We study the relationship between the mid-IR (5-18 Ám)
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 H and
the UV luminosity. The comparison of resolved images reveals a remarkable similarity
in the H and mid-IR morphologies, with prominent HII regions at both frequencies.
The mid-IR images, however, show in addition a di use emission not associated with HII
regions nor with the di use H 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. H , UV
and far-IR luminosity relation is mostly due to metallicity e ects, 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.