Testing [O II] $\lambda 3727$ as a Star Formation Rate Tracer in Quasar Host Galaxies

Abstract

The [O II] $\lambda 3727$ emission line is a widely used star formation rate (SFR) tracer. However, its application to type I quasars is not straightforward, because the line can be affected by dust extinction, metallicity and contamination from the AGN narrow-line region (NLR). We test the reliability of [O II] SFRs using a sample of 202 SDSS and PG quasars, by comparing [O II] SFRs and reference far-infrared (FIR) SFRs derived from multiwavelength SED decomposition. We measure [O II], [O III], and narrow Balmer emission lines by spectral fitting. Then, we calculate [O II] SFRs after correcting dust extinction and metallicity. We then compare these SFRs with the FIR SFRs, both with and without subtracting the AGN contribution estimated from [O III]. After this correction, the median offset between [O II] and FIR SFRs is $-0.20\pm 0.72$ dex for the full analysis sample and $-0.17\pm 0.69$ dex for sources with S/N $>5$ in both [O II] and [O III]. Without subtracting the AGN contribution, the corresponding offsets are $0.00\pm 0.69$ and $0.12\pm 0.66$ dex. We conclude that [O II] is useful as a statistical SFR tracer for quasar host galaxies, but individual objects still require careful treatment of AGN contamination, extinction, metallicity, aperture effects, and redshift-dependent systematics.

25 pages, 8 figures