Hot-Dust-Poor Type 1 Active Galactic Nuclei in the COSMOS Survey

TL;DR

This study identifies a significant class of type 1 AGNs with weak near-infrared emission, revealing insights into their dust covering factors and accretion disk structures across different redshifts.

Contribution

It reports the discovery of hot-dust-poor AGNs and quantifies their properties, highlighting their increasing prevalence with redshift and implications for AGN unification models.

Findings

Hot-dust-poor AGNs constitute up to 20% at z > 2.

Their torus covering factor is significantly smaller than in typical AGNs.

Outer accretion disk edges are estimated at 10-23 times gravitational stability radii.

Abstract

We report a sizable class of type 1 active galactic nuclei (AGNs) with unusually weak near-infrared (1-3{\mu}m) emission in the XMM-COSMOS type 1 AGN sample. The fraction of these "hot-dust-poor" AGNs increases with redshift from 6% at lowredshift (z < 2) to 20% at moderate high redshift (2 < z < 3.5). There is no clear trend of the fraction with other parameters: bolometric luminosity, Eddington ratio, black hole mass, and X-ray luminosity. The 3{\mu}m emission relative to the 1{\mu}m emission is a factor of 2-4 smaller than the typical Elvis et al. AGN spectral energy distribution (SED), which indicates a "torus" covering factor of 2%-29%, a factor of 3-40 smaller than required by unified models. The weak hot dust emission seems to expose an extension of the accretion disk continuum in some of the source SEDs. We estimate the outer edge of their accretion disks to lie at $(0.3-2.0) /times 10^4$ Schwarzschild radii, ~10-23 times the gravitational stability radii. Formation scenarios for these sources are discussed.