Distance to the Brick cloud using stellar kinematics

TL;DR

This study uses stellar kinematics to determine the distance to the Brick molecular cloud, concluding it is within the central molecular zone, contrary to previous claims placing it outside the nuclear stellar disk.

Contribution

It introduces a kinematic method to constrain the distance to molecular clouds using proper motions of stars, clarifying the Brick's position relative to the nuclear stellar disk.

Findings

The Brick is located inside the nuclear stellar disk.

Stars behind the Brick are blocked, revealing only certain kinematic components.

The Brick is part of the central molecular zone.

Abstract

Context. The central molecular zone at the Galactic center is currently being studied intensively to understand how star formation proceeds under the extreme conditions of a galactic nucleus. Knowing the position of molecular clouds along the line of sight toward the Galactic center has had important implications in our understanding of the physics of the gas and star formation in the central molecular zone.It was recently claimed that the dense molecular cloud G0.253 + 0.016 (the Brick) has a distance of $\sim$7.20 kpc from the Sun. That would place it outside of the central molecular zone, and therefore of the nuclear stellar disk, but still inside the Bulge. Aims. Theoretical considerations as well as observational studies show that stars that belong to the nuclear stellar disk have different kinematics from those that belong to the inner Bulge. Therefore, we aim to constrain the distance to the Brick by studying the proper motions of the stars in the area. Results. The stellar population seen toward the nuclear stellar disk shows the following three kinematic components: 1) Bulge stars with an isotropic velocity dispersion of $\sim$3.5 micro-arc second per year; 2) eastward moving stars on the near side of the nuclear stellar disk; and 3) westward moving stars on the far side of the nuclear stellar disk. We clearly see all three components toward the comparison field. However, toward the Brick, which blocks the light from stars behind it, we can only see kinematic components 1) and 2). Conclusions. While the Brick blocks the light from stars on the far side of the nuclear stellar disk, the detection of a significant component of eastward streaming stars implies that the Brick must be located inside the nuclear stellar disk and, therefore, that it forms part of the central molecular zone.