Spatially Resolved HCN Absorption Features in the Circumnuclear Region of NGC 1052

TL;DR

This study reports the first VLBI detection of spatially resolved HCN molecular absorption in NGC 1052, revealing infall motion within the circumnuclear torus at parsec scales.

Contribution

It provides the first high-resolution spatial mapping of HCN absorption in an active galactic nucleus, demonstrating infall motion and detailed structure of the circumnuclear molecular gas.

Findings

HCN absorption detected against the nuclear jet structure.

Absorption features indicate infall motion towards the central engine.

Absorbing gas is localized on the receding jet side, on parsec scales.

Abstract

We present the first VLBI detection of HCN molecular absorption in the nearby active galactic nucleus NGC 1052. Utilizing the 1 milliarcsecond resolution achieved by the Korean VLBI Network, we have spatially resolved the HCN absorption against a double-sided nuclear jet structure. Two velocity features of HCN absorption are detected significantly at the radial velocity of 1656 and 1719 km/s, redshifted by 149 and 212 km/s with respect to the systemic velocity of the galaxy. The column density of the HCN molecule is estimated to be 10^{15}-10^{16} cm^{-2}, assuming the excitation temperature of 100-230 K. The absorption features show high optical depth localized on the receding jet side, where the free-free absorption occurred due to the circumnuclear torus. The size of the foreground absorbing molecular gas is estimated to be on approximately one-parsec scales, which agrees well with the approximate size of the circumnuclear torus. HCN absorbing gas is likely to be several clumps smaller than 0.1 parsec inside the circumnuclear torus. The redshifted velocities of the HCN absorption features imply that HCN absorbing gas traces ongoing infall motion inside the circumnuclear torus onto the central engine.