[O II] as a tracer of current star formation

[O II] lambda 3727 is often used as a tracer of star formation at intermediate redshifts (z greater than or similar to 0.4), where H alpha 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]/H alpha emission-line ratio as a function of galaxy luminosity at low redshift. We find that the observed [O II]/H alpha ratio varies by a factor of 7 at luminosities near M-B*. The [O II]/H alpha ratio is inversely correlated with luminosity. The scatter in the [O II]/H alpha 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 H alpha cannot be observed, high signal-to-noise ratio H beta fluxes are much preferable to [O II] fluxes for deriving star formation rates. We present several purely empirical corrections for extinction.