Local SDSS galaxies in the Herschel Stripe82 survey: A critical
  assessment of optically-derived star-formation rates
  We study a set of 3319 galaxies in the redshift interval 0.04 < z < 0.15 with
far-infrared (FIR) coverage from the Herschel Stripe 82 survey (HerS), and
emission-line measurements, redshifts, stellar masses and star-formation rates
(SFRs) from the SDSS (DR7) MPA/JHU database. About 40% of the sample are
detected in the Herschel/SPIRE 250 micron band. Total infrared (TIR)
luminosities derived from HerS and ALLWISE photometry allow us to compare
infrared and optical estimates of SFR with unprecedented statistics for diverse
classes of galaxies. We find excellent agreement between TIR-derived and
emission line-based SFRs for H II galaxies. Other classes, such as active
galaxies and evolved galaxies, exhibit systematic discrepancies between optical
and TIR SFRs. We demonstrate that these offsets are attributable primarily to
survey biases and the large intrinsic uncertainties of the D4000- and
colour-based optical calibrations used to estimate the SDSS SFRs of these
galaxies. Using a classification scheme which expands upon popular
emission-line methods, we demonstrate that emission-line galaxies with
uncertain classifications include a population of massive, dusty, metal-rich
star-forming systems that are frequently neglected in existing studies. We also
study the capabilities of infrared selection of star-forming galaxies. FIR
selection reveals a substantial population of galaxies dominated by cold dust
which are missed by the long-wavelength WISE bands. Our results demonstrate
that Herschel large-area surveys offer the means to construct large, relatively
complete samples of local star-forming galaxies with accurate estimates of SFR
that can be used to study the interplay between nuclear activity and
star-formation.
