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

TL;DR

This study presents sub-parsec ALMA observations of the dense molecular gas in the NGC 1068 nucleus, focusing on the HCN and HCO+ 4–3 transitions to resolve the dusty molecular torus. By employing the longest-baseline configuration, the authors achieve ≲1 pc scale mapping and reveal how the torus gas dynamics vary between inner and outer regions, including persistent counter-rotation signals. They test a physically motivated scenario where a compact dense gas clump collides with the western torus, finding that this model reproduces the observed kinematics and line brightness patterns across new and archival datasets. The results refine our understanding of torus dynamics in AGN and offer a concrete mechanism for counter-rotation that complements previous J=3–2 findings, with implications for torus fueling and AGN unification.

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.