¹²CO and ¹³CO observation of the low-metallicity dwarf galaxy DDO 154

The conversion factor from carbon monoxide (CO) intensity to molecular gas mass is a source of large uncertainty in understanding gas and its relation to star formation in galaxies. In particular, the conversion factor in low-metallicity environments have remained elusive, as currently only two galaxies have been detected in any CO isotopes in environments with 12 + log (O/H) < 8.0. Here we report (CO)-C-12 (J = 1-0) and (CO)-C-13 (J = 1-0) observations towards a star-forming region in DDO 154, a low-metallicity dwarf irregular galaxy at 12 + log (O/H) = 7.67. This is a re-observation of a previous non-detection at higher angular and velocity resolution. No significant emission was detected. By estimating the molecular gas mass from associated star formation, we find that DDO 154 has a conversion factor of more than 10(3) times the Milky Way. Alternatively, if we estimate molecular mass using dust continuum emission, the conversion factor is at least 2 orders of magnitude larger than the Milky Way. These estimates signify a large amount of CO-dark molecular gas in this galaxy.