Distribution of the CO(2−1) derived molecular gas mass surface densities using the MW α_CO (continuous black histogram) and a varying α_CO that follows the stellar mass density distribution (dashed blue histogram), using the relation given in Sandstrom et al. (2013). The vertical dashed and continuous red lines show the average density at the galaxy centers, estimated over the central ≃1 kpc², assuming the variable α_CO and the MW α_CO, respectively.