We report on observations of ^ { 12 } CO ( 1 \rightarrow 0 ) emission from the chemically young starburst galaxy Mrk 109 .
These observations were part of a study to constrain the O _ { 2 } /CO ratio in metal–deficient galaxies , which were motivated by theoretical work that suggests the possible enhancement of the O _ { 2 } /CO ratio in chemically young systems .
Five low metallicity ( Z \leq 0.5 Z _ { \odot } ) IRAS galaxies at redshifts z > 0.02 ( required to shift the 118.75 GHz ^ { 16 } O _ { 2 } line away from the atmospheric line ) were searched for CO emission .
We detected the CO line in only Mrk 109 .
From O _ { 2 } observations of Mrk 109 , we achieved an upper limit for the O _ { 2 } column density abundance ratio of N ( { O } _ { 2 } ) / 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 ( A _ { V } \ga 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 ( { H } _ { 2 } ) \simeq 4 \times 10 ^ { 9 } M _ { \sun } , and the CO data are consistent with a central starburst induced by the interaction with a nearby companion .