2015ApJ...811L..23K

We report Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) spectroscopy in the redshifted molecular oxygen (O₂) 56.265 and 424.763 GHz transitions from the z = 0.88582 gravitational lens toward PKS 1830-21. The ALMA non-detection of O₂424.763 GHz absorption yields the 3σ upper limit N(0₂) ≤ 5.8 x 10¹⁷/cm2 on the O₂ column density, assuming that the O₂level populations are thermalized at the gas kinetic temperature of 80 K. The VLA spectrum shows absorption by the CH₃CHO 56.185 and 56.265 GHz lines, with the latter strongly blended with the O₂56.265 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 O₂56.265 GHz absorption in the residual spectrum. The non-detection of redshifted O₂56.265 GHz absorption in the CH₃CHO-subtracted VLA spectrum yields N(0₂) ≤ 2.3 x 10¹⁷/cm2 . Our 3σ limits on the O₂ abundance relative to H₂are then X(0₂) ≤ 9.1 x 10^–6 (VLA) and X(0₂) ≤ 2.3 x 10^–5 (ALMA). These are 5-15 times lower than the best previous constraint on the O₂abundance in an external galaxy. The low O₂abundance 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.