[O II] As a Tracer of Current Star Formation

[O II] λ3727 is often used as a tracer of star formation at intermediate redshifts (z ≳ 0.4), where Hα is not easily observed. We use the spectrophotometric data of the Nearby Field Galaxy Survey to investigate the range and systematic variation in the observed [O II]/Hα emission-line ratio as a function of galaxy luminosity at low redshift. We find that the observed [O II]/Hα ratio varies by a factor of 7 at luminosities near M_B*. The [O II]/Hα ratio is inversely correlated with luminosity. The scatter in the [O II]/Hα ratio and the dependence of the ratio on luminosity are due in equal parts to reddening and to the metallicity-dependent excitation of the interstellar medium. The uncertainty in star formation rates derived from [O II] fluxes is therefore large. If Hα cannot be observed, high signal-to-noise ratio Hβ fluxes are much preferable to [O II] fluxes for deriving star formation rates. We present several purely empirical corrections for extinction.