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

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 (similar to 1.22 mu m), H (similar to 1.63 mu m), K (similar to 2.19 mu m), L (similar to 3.45 mu m), and N bands (similar to 10.6 mu m), plus multiple measurements at 20-40 mu m. At 1-4 mu m, the hot dust reverberation signals come from two distinct dust populations at separate radii (similar to 0.033 pc and similar to 0.076 pc), with temperatures of similar to 1500-2500 K and similar to 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 similar to 25 yr. The emission by hot dust warmed directly by the optical/UV AGN output has increased gradually by similar to 4% yr(-1), indicating a possibly growing torus. A third dust component at similar to 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 similar to 10 mu m with an inferred radius for the warm dust of similar to 2.2-3.1 pc. The lack of variability at 20-40 mu 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.

Automated summary

We conducted comprehensive IR reverberation mapping on the archetypical Type 1 AGN in NGC 4151, revealing distinct dust populations and a potential growth of the torus over 25 years. The lack of variability at 20-40 mu m suggests far-IR emission from extended regions, and the torus properties do not align with predictions from clumpy torus models, possibly indicating a non-clumpy or flared torus or polar wind as the origin of longer-wavelength emission.