The Dust-to-Gas Ratio and the Role of Radiation Pressure in Luminous, Obscured Quasars

TL;DR

This paper investigates the relationship between dust-to-gas ratios, radiation pressure, and obscuration in quasars, revealing that luminous obscured quasars often have high Eddington ratios, challenging previous feedback models at high luminosities.

Contribution

It provides a comprehensive analysis of obscuration and Eddington ratios across different luminosities, highlighting the role of radiation pressure and evolutionary timescales in luminous quasars.

Findings

Luminous obscured quasars at high redshift have high Eddington ratios.

Moderate luminosity AGN show lower Eddington ratios and different obscuration properties.

Extended obscuration and short luminous phases explain the observed quasar fractions.

Abstract

The absence of high Eddington ratio, obscured Active Galactic Nuclei (AGN) in local ($z\lesssim0.1$) samples of moderate luminosity AGN has generally been explained to result from radiation pressure on the dusty gas governing the level of nuclear ($\lesssim10$pc) obscuration. However, very high accretion rates are routinely reported among obscured quasars at higher luminosities, and may require a different feedback mechanism. We compile constraints on obscuration and Eddington ratio for samples of X-ray, optical, infrared, and submm selected AGN at quasar luminosities. Whereas moderate luminosity, obscured AGN in the local universe have a range of lower Eddington ratios ($f_{\rm Edd} \sim 0.001-0.1$), the most luminous ($L_{\rm bol} \gtrsim 10^{46} $erg s$^{-1}$) IR/submm-bright, obscured quasars out to $z\sim3$ commonly have very high Eddington ratios ($f_{\rm Edd} \sim 0.1-1$). This apparent lack of radiation pressure feedback in luminous obscured quasars is likely coupled with AGN timescales, such that a higher fraction of luminous obscured quasars are seen due to the short timescale for which quasars are most luminous. Adopting quasar evolutionary scenarios, extended ($\sim10^{2-3}$pc) obscuration may work together with the shorter timescales to explain the observed fraction of obscured, luminous quasars, while outflows driven by radiation pressure will slowly clear this material over the AGN lifetime.