Gas-to-dust mass ratio for both the reference sample, consisting of HERACLES (red diamonds) and FCRAO (red squares) sources, and our (U)LIRG sample (black circles) as a function of Σ_SFR. Horizontal lines show the median value for each sample, and shaded areas indicate their median absolute deviations. Top panels: M_dust has been obtained leaving β as a free parameter in the SED fit and using the fitted T_dust to derive dust mass. Middle panels: emissivity index has been fixed to β = 1.8, but M_dust is still obtained from the fitted dust temperatures. Finally, in the bottom panels, emissivity is fixed to 1.8, and dust masses are obtained assuming a fixed 25 K dust temperature (see main text for further details). Left panels: results of using a Milky Way CO-to-H₂ conversion factor (α = 4). To make the gas-to-dust mass ratio of (U)LIRGs consistent with the reference sample, α has been modified to the value quoted in the right panels. For the remainder of this work, a fixed dust emissivity of β = 1.8 is assumed, and we adopt a CO-to-H₂ factor of α = 1.8 for (U)LIRGs, as required to make the median gas-to-dust mass ratio of (U)LIRGs match that of the reference sample of (non-LIRG) spiral galaxies.