The Nature of the Second Parameter in the IRX-beta Relation for Local
  Galaxies
  We present an analysis of 98 galaxies of low-dust content, selected from the
Spitzer Local Volume Legacy survey, aimed at examining the relation between the
ultraviolet (UV) color, beta, and dust attenuation in normal star-forming
galaxies. The IRX-beta diagram relates the total dust attenuation in a galaxy,
traced by the far-IR (FIR) to UV ratio, to the observed UV color, indicated by
beta. Previous research has indicated that while starburst galaxies exhibit a
relatively tight IRX-beta relation, normal star-forming galaxies do not, and
have a much larger spread in the total-IR to far-UV (FUV) luminosity for a
fixed UV color. We examine the role that the age of the stellar population
plays as the "second parameter" responsible for the observed deviation and
spread of star-forming galaxies from the starburst relation. We model the FUV
to FIR spectral energy distribution (SED) of each galaxy according to two broad
bins of star formation history (SFH): constant and instantaneous burst. We find
clear trends between stellar population mean age estimators
(extinction-corrected FUV/NIR, U-B, and EW(H-alpha)) and the UV color, beta;
the trends are mostly driven by the galaxies best-described by instantaneous
burst populations. We also find a significant correlation between beta and the
mean age directly determined from the best-fit instantaneous models. As already
indicated by other authors, the UV attenuation in star-forming galaxies may not
be recovered with the UV color alone and is highly influenced by the stellar
population's mean age and SFH. Overall, the scatter in the IRX-beta diagram is
better correlated with beta than with the perpendicular distance.
