CO (3-2) and CO (2-1) detections in a z = 1.3 hyper-luminous starburst galaxy

Abstract

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 Kkms -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. © 2006. Astronomical Society of Japan.