Mid - IR emission of galaxies in the Virgo cluster and in the Coma supercluster . IV . The nature of the dust heating sources

We study the relationship between the mid-IR (5-18 µm) emission of late-type galaxies and various other star formation tracers in order to investigate the nature of the dust heating sources in this spectral domain. The analysis is carried out using a sample of 123 normal, late-type, nearby galaxies with available data at several frequencies. The mid-IR luminosity (normalized to the H-band luminosity) correlates better with the far-IR luminosity than with more direct tracers of the young stellar population such as the Hα and the UV luminosity. The comparison of resolved images reveals a remarkable similarity in the Hα and mid-IR morphologies, with prominent HII regions at both frequencies. The mid-IR images, however, show in addition a diffuse emission not associated with HII regions nor with the diffuse Hα emission. This evidence indicates that the stellar population responsible for the heating of dust emitting in the mid-IR is similar to that heating big grains emitting in the far-IR, including relatively evolved stars responsible for the non-ionizing radiation. The scatter in the mid-IR vs. Hα, UV and far-IR luminosity relation is mostly due to metallicity effects, with metal-poor objects having a lower mid-IR emission per unit star formation rate than metal-rich galaxies. Our analysis indicates that the mid-IR luminosity is not an optimal star formation tracer in normal, late-type galaxies.