Metallicity dependence of the CO-to-H ₂ conversion factor. α _CO is derived from the ratio of observed SFR scaled by a constant H ₂ depletion time, τ _dep = 1.8 Gyr, and the observed CO luminosity, L _CO. Bigger symbols show galaxy-integrated measurements of dwarf galaxies from this work; smaller symbols show data for our literature compilation with starbursts highlighted by stars. The horizontal dashed line shows the Galactic conversion factor, and the diagonal lines show regression fits: to all galaxies (dotted line), to all non-starburst galaxies (dashed line), and exclusively to the HERACLES sample (solid line).