The Dusty Nuclear Torus in NGC 4151: Constraints from Gemini Near-Infrared Integral Field Spectrograph Observations

TL;DR

This study uses near-infrared spectroscopy of NGC 4151's nucleus to identify a hot dust component likely constituting the inner wall of the dusty torus or a dusty wind, providing constraints on its properties.

Contribution

It presents the first detailed near-infrared spectral analysis of the nuclear dust in NGC 4151, constraining its temperature, mass, and size, supporting the dusty torus model.

Findings

Blackbody component at ~1285 K dominates near-IR spectrum.

Dust mass estimated at ~7 x 10^-4 solar masses.

Upper limit of dust distance from nucleus is 4 parsecs.

Abstract

We have used a near-infrared nuclear spectrum (covering the Z, J, H and K bands) of the nucleus of NGC 4151 obtained with the Gemini Near-infrared Integral Field Spectrograph (NIFS) and adaptive optics, to isolate and constrain the properties of a near-IR unresolved nuclear source whose spectral signature is clearly present in our data. The near-IR spectrum was combined with an optical spectrum obtained with the Space Telescope Imaging Spectrograph which was used to constrain the contribution of a power-law component. After subtraction of the power-law component, the near-IR continuum is well fitted by a blackbody function, with $T=1285\pm50 $K, which dominates the nuclear spectrum -- within an aperture of radius 0$\farcs$3 -- in the near-IR. We attribute the blackbody component to emission by a dusty structure, with hot dust mass $M_{\rm HD}=(6.9\pm 1.5) \times10^{-4} {\rm M_\odot}$, not resolved by our observations, which provide only an upper limit for its distance from the nucleus of 4 pc. If the reddening derived for the narrow-line region also applies to the near-IR source, we obtain a temperature $T=1360\pm50 $K and a mass $M_{\rm HD}=(3.1\pm 0.7) \times10^{-4} {\rm M_\odot}$ for the hot dust. This structure may be the inner wall of the dusty torus postulated by the Unified Model or the inner part of a dusty wind originating in the accretion disk.