Molecular gas in Markarian 109: Constraining the O-2/CO ratio in chemically young galaxies

We report on observations of (CO)-C-12 (1 --> 0) emission from the chemical ly young starburst galaxy Mrk 109. These observations were part of a study to constrain the O-2/CO ratio in metal-deficient galaxies and were motivate d by theoretical work that suggests the possible enhancement of the O-2/CO ratio in chemically young systems. Five low-metallicity (Z less than or equ al to 0.5 Z.) IRAS galaxies at redshifts z > 0.02 (required to shift the 11 8.75 GHz O-16(2) line away from the atmospheric line) were searched for CO emission. We detected the CO line only in Mrk 109. From O-2 observations of Mrk 109 we achieved an upper limit for the O-2 column density abundance ra tio of N(O-2)/N(CO) < 0.31. These results provide useful constraints for th e theoretical models of chemically young galaxies. We argue that either mos t of the molecular gas in Mrk 109 does not reside in dark clouds (A(v) grea ter than or similar to 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) similar or equal to 4 x 10(9) M., and th e CO data are consistent with a central starburst induced by the interactio n with a nearby companion.