Ratios of integrated intensities HCO⁺/CO vs. HCN/CO for M 33 (red points: this work) and for M 31 (green points: BR05). Upper/lower limits are denoted by arrows. Linear least squares fits to data from M 33 (red solid line) and M 31 (BR05, black dashed) are shown. Both fits exclude points with upper limits. The solid black line shows the angle bisector where I(HCO⁺) = I(HCN). The gray shaded areas display the range of the HCN/CO found in the disk of the Milky Way (MW) by (Helfer & Blitz 1997, HE97) (darker gray) and in a sample of normal spiral galaxies (GA04b) (lighter gray).