The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 μm surface brightness ratios

We examined variations in the 160/250 and 250/350 mu m surface brightness ratios within 24 nearby (<30 Mpc) 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 250/350 mu m ratios to H alpha and 24 mu m emission, which trace the light from star-forming regions, and 3.6 mu m 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 <= 160 mu m (and in rare cases potentially at <= 350 mu m) originates from dust heated by star-forming regions. In another subset, we found that the emission at >= 250 mu m (and sometimes at >= 160 mu m) 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.