Molecular depletion times and the CO-to-H2 conversion factor in metal-poor galaxies

Tracing molecular hydrogen content with carbon monoxide in low-metallicity galaxies has been exceedingly difficult. Here we present a new effort, with IRAM 30-m observations of (CO)-C-12(1-0) of a sample of 8 dwarf galaxies having oxygen abundances ranging from 12 + log(O/H) similar to 7.7 to 8.4. CO emission is detected in all galaxies, including the most metal-poor galaxy of our sample (0.1 Z(circle dot)); to our knowledge this is the largest number of (CO)-C-12(1-0) detections ever reported for galaxies with 12 + log(O/H) less than or similar to 8 (0.2 Z(circle dot)) outside the Local Group. We calculate stellar masses, M-star, and star-formation rates (SFRs), and analyze our results in conjunction with galaxy samples from the literature. Extending previous results for a correlation of the molecular gas depletion time, tau(dep), with M-star and specific SFR (sSFR), we find a variation in tau(dep) of a factor of 200 or more (from less than or similar to 50 Myr to similar to 10 Gyr) over a spread of similar to 10(3) in sSFR and M-star. We exploit the variation of tau(dep) to constrain the CO-to-H-2 mass conversion factor alpha(CO) at low metallicity, and assuming a power-law variation find alpha(CO) proportional to Z/Z(circle dot)(-2), similar to results based on dust continuum measurements compared with gas mass. By including HI measurements, we show that the fraction of total gas mass relative to the baryonic mass is higher in galaxies that are metal poor, of low mass, and with high sSFR. Finally, comparisons of the data with star-formation models of the molecular gas phases show that the models are generally quite successful, but at metallicities Z/Z(circle dot) less than or similar to 0.2, there are some discrepancies.