Constraints on the star formation rate in active galaxies

The [O (II)] lambda 3727 emission line is often used as an indicator of star formation rate in extragalactic surveys, and it can be an equally effective tracer of star formation in systems containing luminous active galactic nuclei (AGNs). In order to investigate the ongoing star formation rate of the host galaxies of AGNs, we measured the strength of [O (II)] and other optical emission lines from a large sample (similar to 3600) of broad-line (type 1) AGNs selected from the Sloan Digital Sky Survey. We performed a set of photoionization calculations to help evaluate the relative contribution of stellar and nonstellar photoionization to the observed strength of [O (II)]. Consistent with the 2005 study by Ho, we find that the observed [O (II)] emission can be explained entirely by photoionization from the AGN itself, with little or no additional contribution from H (II) regions. This indicates that the host galaxies of type 1 AGNs experience very modest star formation concurrent with the optically active phase of the nucleus. By contrast, we show that the sample of "type 2'' quasars selected from the Sloan Digital Sky Survey does exhibit substantially stronger [O (II)] emission, consistent with an elevated level of star formation, a result that presents a challenge to the simplest form of the AGN unification model.