2026ApJ...997..322S

The origin of the multiphase gas within the Fermi/eROSITA bubbles is crucial for understanding Galactic center (GC) feedback. We use HI4PI data to investigate the kinematics and physical properties of high-velocity clouds (HVCs) toward the GC region (l = +25° to -10°). Our results reveal that the HVCs exhibit a distinct asymmetric distribution, closely associated with bar-driven tilted dust lanes and distorted overshooting streams. We propose that powerful nuclear outflows interact with these gas-rich, off-plane structures, striping and entraining cold gas from the outer Galactic regions (R_GC ∼ 0.5-1.7 kpc) rather than solely from the central molecular zone (CMZ; R_GC ≲ 0.3 kpc). In this scenario, as the Galactic bar drives gas inflows along the dust lanes, nuclear outflows simultaneously break through the CMZ, sweeping up and ablating cold gas from the boundary layer of these preexisting structures. This process naturally accounts for the observed high turbulence, complex spectral signatures, and anomalous spatial-kinematic gas patterns, as well as multiwavelength asymmetries of the bubbles. The HVCs are accelerated to about 230-340 km s^–1 over a dynamical time of ∼3-6 Myr. When the multiphase, inhomogeneous composition of the gas is included, the estimated gas outflow rate in on the order of ∼1 M_⊙ yr^–1. This value is comparable to the bar-driven inflow rate, indicating a tightly coupled gas cycle in the inner Galaxy. Our research highlights the critical role of bar-driven gas dynamics and nuclear feedback in the secular evolution of the Milky Way, offering a valuable paradigm for investigating the gas outflow-inflow cycle in external galaxies.