A Comparison of Uit Far-ultraviolet and Hα Star Formation Rates

We have used archival ultraviolet (UV) imaging of 50 nearby star-forming galaxies obtained with the Ultraviolet Imaging Telescope (UIT) to derive integrated near-UV and far-UV magnitudes, and have combined these data with Hα, far-infrared, and thermal radio continuum measurements to explore the consistency of UV and Hα star formation rates (SFRs). In agreement with previous studies, we find that the UV and Hα SFRs are qualitatively consistent, even before corrections for extinction are applied. The uncorrected UV SFRs are systematically lower by a factor of 1.5 (with a factor of two scatter) among luminous galaxies with SFR & 1 M⊙ yr−1, indicating a higher effective attenuation of the far-UV radiation. Among less luminous galaxies there is no significant offset between the Hα and far-UV SFR scales. This behavior is consistent with those of higher-redshift samples observed by Sullivan et al., Glazebrook et al., and Yan et al. for comparable ranges of galaxy luminosities and absolute SFRs. Far-infrared and thermal radio continuum data available for a subset of our sample allow us to estimate the attenuation in the UV and at Hα independently. The UV and Hα attenuations appear to be correlated, and confirm systematically higher attenuations in the UV. Although the galaxies in our sample show modest levels of attenuation (with median values of 0.9 mag at Hα and 1.4 mag at 1550 Å), the range across the sample is large, ∼4 mag for Hα and &5 mag in the far-UV (1550 Å). This indicates that the application of a single characteristic extinction correction to Hα or UV SFRs is only realistic for large, well-defined and well-studied galaxy samples, and that extinction bias may be important for UV or emission-line selected samples of star-forming galaxies. Subject headings: galaxies : general — galaxies : photometry — galaxies : stellar content — galaxies : evolution — ultraviolet : galaxies — dust, extinction