The Dusty Heart of NGC 4151 Revealed by λ ∼ 1–40 μm Reverberation Mapping and Variability: A Challenge to Current Clumpy Torus Models

Abstract

We probe the dusty environment of the archetypical Type 1 active galactic nucleus (AGN) in NGC 4151 with comprehensive IR reverberation mapping over several decades, in the J (∼1.22 μ m), H (∼1.63 μ m), K (∼2.19 μ m), L (∼3.45 μ m), and N bands (∼10.6 μ m), plus multiple measurements at 20–40 μ m. At 1–4 μ m, the hot dust reverberation signals come from two distinct dust populations at separate radii (∼0.033 pc and ∼0.076 pc), with temperatures of ∼1500–2500 K and ∼900–1000 K, consistent with the expected properties of sublimating graphite and silicate dust grains. The domination of the torus infrared output by carbon and silicate grains near their sublimation temperatures and radii may account for the general similarity of AGN near-IR spectral energy distributions. The torus inner edge defined by the hottest dust remains at roughly the same radius independent of the AGN optical luminosity over ∼25 yr. The emission by hot dust warmed directly by the optical/UV AGN output has increased gradually by ∼4% yr −1 , indicating a possibly growing torus. A third dust component at ∼700 K does not seem to participate directly in the IR reverberation behavior, and its emission may originate deep in the circumnuclear torus. We find a reverberation signal at ∼10 μ m with an inferred radius for the warm dust of ∼2.2–3.1 pc. The lack of variability at 20–40 μ m indicates that the far-IR emission comes from even more extended regions. The torus properties revealed by dust reverberation analysis are inconsistent with predictions from pure clumpy torus models. Instead, the longer-wavelength emission possibly originates in a flared torus or the polar wind.