Scatter plots showing Σ _gas as a function of Σ _dust for M 31. The columns show the relationship between dust ( x-axis) and (left) H i, (middle) H ₂ derived from CO using our best-fit α _CO, and (right) total (H i + H ₂) gas. Our best-fit α _CO yields reasonable linear scalings between total gas and dust in each system (the red line shows the median δ _GDR), usually a clear improvement on the relationship between dust and either phase alone.