The Nature of the Second Parameter in the IRX-β 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 and dust attenuation in normal star-forming galaxies. The IRX-β 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 β. Previous research has indicated that while starburst galaxies exhibit a relatively tight IRX-β 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 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(Hα)) and the UV color β the trends are mostly driven by the galaxies best-described by instantaneous burst populations. We also find a significant correlation between β 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-β diagram is better correlated with β than with the perpendicular distance, d_p .