Scaling relations for CO luminosity. Galaxy-integrated CO(1–0) luminosity as a function of B-band magnitude (top) and metallicity (middle), and the ratio of L _CO and L _B as a function of metallicity (bottom). Bigger symbols show stacking results of this work; smaller symbols show a compilation of literature measurements. Color highlights metallicity. The solid line in the top panel shows a constant L _CO/ L _B ratio intersecting the bright galaxies. The trends are correlated by the well-established luminosity–metallicity relation.