Dissecting the origin of the submillimetre emission in nearby galaxies with Herschel and LABOCA

We model the infrared to submillimetre spectral energy distribution of 11 nearby galaxies of the Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel sample using Spitzer and Herschel data and compare model extrapolations at 870 μm (using different fitting techniques) with Large APEX BOlometer CAmera (LABOCA) 870 μm observations. We investigate how the differences between predictions and observations vary with model assumptions or environment. At global scales, we find that modified blackbody models using realistic cold emissivity indices (β_c = 2 or 1.5) are able to reproduce the 870 μm observed emission within the uncertainties for most of the sample. Low values (β_c < 1.3) would be required in NGC 0337, NGC 1512 and NGC 7793. At local scales, we observe a systematic 870 μm excess when using β_c = 2.0. The β_c = 1.5 or the Draine & Li (2007) models can reconcile predictions with observations in part of the discs. Some of the remaining `excesses' occur towards the centres and can be partly or fully accounted for by non-dust contributions such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In three non-barred galaxies, the remaining excesses rather occur in the disc outskirts. This could be a sign of a flattening of the submm slope (and decrease of the effective emissivity index) with radius in these objects.