CO Emission Delineating the Interface between the Milky Way Nuclear Wind
  Cavity and the Gaseous Disk
  Based on the MWISP survey, we study high-z CO emission toward the tangent
points, in which the distances of the molecular clouds (MCs) are well
determined. In the region of l=12-26 deg and |b|<5.1 deg, a total of 321 MCs
with |z|> 110 pc are identified, of which nearly 30 extreme high-z MCs (EHMCs
at |z|> 260 pc) are concentrated in a narrow region of R_GC=2.6-3.1 kpc. The
EHMC concentrations, together with other high-z MCs at R_GC=2.3-2.6 kpc,
constitute molecular crater-wall structures surrounding the edges of the HI
voids that are physically associated with the Fermi bubbles. Intriguingly, some
large high-z MCs, which lie in the crater-walls above and below the Galactic
plane, show cometary structures with the head toward the plane, favouring the
scenario that the entrained molecular gas moves with the multi-phase flows from
the plane to the high-z regions. We suggest that the Milky Way nuclear wind has
a significant impact on the Galactic gaseous disk. The powerful nuclear wind at
~3-6 Myr ago is likely responsible for the observational features, (1) the
enhanced CO gas lying in the edges of the HI voids, (2) the deficiency of
atomic and molecular gas within R_GC<3 kpc, (3) the possible connection between
the EHMC concentrations and the 3-kpc arm, and (4) the elongated high-z MCs
with the tail pointing away from the Galactic plane.
