Probing the innermost dusty structure in AGN with mid-IR and near-IR interferometers

TL;DR

This study uses mid-IR and near-IR interferometry to map the dust structure in Type 1 AGNs, revealing luminosity-dependent size and distribution variations at sub-parsec scales.

Contribution

First systematic interferometric analysis of Type 1 AGNs showing how dust distribution scales with luminosity and differs between mid-IR and near-IR emissions.

Findings

Mid-IR size scales with luminosity and becomes more compact at higher luminosities.

Mid-IR brightness distribution follows a steeper power-law in more luminous sources.

Near-IR emission originates from a distinct inner dust rim, possibly steeper in jet-launching AGNs.

Abstract

With mid-IR and near-IR long-baseline interferometers, we are now mapping the radial distribution of the dusty accreting material in AGNs at sub-pc scales. We currently focus on Type 1 AGNs, where the innermost region is unobscured and its intrinsic structure can be studied directly. As a first systematic study of Type 1s, we obtained mid-/near-IR data for small samples over ~3-4 orders of magnitudes in UV luminosity L of the central engine. Here we effectively trace the structure by observing dust grains that are radiatively heated by the central engine. Consistent with a naive expectation for such dust grains, the dust sublimation radius R_in is in fact empirically known to be scaling with L^1/2 from the near-IR reverberation measurements, and this is also supported by our near-IR interferometry. Utilizing this empirical relationship, we normalize the radial extent by R_in and eliminate the simple L^1/2 scaling for a direct comparison over the samples. We then find that, in the mid-IR, the overall size in units of R_in seems to become more compact in higher luminosity sources. More specifically, the mid-IR brightness distribution is rather well described by a power-law, and this power-law becomes steeper in higher luminosity objects. The near-IR flux does not seem to be a simple inward extrapolation of the mid-IR power-law component toward shorter wavelengths, but it rather comes from a little distinct brightness concentration at the inner rim region of the dust distribution. Its structure is not well constrained yet, but there is tentative evidence that this inner near-IR-emitting structure has a steeper radial distribution in jet-launching objects. All these should be scrutinized with further observations.