ALMA 0.1-0.2 arcsec resolution imaging of the NGC 1068 nucleus - compact dense molecular gas emission at the putative AGN location

TL;DR

This study uses high-resolution ALMA imaging to detect dense molecular gas at the AGN location in NGC 1068, revealing a compact torus and jet-related features, advancing understanding of AGN environments.

Contribution

First high-resolution ALMA observations distinctly identify dense molecular gas at the AGN core, measuring its size, mass, and molecular ratios, providing new insights into the AGN's molecular environment.

Findings

Dense molecular gas detected at the AGN location (~10 pc size)

High HCN-to-HCO+ flux ratios throughout the nucleus

Continuum emission aligned with the radio jet and star formation region

Abstract

We present the results of our ALMA Cycle 2 high angular resolution (0.1-0.2 arcsec) observations of the nuclear region of the nearby well-studied type-2 active galactic nucleus (AGN), NGC 1068, at HCN J=3-2 and HCO+ J=3-2 emission lines. For the first time, due to a higher angular resolution than previous studies, we clearly detected dense molecular gas emission at the putative AGN location, identified as a ~1.1 mm (~266 GHz) continuum emission peak, by separating this emission from brighter emission located at 0.5-2.0 arcsec on the eastern and western sides of the AGN. The estimated intrinsic molecular emission size and dense molecular mass, which are thought to be associated with the putative dusty molecular torus around an AGN, were ~10 pc and ~several x 10^5 Msun, respectively. HCN-to-HCO+ J=3-2 flux ratios substantially higher than unity were found throughout the nuclear region of NGC 1068. The continuum emission displayed an elongated morphology along the direction of the radio jet located at the northern side of the AGN, as well as a weak spatially resolved component at ~2.0 arcsec on the southwestern side of the AGN. The latter component most likely originated from star formation, with the estimated luminosity more than one order of magnitude lower than the luminosity of the central AGN. No vibrationally excited (v2=1f) J=3-2 emission lines were detected for HCN and HCO+ across the field of view.