Molecular Gas in Markarian 109: Constraining the O2/CO Ratio in Chemically Young Galaxies

We report on observations of 12CO (1 → 0) emission from the chemically young starburst galaxy Mrk 109. These observations were part of a study to constrain the O2/CO ratio in metal-deficient galaxies and were motivated by theoretical work that suggests the possible enhancement of the O2/CO ratio in chemically young systems. Five low-metallicity (Z ≤ 0.5 Z☉) IRAS galaxies at redshifts z > 0.02 (required to shift the 118.75 GHz16O2 line away from the atmospheric line) were searched for CO emission. We detected the CO line only in Mrk 109. From O2 observations of Mrk 109 we achieved an upper limit for the O2 column density abundance ratio of N(O2)/N(CO) < 0.31. These results provide useful constraints for the theoretical models of chemically young galaxies. We argue that either most of the molecular gas in Mrk 109 does not reside in dark clouds (AV ≳ 5) or the standard equilibrium chemistry models are inadequate for metal-poor systems. The molecular gas mass implied by the CO observations of Mrk 109 is M(H2) ≃ 4 × 109 M☉, and the CO data are consistent with a central starburst induced by the interaction with a nearby companion.