CO (3–2) and CO (2–1) Detections in a z = 1.3 Hyper-Luminous Starburst Galaxy

We present an ~ 2^ resolution image of the CO (2–1) emission and an ~ 4^ resolution image of the CO (3–2) emission in the $z=1.3$ hyper-luminous starburst galaxy MIPS J142824.0$+$352619, obtained at the Nobeyama Radio Observatory. Our new CO (3–2) and CO (2–1) emission detections yield L'CO\,(3 -- 2) = 1.4 \; (± 0.5) × 10¹¹ and L'CO\,(2 --1) = 1.2 \; (± 0.3) × 10¹¹ \,K \,km \,s⁻¹ \,pc², which translate to a molecular gas mass of ~ 10¹¹ \,M_, assuming a CO to H₂ conversion factor appropriate for ULIRGs. The derived high CO luminosities place this source as being one of the brightest galaxies detected in CO to date. The CO (3–2) and CO (2–1) derived redshifts are 1.3248 ± 0.0002 and 1.3250 ± 0.0002. If the bulk of the molecular gas traced in these lines is completely thermalized, as suggested by L'CO\,(3 -- 2) / L'CO\,(2 --1) ~ 1, a constraint to the gas density of n 10³ \,cm⁻³ is obtained from our Large Velocity Gradient (LVG) analysis. If a significant fraction of the bright FIR luminosity of LFIR = (3.2 ± 0.7) × 10¹³ \,L_ arises from starburst activity, then the Star Formation Efficiency (SFE) is 320 \,L_ \,M_ ⁻¹. The size constraint of 1 .3 given by the CO (2–1) map provides an upper limit to the lensing magnification factor of μ 8, which further supports the earlier claim that the magnification of this source is only modest.