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The Origin of the 4.5 micron Excess from Dwarf Galaxies

Dwarf galaxies tend to have redder [3.6 micron] - [4.5 micron] Spitzer broadband colors than spirals. To investigate this effect, for a large sample of dwarf galaxies we combine Spitzer fluxes with data at other wavelengths and compare to population synthesis models. Lower metallicity systems are found to have redder [3.6] - [4.5] colors on average, but with considerable scatter. The observed range in [3.6] - [4.5] color is too large to be accounted for solely by variations in stellar colors due to age or metallicity differences; interstellar effects must contribute as well. For the reddest systems, the 4.5 micron luminosity may not be a good tracer of stellar mass. We identify three factors that redden this color in dwarfs. First, in some systems, strong Br-alpha emission contributes significantly to the 4.5 micron emission. Second, in some cases high optical depths lead to strong reddening of the starlight in the Spitzer bands. Third, in some galaxies, the nebular continuum dominates the 4.5 micron flux, and in extreme cases, the 3.6 micron flux as well. The harder UV radiation fields in lower metallicity systems produce both more gaseous continuum in the infrared and more Br-alpha per star formation rate. The combination of these three factors can account for the 4.5 micron excess in our sample galaxies, thus it is not necessary to invoke a major contribution from hot dust to the 4.5 micron band. However, given the uncertainties, we are not able to completely rule out hot dust emission at 4.5 micron. More spectroscopic observations in the 3 - 5 micron range are needed to disentangle these effects.