A circumnuclear disk of atomic hydrogen in Centaurus A

TL;DR

This study uses new ATCA observations to reveal that atomic hydrogen in Centaurus A's core forms a circumnuclear disk, challenging previous interpretations of gas infall into the AGN.

Contribution

First detection of blueshifted HI absorption against Centaurus A's core, indicating a circumnuclear disk rather than gas infall.

Findings

HI absorption is detected against the radio core at blueshifted velocities.

The broad redshifted absorption component suggests complex kinematics.

The HI distribution resembles the molecular circumnuclear disk.

Abstract

We present new observations, performed with the Australia Telescope Compact Array, of the HI absorption in the central regions of Centaurus A. For the first time, absorption is detected against the radio core at velocities blueshifted with respect to the systemic velocity. Moreover, the data show that the nuclear redshifted absorption component is broader than reported before. With these new results, the kinematics of the HI in the inner regions of Cen A appears very similar to that observed in emission for the molecular circumnuclear disk. This suggests that the central HI absorption is not, as was previously claimed, evidence of gas infall into the AGN, but instead is due to a cold, circumnuclear disk.