Star-Forming or Starbursting? The Ultraviolet Conundrum
  Compared to starburst galaxies, normal star forming galaxies have been shown
to display a much larger dispersion of the dust attenuation at fixed reddening
through studies of the IRX-beta diagram (the IR/UV ratio "IRX" versus the UV
color "beta"). To investigate the causes of this larger dispersion and attempt
to isolate second parameters, we have used GALEX UV, ground-based optical, and
Spitzer infrared imaging of 8 nearby galaxies, and examined the properties of
individual UV and 24 micron selected star forming regions. We concentrated on
star-forming regions, in order to isolate simpler star formation histories than
those that characterize whole galaxies. We find that 1) the dispersion is not
correlated with the mean age of the stellar populations, 2) a range of dust
geometries and dust extinction curves are the most likely causes for the
observed dispersion in the IRX-beta diagram 3) together with some potential
dilution of the most recent star-forming population by older unrelated bursts,
at least in the case of star-forming regions within galaxies, 4) we also
recover some general characteristics of the regions, including a tight positive
correlation between the amount of dust attenuation and the metal content.
Although generalizing our results to whole galaxies may not be immediate, the
possibility of a range of dust extinction laws and geometries should be
accounted for in the latter systems as well.
