2015MNRAS.448..135B

We examined variations in the 160/250 and 250/350µm surface brightness ratios within 24 nearby (<30Mpc) face-on spiral galaxies observed with the Herschel Space Observatory to identify the heating mechanisms for dust emitting at these wavelengths. The analysis consisted of both qualitative and quantitative comparisons of the 160/250 and 250/350µm ratios to Hα and 24µm emission, which trace the light from star-forming regions, and 3.6µm emission, which traces the light from the older stellar populations of the galaxies. We find broad variations in the heating mechanisms for the dust. In one subset of galaxies, we found evidence that emission at ≤ 160µm (and in rare cases potentially at ≤ 350µm) originates from dust heated by star-forming regions. In another subset, we found that the emission at ≥ 250µm (and sometimes at ≥ 160µm) originates from dust heated by the older stellar population. In the rest of the sample, either the results are indeterminate or both of these stellar populations may contribute equally to the global dust heating. The observed variations in dust heating mechanisms do not necessarily match what has been predicted by dust emission and radiative transfer models, which could lead to overestimated dust temperatures, underestimated dust masses, false detections of variability in dust emissivity, and inaccurate star formation rate measurements.