Distance to the Brick cloud using stellar kinematics
  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.
