ALMA Sub-parsec Resolution Dense Molecular Line Observations of the NGC 1068 Nucleus

TL;DR

This study uses ALMA's high-resolution observations to analyze the dense molecular gas structure and dynamics in the nucleus of NGC 1068, revealing a complex, counter-rotating torus with implications for AGN fueling.

Contribution

First high-resolution ALMA observations of HCN and HCO+ lines at sub-parsec scale in NGC 1068, confirming counter-rotation and proposing a collision model for the torus dynamics.

Findings

Detection of bright HCN J=4-3 emission in the torus

Confirmation of counter-rotation in the nuclear region

A model involving a dense gas clump collision explains observed dynamics

Abstract

We present the results of our ALMA observations of the dense molecular HCN J=4-3 and HCO$^{+}$ J=4-3 lines at $\lesssim$1 pc ($\lesssim$14 mas) resolution in the nuclear region of the nearby ($\sim$14 Mpc) well-studied AGN NGC 1068. Both emission lines are clearly detected around the AGN along an almost east-west direction, which we ascribe to the dusty molecular torus. The HCN J=4-3 emission is brighter than the HCO$^{+}$ J=4-3 emission in the compact ($\lesssim$3-5 pc) torus region. Apparent counter-rotation between the inner ($\lesssim$2 pc) and outer ($\gtrsim$2 pc) parts of the western torus, previously seen in $\sim$1.5 pc-resolution HCN J=3-2 and HCO$^{+}$ J=3-2 data, is also confirmed in our new $\lesssim$1 pc-resolution HCN J=4-3 and HCO$^{+}$ J=4-3 data. We apply a physically counter-rotating torus model, in which a compact dense gas clump collided with the western side of the existing rotating torus from the opposite direction, and we find that this model largely reproduces the observed properties of the combined new $\lesssim$1 pc-resolution HCN J=4-3 and HCO$^{+}$ J=4-3 data, and the previously obtained $\lesssim$1.5 pc-resolution HCN J=3-2 and HCO$^{+}$ J=3-2 data.