Accretion Rates of Red Quasars from the Hydrogen P$β$ line

Abstract

Red quasars are thought to be an intermediate population between merger-driven star-forming galaxies in dust-enshrouded phase and normal quasars. If so, they are expected to have high accretion ratios, but their intrinsic dust extinction hampers reliable determination of Eddington ratios. Here, we compare the accretion rates of 16 red quasars at $z \sim 0.7$ to those of normal type 1 quasars at the same redshift range. The red quasars are selected by their red colors in optical through near-infrared (NIR) and radio detection. The accretion rates of the red quasars are derived from the P$β$ line in NIR spectra, which is obtained by the SpeX on the Infrared Telescope Facility (IRTF) in order to avoid the effects of dust extinction. We find that the measured Eddington ratios ($L_{\rm bol}$/$L_{\rm Edd} \simeq 0.69$) of red quasars are significantly higher than those of normal type 1 quasars, which is consistent with a scenario in which red quasars are the intermediate population and the black holes of red quasars grow very rapidly during such a stage.