A New Constraint On The Molecular Oxygen Abundance At Z ∼ 0.886

Abstract

We report Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter Array (ALMA) spectroscopy in the redshifted molecular oxygen (O₂) 56.265~GHz and 424.763~GHz transitions from the z=0.88582 gravitational lens towards PKS\,1830−21. The ALMA non-detection of O₂ 424.763~GHz absorption yields the 3σ upper limit N(O₂)≤5.8×1017~cm^-2 on the O₂ column density, assuming that the O₂ level populations are thermalized at the gas kinetic temperature of 80~K. The VLA spectrum shows absorption by the CH₃CHO 56.185~GHz and 56.265~GHz lines, with the latter strongly blended with the O₂ 56.265~GHz line. Since the two CH₃CHO lines have the same equilibrium strength, we used the known CH₃CHO 56.185~GHz line profile to subtract out the CH₃CHO 56.265~GHz feature from the VLA spectrum, and then carried out a search for O₂ 56.265~GHz absorption in the residual spectrum. The non-detection of redshifted O2 56.265~GHz absorption in the CH₃CHO-subtracted VLA spectrum yields N(O2)≤2.3×1017~cm^-2 . Our 3σ limits on the O2 abundance relative to H₂ are then X(O₂)≤9.1×10−6 (VLA) and X(O₂)≤2.3×10^-5 (ALMA). These are 5−15 times lower than the best previous constraint on the O₂ abundance in an external galaxy. The low O₂ abundance in the z=0.88582 absorber may arise due to its high neutral carbon abundance and the fact that its molecular clouds appear to be diffuse or translucent clouds with low number density and high kinetic temperature.