A simple connection between the near- and mid-infrared emission of
  galaxies and their star-formation rates
  We have measured the near-infrared colors and the fluxes of individual pixels
in 68 galaxies common to the Spitzer Infrared Nearby Galaxies Survey and the
Large Galaxy Atlas Survey. Each galaxy was separated into regions of
increasingly red near-infrared colors. In the absence of dust extinction and
other non-stellar emission, stellar populations are shown to have relatively
constant NIR colors, independent of age. In regions of high star formation, the
average intensity of pixels in red-excess regions (at 1.25, 3.6, 4.5, 5.6, 8.0
and 24 micron) scales linearly with the intrinsic intensity of Halpha emission,
and thus with the star-formation rate within the pixel. This suggests that most
NIR-excess regions are not red because their light is being depleted by
absorption. Instead, they are red because additional infrared light is being
contributed by a process linked to star-formation. This is surprising because
the shorter wavelength bands in our study (1.25 micron-5.6 micron) do not probe
emission from cold (10-20 K) and warm (50-100 K) dust associated with
star-formation in molecular clouds. However, emission from hot dust (700-1000
K) and/or Polycyclic Aromatic Hydrocarbon molecules can explain the additional
emission seen at the shorter wavelengths in our study. The contribution from
hot dust and/or PAH emission at 2-5micron and PAH emission at 5.6 and 8.0
micron scales linearly with warm dust emission at 24 micron and the intrinsic
Halpha emission. Since both are tied to the star-formation rate, our analysis
shows that the NIR excess continuum emission and PAH emission at ~1-8 micron
can be added to spectral energy distribution models in a very straight-forward
way, by simply adding an additional component to the models that scales
linearly with star-formation rate.
