A redline starburst: CO(2-1) observations of an Eddington-limited galaxy reveal star formation at its most extreme

We report observations of the CO(2-1) emission of SDSS J1506+54, a compact (r(e) approximate to 135 pc) starburst galaxy at z = 0.6. SDSS J1506+ 54 appears to be forming stars close to the limit allowed by stellar radiation pressure feedback models: the measured L-IR/L-CO' approximate to 1500 is one of the highest measured for any galaxy. With its compact optical morphology but extended low surface brightness envelope, post-starburst spectral features, high infrared luminosity (L-IR > 10(12.5) L-circle dot), low gas fraction (M-H2 /M-star approximate to 15%), and short gas depletion time (tens of Myr), we speculate that this is a feedback-limited central starburst episode at the conclusion of a major merger. Taken as such, SDSS J1504+54 epitomizes the brief closing stage of a classic model of galaxy growth: we are witnessing a key component of spheroid formation during what we term a "redline" starburst.