ALMA View of the Galactic Center Mini-spiral: Ionized Gas Flows around Sagittarius A*

TL;DR

This study used ALMA to observe the Galactic Center Mini-spiral, revealing detailed ionized gas kinematics, deviations from Keplerian orbits, and potential star formation activity near Sgr A* in unprecedented detail.

Contribution

First large-scale ALMA mosaic of the GCMS revealing ionized gas kinematics, orbit deviations, and star formation indicators near Sgr A*.

Findings

Ionized gas streamers deviate from Keplerian orbits in outer regions.

The Bar streamer follows a Keplerian orbit with high eccentricity (~0.8).

Electron temperature and density increase toward Sgr A*, indicating external pressure effects.

Abstract

We have performed the observation of the "Galactic Center Mini-spiral(GCMS)" in H42alpha recombination line as a part of the first large-scale mosaic observation in the Sagittarius A complex using Atacama Millimeter/sub-millimeter Array (ALMA). We revealed the kinematics of the ionized gas streamers of the GCMS. We found that the ionized gas streamers of the Northern Arm(NA) and Eastern Arm(EA) in their outer regions somewhat deviate from the Keplerian orbits which were derived previously from the trajectories in the inner regions. In addition, we found that the streamer corresponding to the Bar of the GCMS has a Keplerian orbit with an eccentricity of e~0.8, which is independent from the Keplerian orbits of the other streamers of the GCMS. We estimated the LTE electron temperature and electron density in the ionized gas streamers. We confirmed the previously claimed tendency that the electron temperatures increase toward Sgr A*. We found that the electron density in the NA and EA also increases with approaching Sgr A* without the lateral expansion of the gas streamers. This suggests that there is some external pressure around the GCMS. The ambient ionized gas may cause the confinement and/or the perturbation for the orbits. There is a good positional correlation between the protostar candidates detected by JVLA at 34 GHz and the ionized gas streamer, Northeastern Arm, newly found by our H42alpha recombination line observation. This suggests that the candidates had formed in the streamer and they were brought to near Sgr A* as the streamer falls.