Young, UV-bright Stars Dominate Dust Heating in Star Forming Galaxies
  In star forming galaxies, dust plays a significant role in shaping the
ultraviolet (UV) through infrared (IR) spectrum. Dust attenuates the radiation
from stars, and re-radiates the energy through equilibrium and non-equilibrium
emission. Polycyclic aromatic hydrocarbons (PAH), graphite, and silicates
contribute to different features in the spectral energy distribution; however,
they are all highly opaque in the same spectral region - the UV. Compared to
old stellar populations, young populations release a higher fraction of their
total luminosity in the UV, making them a good source of the energetic UV
photons that can power dust emission. However, given their relative abundance,
the question of whether young or old stellar populations provide most of these
photons that power the infrared emission is an interesting question. Using
three samples of galaxies observed with the Spitzer Space Telescope and our
dusty radiative transfer model, we find that young stellar populations (on the
order of 100 million years old) dominate the dust heating in star forming
galaxies, and old stellar populations (13 billion years old) generally
contribute less than 20% of the far-IR luminosity.
