Neutral Gas within 20,000 Schwarzschild radii of Sagittarius A*

TL;DR

This study uses ALMA observations of hydrogen recombination lines to detect and quantify neutral gas near Sagittarius A*, revealing dynamic ionization effects caused by the star S0-2's close approach.

Contribution

It provides the first measurement of neutral gas mass within 20,000 Schwarzschild radii of Sgr A* using recombination line variability.

Findings

Detected at least (6.6 +- 3.3) x 10^{-6} solar masses of neutral gas.

Observed a 20% increase in H30alpha line flux prior to S0-2's pericenter.

Recombination line variability indicates ionization effects from S0-2.

Abstract

Murchikova et al 2019 discovered a disk of cool ionized gas within 20,000 Schwarzschild radii of the Milky Way's Galactic Center black hole Sagittarius A*. They further demonstrated that the ionizing photon flux in the region is enough to keep the disk ionized, but there is not ample excess of this radiation. This raised the possibility that some neutral gas could also be in the region shielded within the cool ionized clumps. Here we present ALMA observations of a broad 1.3 millimeter hydrogen recombination line H30alpha: n = 31 -> 30, conducted during the flyby of the S0-2 star by Sgr A*. We report that the velocity-integrated H30alpha line flux two month prior to the S0-2 pericenter passage is about 20% larger than it was one month prior to the passage. The S0-2 is a strong source of ionizing radiation moving at several thousand kilometers per second during the approach. Such a source is capable of ionising parcels of neural gas along its trajectory, resulting in variation of the recombination line spectra from epoch to epoch. We conclude that there are at least (6.6 +- 3.3) x 10^{-6} Msun of neutral gas within 20,000 Schwarzschild radii of Sgr A*.