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.

Revealing Gas Inflows Toward the Galactic Central Molecular Zone

“…The cold atomic and molecular gas related to the Galactic nuclear winds has attracted even more attention very recently due to the cold gaseous substances found in the FBs from largescale H I and CO studies (e.g., Lockman & McClure-Griffiths 2016;Di Teodoro et al 2018Lockman et al 2020;Noon et al 2023;Veena et al 2023). Using the CO data, we also find that the Galactic nuclear winds have a significant impact on the Galactic gaseous disk, leading to material accumulation at the 3 kpc crater walls (Su et al 2022) and gas deficiency in the region of 0.3 kpc R GC  3.1 kpc (here R GC is the Galactocentric distance; e.g., Burton 1976;Dickey & Lockman 1990; Dame et al 2001;Kalberla & Kerp 2009;Su et al 2021). The mass-loading rate of the outflows related to the Galactic nuclear winds is estimated to be ∼2-4 M e yr −1 (Su et al 2022).…”

“…Using the CO data, we also find that the Galactic nuclear winds have a significant impact on the Galactic gaseous disk, leading to material accumulation at the 3 kpc crater walls (Su et al 2022) and gas deficiency in the region of 0.3 kpc R GC  3.1 kpc (here R GC is the Galactocentric distance; e.g., Burton 1976;Dickey & Lockman 1990; Dame et al 2001;Kalberla & Kerp 2009;Su et al 2021). The mass-loading rate of the outflows related to the Galactic nuclear winds is estimated to be ∼2-4 M e yr −1 (Su et al 2022). Considering additional warm and hot gas in the FBs (Miller & Bregman 2016;Krishnarao et al 2020;Predehl et al 2020;Zhang & Guo 2021), the actual mass-loading rate of the Galactic nuclear winds may be even higher than what is inferred from the cold atomic and molecular gas alone.…”

“…The overall distribution of the WGL shows that the current mass transport is relatively continuous, suggesting that the gas in the CMZ can accumulate rapidly over a period of several tens of Myr. However, the gas inflow rate to the CMZ seems to be comparable to the outflow rate (e.g., ∼1.5 ± 0.5 M e yr −1 ; see details in Su et al 2022) driven by the Galactic nuclear winds after applying the updated lower X-factor above. The general equilibrium between the outflow and the inflow rate likely indicates that the accumulation of gas in the CMZ may take a relatively longer time of hundreds of Myr to several Gyr.…”

“…This result agrees with the CO observation that the WGL cannot continue to maintain a positive inflow velocity at the point of l ∼ 15°(i.e., the gas should have v LSR  0 km s −1 at l ∼ 15°for v //  0 km s −1 ; see Section 3.2.3). Given the deficiency of atomic and molecular gas within the 3 kpc ring (e.g., Burton 1976;Dame et al 2001;Su et al 2022), on the other hand, it is reasonable to define a minimum length of the WGL of  3.1 kpc. Given the uncertainties of the length of R bar ≈ 3.1-3.6 kpc and the distance to the bar end of 4.9-5.2 kpc, the estimation of f bar is in the range 20°-26°.…”