The First Spatially Resolved Mid‐Infrared Spectra of NGC 1068 Obtained at Diffraction‐limited Resolution with the Keck I Telescope Long Wavelength Spectrometer

Abstract

We present spatially resolved mid-infrared (mid-IR) spectra of NGC 1068 with a diffraction-limited resolution of 025 using the Long Wavelength Spectrometer (LWS) at the Keck I telescope. The mid-IR image of NGC 1068 is extended along the north-south direction. Previous imaging studies have shown that the extended regions are located inside the ionization cones, indicating that the mid-IR emission arises perhaps from the inner regions of the narrow-line clouds instead of the proposed dusty torus itself. The spatially resolved mid-IR spectra were obtained at two different slit position angles, +80 and -130 across the elongated regions in the mid-IR. From these spectra, we found only weak silicate absorption toward the northern extended regions but strong absorption in the nucleus and the southern extended regions. This is consistent with a model of a slightly inclined cold obscuring torus that covers much of the southern regions but is behind the northern extension. While a detailed analysis of the spectra requires a radiative transfer model, the lack of silicate emission from the northern extended regions prompts us to consider a dual dust population model as one of the possible explanations in which a different dust population exists in the ionization cones compared to that in the dusty torus. Dust inside the ionization cones may lack small silicate grains, giving rise to only a featureless continuum in the northern extended regions, while dust in the dusty torus has plenty of small silicate grains to produce the strong silicate absorption lines toward the nucleus and the southern extended regions.