Dust-induced Systematic Errors in Ultraviolet-derived Star Formation Rates

Rest-frame far-ultraviolet (FUV) luminosities form the ‘backbone’ of our understanding of star formation at all cosmic epochs. These luminosities are typically corrected for dust by assuming that the tight relationship between the UV spectral slopes (β) and the FUV attenuations (AFUV) of starburst galaxies applies for all starforming galaxies. Data from seven independent UV experiments demonstrates that quiescent, ‘normal’ starforming galaxies deviate substantially from the starburst galaxy β–AFUV correlation, in the sense that normal galaxies are redder than starbursts. Spatially resolved data for the Large Magellanic Cloud suggests that dust geometry and properties, coupled with a small contribution from older stellar populations, cause deviations from the starburst galaxy β–AFUV correlation. Folding in data for starbursts and ultra-luminous infrared galaxies, it is clear that neither rest-frame UV-optical colors nor UV/Hα significantly help in constraining the UV attenuation. These results argue that the estimation of SF rates from rest-frame UV and optical data alone is subject to large (factors of at least a few) systematic uncertainties because of dust, which cannot be reliably corrected for using only UV/optical diagnostics. Subject headings: ultraviolet: galaxies — dust, extinction — galaxies: general — galaxies: stellar content