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/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 O2 424.763 GHz absorption yields the upper limit 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 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 cm-2. Our limits on the O2 abundance relative to H2 are then (VLA) and (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.