The 3-5 micron Spectrum of NGC 1068 at High Angular Resolution: Distribution of Emission and Absorption Features across the Nuclear Continuum Source

TL;DR

This study uses high-resolution 3-5 micron spectroscopy to analyze the nuclear region of NGC 1068, revealing the distribution of emission and absorption features, shock-ionization of coronal lines, and dust environment characteristics close to the AGN.

Contribution

It provides detailed spatial mapping of emission and absorption features in NGC 1068's nucleus at high resolution, linking shock-ionization and dust properties to the AGN environment.

Findings

Coronal line intensities peak near radio knot C, indicating shock-ionization.

The 3.4 micron hydrocarbon absorption and 10 micron silicate absorption are spatially correlated.

The dust environment resembles Galactic diffuse clouds, close to the central engine.

Abstract

We report moderate resolution 3-5 micron spectroscopy of the nucleus of NGC 1068 obtained at 0.3 arcsec (20 pc) resolution with the spectrograph slit aligned approximately along the ionization cones of the AGN. The deconvolved FWHM of the nuclear continuum source in this direction is 0.3 arcsec. Four coronal lines of widely different excitations were detected; the intensity of each peaks near radio knot C, approximately 0.3 arcsec north of the infrared continuum peak, where the radio jet changes direction. Together with the broadened line profiles observed near that location, this suggests that shock-ionization is the dominant excitation mechanism of the coronal lines. The depth of the 3.4 micron hydrocarbon absorption is maximum at and just south of the continuum peak, similar to the 10 micron silicate absorption. That and the similar and rapid variations of the optical depths of both features across the nucleus suggest that substantial portions of both arise in a dusty environment just in front of the continuum source(s). A new and tighter limit is set on the column density of CO. Although clumpy models of the dust screen might explain the shallowness of the silicate feature, the presence of the 3.4 micron feature and the absence of CO are strongly reminiscent of Galactic diffuse cloud environments and a consistent explanation for them and the observed silicate feature is found if all three phenomena occur in such an environment, existing as close as 10 pc from the central engine.