[O II] emission in quasar host galaxies: Evidence for a suppressed star formation efficiency

The [O (II)] lambda 3727 line, a commonly used estimator of star formation rate in extragalactic surveys, should be an equally effective tracer of star formation in the host galaxies of quasars, whose narrow-line regions are expected to produce weak low-ionization emission. Quasar spectra generally show little or no [O (II)] emission beyond that expected from the active nucleus itself. The inferred star formation rates in optically selected quasars are typically below a few M-circle dot yr(-1), and some are significantly less. Quasars do not appear to occur coevally with starbursts. Recent observations, on the other hand, reveal abundant molecular gas in low-redshift quasars. These two results suggest that the star formation efficiency in quasar host galaxies is somehow suppressed during the active phase of the nucleus. The low star formation rates also imply that the nonstellar nucleus powers the bulk of the thermal infrared emission in radio-quiet quasars.