0.19, E(B-V) 0.07, log(L IR /L UV ) < −1.2), indicating that we are witnessing the very early phase of an intense starburst that has had no time to enrich the ISM. BOSS-EUVLG1 might represent a short-lived (<100 Myrs), yet important phase of star-forming galaxies at high redshift that has been missed in previous surveys. Within a galaxy evolutionary scheme, BOSS-EUVLG1 could likely represent the very initial phases in the evolution of massive quiescent galaxies, even before the dusty star-forming phase.