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.
