The dusty nuclear torus in NGC 4151 : constraints from gemini near-infrared integral field spectrograph observations

Abstract

We have used a near-infrared (near-IR) 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 spectrumwas 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 ± 50 K, which dominates the nuclear spectrum—within an aperture of radius 0".3—in the near-IR. We attribute the blackbody component to emission by a dusty structure, with hot dust mass MHD = (6.9 ± 1.5) × 10−4 Mʘ , 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±50 K and a mass MHD = (3.1±0.7)×10−4 Mʘ 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.