Preprint typeset using LATEX style emulateapj v. 11/10/09 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 1.25µm, 3.6µm, 4.5µm, 5.6µm, 8.0µm and 24µm) scales linearly with the intrinsic intensity of Hα 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 (1.25µm–5.6µm) 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 2µm–5µm and PAH emission at 5.6µm and 8.0µm scales linearly with warm dust emission at 24µm and the intrinsic Hα emission. Since both are tied to the star-formation rate, our analysis shows that the NIR excess continuum emission and PAH emission at ∼ 1−8µm 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. 1