2020MNRAS.494L..48P

Recent analyses of Herschel observations suggest that in nearby disc galaxies the dust mass opacity at 500 µm, κ₅₀₀, decreases with increasing gas surface density, Σ_ISM. This apparent anticorrelation between κ₅₀₀ and Σ_ISM is opposite to the behaviour expected from theoretical dust evolution models; in such models, dust in denser, cooler regions (i.e. regions of increased Σ_ISM) tends to grow and therefore to have increased κ₅₀₀. We show, using a toy model, that the presence of a range of dust temperatures along the line of sight can lead to spuriously low estimated values of κ₅₀₀. If in regions of higher Σ_ISM the range of dust temperatures extends to lower values (as seems likely), the magnitude of this effect may be sufficient to explain the apparent anticorrelation between κ₅₀₀ and Σ_ISM. Therefore there may not be any need for spatial variation in the intrinsic dust properties that run counter to theoretical expectations.