We report observations of the CO ( 2–1 ) emission of SDSS J1506+54 , a compact ( r _ { e } \approx 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 ^ { \prime } _ { CO } \approx 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 _ { \odot } ) , low gas fraction ( M _ { H _ { 2 } } / M _ { \star } \approx 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 .