How is Cold Gas Loaded into Galactic Nuclear Outflows?

TL;DR

This work addresses how cold gas is loaded into the Milky Way's nuclear outflows and couples bar-driven inflows to multiphase winds. By combining HI4PI and MWISP data with a Gaussian-derivative clustering approach, it shows that high-velocity clouds near the GC largely originate from off-plane, warped gas structures at 0.5–1.7 kpc, which are entrained by winds that break through the CMZ. The proposed mechanism involves winds stripping and entraining gas from the boundaries of tilted dust lanes and distorted overshooting streams, yielding mass loading on the order of 0.6–1 Msun/yr and outflow speeds up to 340 km/s, comparable to the inflow rate. This framework explains the observed turbulence, multiphase gas, and asymmetries of the Fermi/eROSITA bubbles and provides a paradigm for gas circulation in the inner Milky Way and in external galaxies.

Abstract

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. Our results reveal that the HVCs exhibit a distinct asymmetric distribution, closely associated with the bar-driven tilted dust lanes and the 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 region of 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 pre-existing 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 over a dynamical time of ~3--6 Myr. When the multiphase, inhomogeneous composition of the gas is included, the estimated gas outflow rate reaches ~1 Msun/yr. 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 gas cycles in external galaxies.

Figures (4)

• Figure 1: Panel (a) illustrates the large-scale tilt of off-plane HVCs toward the GC. The contours show the intensity of the HVC, and the colors represent its extreme velocities at $|b|\hbox{$>$ $\sim$}2\fdg0$. The blue color traces the approaching HVCs (starting from 1 K km s$^{-1}$ with a step of 4 K km s$^{-1}$), while the red is for receding HVCs (starting from 2 K km s$^{-1}$ with a step of 12 K km s$^{-1}$). Two HVCs, labeled MW C1 and MW C2 2020Natur.584..364D, are marked in the figure. Note that the extreme negative-velocity HVCs in the range $l\hbox{$<$ $\sim$}0^{\circ}$ and $b=-3\fdg5$ to $-6\fdg5$ are not shown due to potential contamination of approaching gas from the far dust lane. The black point marks the MWISP detected CO cloud at ($l=19\fdg56, b=-6\fdg77$). Panel (b) displays the tilted gas flows driven by the Galactic bar within $l=[+6^{\circ},-6^{\circ}]$ and $b=[-3\fdg5, +2^{\circ}]$. Along the tilted dust lanes, the streams flow towards the CMZ. Red indicates receding atomic gas from the near dust lane, while blue shows approaching gas from the far dust lane. Superimposed yellow contours are molecular inflows from MWISP CO data 2024ApJ...971L...6S. Panel (c) presents the longitude-velocity diagram of H i gas, highlighting anomalous velocity zones across different Galactic longitude regions. The red and blue lines mark the lower and upper velocity integration limits for the receding and approaching gas structures identified in panel (b), respectively. Nearly all the high-latitude HVCs that we have identified fall within these anomalous spatial and velocity ranges (Figure \ref{['fig:f2']}). The orange and red dots denote the morphology of the Fermi bubbles (the interior of hot, shocked wind) and the associated shell structures 2010ApJ...724.1044S2016ApJ...829....9M, respectively. The green dots mark the 430 pc bipolar radio bubbles, seen in the MeerKAT 1.284 GHz map towards the GC 2019Natur.573..235H.
• Figure 2: Panel (a) displays the velocity distribution of extreme negative-velocity gas in the near side region, where the blue arrow indicates progressively decreasing velocities (accelerating approach) with increasing $l$ and decreasing $b$. Panel (b) shows the velocity field of extreme positive-velocity gas in the far side region, with the red arrow marking gradually increasing velocities (accelerating recession) as $l$ decreases and $b$ increases.
• Figure 3: Panel (a)--(c) show typical HVC spectral line profiles, while panel (d) displays H i and CO spectral profiles of the receding inflow gas from the near dust lane 2024ApJ...971L...6S2025ApJ...984..109S.
• Figure 4: Illustration of the gas circulation (i.e., inflows and outflows) toward the inner Galactic region of $R_{\rm GC}\hbox{$<$ $\sim$}3.0-3.4$ kpc. Red and blue arrows indicate receding and approaching gas flows toward the CMZ, respectively. The color blocks depict the fragmented HVCs driven by Galactic nuclear outflows. The four quadrants mark observational characteristics of multiphase gas in term of mass-loading, cooling efficiency, and obscuration effects. Galactic bar perturbations induce a globally tilted gas distribution, which in turn influences the large-scale symmetric patterns of the Galactic outflow structures in observations. The orange solid arrows indicate the shock front, which corresponds to the boundary of the heavily obscured soft X-ray emission region near the base of the plane 2016ApJ...829....9M.