ALMA View of the Circum-nuclear Disk of the Galactic Center; Tidally-disrupted Molecular Clouds falling to the Galactic Center

TL;DR

This study uses ALMA to image the Circum-Nuclear Disk of the Milky Way, revealing its structure, dynamics, and interactions with infalling molecular clouds, shedding light on material transfer to the galactic center.

Contribution

First high-resolution ALMA observations of the CND across multiple molecular lines, identifying infalling clouds and their potential role in feeding the galactic nucleus.

Findings

CND is a torus-like structure with radii ~1.5-2 pc.

Detected molecular clouds falling into the CND, some rotating oppositely.

Falling clouds are disrupted by tidal shear and ionized by UV radiation.

Abstract

We present the high angular resolution and high sensitivity images of the "Circum-Nuclear Disk (CND)" and its surrounding region of Milky Way Galaxy in the CS J=2-1, SiO v=0 J=2-1, H^13CO^+ J=1-0, C^34S J=2-1, and CH_3OH J_{K_a, K_c}=2_{1,1}-1_{1,0}A_{--} emission lines using ALMA. The CND is recognized as a torus-like molecular gas with gaps in these emission lines except for the CH_3OH emission line. The inner and outer radii of the CND are estimated to be R_in~1.5 and R_out~2 pc, respectively. The velocities of the rotation and radial motion are estimated to be V_rot~115 km s^-1 and V_rad~23 km s^-1, respectively. The LTE molecular gas mass is estimated to be M_LTE~3x10^4 Mo. We also found some anomalous molecular clouds in the surrounding region. One of the molecular clouds is positionally connected to a part of the CND adjacent to the "Western Arc". However, the cloud is seen to rotate in the opposite direction to the CND. The molecular cloud would be falling currently from the outer region to the CND and being disrupted by the tidal shear of Sagittarius A*(Sgr A*) because the velocity is not yet assimilated into that of the CND. Another molecular cloud is continuously connected to the tip of the "Eastern Arm (EA)". The velocity of this cloud is consistent with that of the ionized gas in the EA. These facts suggest that the molecular cloud is falling from the outer region to the vicinity of Sgr A*, being disrupted by the tidal shear, and ionized by strong UV emission from the Central Cluster because the impact parameter of the cloud is smaller than the first cloud. These falling clouds would play an important role in transferring material from the outer region to the CND and/or the vicinity of Sgr A*.