Recalibration of [O II] $\lambda 3727$ as a Star Formation Rate Estimator for Active and Inactive Galaxies

We investigate the use of the [O II] $\lambda3727$ emission line as a star formation rate (SFR) estimator using Sloan Digital Sky Spectra for nearly 100,000 star-forming galaxies and 5,500 galaxies with narrow-line active galactic nuclei. Consistent with previous work, we find that the [O II]/H$\alpha$ ratio in star-forming galaxies depends strongly on gas-phase metallicity. Using metallicities derived from the [N II] $\lambda 6584$/[O II] $\lambda 3727$ method, we refine a metallicity-dependent SFR estimator based on [O II] that is calibrated within a scatter of 0.056 dex against the more commonly used SFR indicator based on H$\alpha$ emission. The scatter increases to only 0.12 dex if the metallicity is estimated using the stellar mass-metallicity relation. With the aim of extending the [O II]-based SFR estimator to active galaxies, we calculate radiation pressure-dominated photoionization models to constrain the amount of [O II] emission arising from the narrow-line region. We use the sample of active galaxies to demonstrate that the SFRs derived from [O II], after accounting for nonstellar contamination, are consistent with independent SFR diagnostics estimated from the stellar continuum of the host galaxies.