A complex dust morphology in the high-luminosity AGN Mrk 876

TL;DR

This study investigates the dust morphology around the high-luminosity AGN Mrk 876 using near-infrared variability, revealing a complex, disk-like dust structure with a larger-than-expected radius, consistent with a flared disk model.

Contribution

First near-IR variable spectrum analysis of Mrk 876, linking dust structure to accretion disk models and proposing a flared disk morphology for hot dust.

Findings

Dust radius exceeds IR response time measurements.

Reduced hot dust variability suggests a secondary dust component or superposition effects.

Dust structure is consistent with a flared, disk-like morphology.

Abstract

Recent models for the inner structure of active galactic nuclei (AGN) advocate the presence of a radiatively accelerated, dusty outflow launched from the outer regions of the accretion disk. Here we present the first near-infrared (near-IR) variable (rms) spectrum for the high-luminosity, nearby AGN Mrk 876. We find that it tracks the accretion disk spectrum out to longer wavelengths than the mean spectrum due to a reduced dust emission. The implied outer accretion disk radius is consistent with the infrared results predicted by a contemporaneous optical accretion disk reverberation mapping campaign and much larger than the self-gravity radius. The reduced flux variability of the hot dust could be either due to the presence of a secondary, constant dust component in the mean spectrum or introduced by the destructive superposition of the dust and accretion disk variability signals or some combination of both. Assuming thermal equilibrium for optically thin dust, we derive the luminosity-based dust radius for different grain properties using our measurement of the temperature. We find that in all cases considered the values are significantly larger than the dust response time measured by IR photometric monitoring campaigns, with the least discrepancy present relative to the result for a wavelength-independent dust emissivity law, i.e. a blackbody, which is appropriate for large grain sizes. This result can be well explained by assuming a flared, disk-like structure for the hot dust.