Accretion Rates of Red Quasars from the Hydrogen P$\beta$ line

TL;DR

This study compares accretion rates of red quasars and normal quasars at similar redshifts, finding red quasars exhibit higher Eddington ratios, indicating rapid black hole growth during their intermediate evolutionary stage.

Contribution

It introduces a method to measure accretion rates of red quasars using the Pβ line in NIR spectra, overcoming dust extinction issues, and provides evidence of their high accretion activity.

Findings

Red quasars have higher Eddington ratios (~0.69) than normal quasars.

Red quasars are likely in a rapid black hole growth phase.

The Pβ line in NIR spectra effectively measures accretion rates despite dust extinction.

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$\beta$ 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.