CO Emission Delineating the Interface between the Milky Way Nuclear Wind Cavity and the Gaseous Disk

Abstract

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° and ∣b∣ ≲ 5.°1, 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 H i 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, favoring the scenario that the entrained molecular gas moves with the multiphase 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 H i 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.