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

TL;DR

This study uses MWISP survey data to analyze high-z CO emission near the Galactic center, revealing structures linked to the Milky Way's nuclear wind and its impact on the gaseous disk, including crater-wall formations and molecular gas dynamics.

Contribution

It identifies high-z molecular clouds associated with the Galactic nuclear wind and describes their spatial distribution and morphology, highlighting the wind's influence on the Galactic gaseous environment.

Findings

High-z molecular clouds form crater-wall structures around HI voids.

Large high-z MCs exhibit cometary shapes with heads toward the Galactic plane.

Features suggest a powerful nuclear wind ~3-6 Myr ago shaped the gas distribution.

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 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.