Figure 3: The conversion factor from CO luminosity to molecular gas mass.

The four red diamonds denote the result of our metal-poor star-forming regions, and all other symbols denote those observations in the literature^9,22,23,24. The error bars of our measurements are the s.d., which are caused by the uncertainties on the CO luminosity, the HI gas mass and the dust mass as well as the dust-to-gas ratio. The lines denote models’ predictions, including the empirical one²⁴ as well as theoretical ones^26,27,28,29. The parameters in those models were set basically following the literature work⁸, including a linear scaling of the dust-to-gas ratio with the metallicity, and a typical gas surface density of 100  pc⁻² in the Milky Way.