Mapping the Dynamics of Cold Gas around Sgr A* through 21 cm Absorption

TL;DR

This paper proposes using 21 cm absorption spectroscopy to study the properties of neutral gas disks around Sgr A* and similar black holes, enabling the detection of disk features and dense gas clumps.

Contribution

It introduces a spectroscopic method to analyze neutral gas disks around galactic nuclei, providing a new way to determine disk parameters and detect dense gas clumps.

Findings

21 cm absorption can reveal disk inclination, optical depth, and temperature.

Spectroscopy can identify gaps in the HI disk.

Dense gas clumps like G2 could be detected via absorption.

Abstract

The presence of a circumnuclear stellar disk around Sgr A* and megamaser systems near other black holes indicates that dense neutral disks can be found in galactic nuclei. We show that depending on their inclination angle, optical depth, and spin temperature, these disks could be observed spectroscopically through 21 cm absorption. Related spectroscopic observations of Sgr A* can determine its HI disk parameters and the possible presence of gaps in the disk. Clumps of dense gas similar to the G2 could could also be detected in 21 cm absorption against Sgr A* radio emission.