Map of regions selected for this study. The gray background image is the 21 cm map of M 33, and the six yellow ellipses indicate the boundary of the radial bins: 1, 2, 3, 4, 5, and 7 kpc. The colored dots indicate all pixels where the CO J = 2−1 integrated emission is above 3σ_CO, and they are color-coded according to the different selected regions described in the text: cyan for the central region (CR), blue for the northern arm (NA), magenta for the southern arm (SA), red for the eastern filament (EF), and green for the interarm (IA).