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

There are probably two scenarios that can explain the observed EHMCs far from the Galactic plane, i.e., (1) the local star formation feedback scenario and (2) the Milky Way nuclear wind scenario. We will discuss the two scenarios described below.

For the local star formation feedback scenario, the feedback from star-forming activities near the Galactic plane has profound effects on the surrounding ISM environment, i.e., changing the physical properties and the distributions of the gas. The energetic processes from massive stellar winds and/or supernova explosions can produce large-scale structures such as supershells, superbubbles, and large-scale chimneys. Especially, the region between l ∼ 20° and 30° is close to the near tip of the Galactic bar, and it hosts intense star-forming regions in the Galactic plane. The region also shows a rich population of H i extraplanar clouds that are not seen in other regions of the Galaxy (the H i scale height in the region of l ∼ 20° is twice the scale height of the corresponding region in l ∼ −20°; see Ford et al. 2008, 2010).

Therefore, the high-z MCs (∣z∣ ≳ 100 pc) in the thick CO disk may be the debris of the disk gas that was blown away by local star formation feedback near the Galactic plane (see, e.g., discussions in Su et al. 2021). For example, the identified high-z MCs at (l ∼ 403, b ∼ −43) are probably associated with the nearby SS 433/W50 system (e.g., z_MC ∼ −400 pc at a distance of 5 kpc; see Su et al. 2018). Is it possible that the EHMCs at l ∼ 20° are from the energetic processes of stellar feedback near the Galactic plane, for example, the Ophiuchus superbubble (see details in Pidopryhora et al. 2007)?

Based on results from the MWISP data, we find that the high-z MCs with very faint CO emission are highly turbulent, indicating that the cold gas is mixing with the warmer gas and the MCs are either dispersing or being assembled by external dynamical processes. These small-sized (1–4 pc) high-z MCs are probably short-lived, e.g., less than their typical internal crossing time of several Myr. The clouds thus cannot move too far from the disk if they are directly from the Galactic midplane, conforming to the observed MCs distribution for the thick CO disk (e.g., σ_z ∼ 110–120 pc; see Su et al. 2021). Here we ignore the case that the high-z molecular gas may condense in situ. Indeed, we find no detailed correlation between the Ophiuchus superbubble in H i emission and the high-z MCs near the tangent point in a region of l ∼ 20°–40° and z = 100–600 pc. On the other hand, the Ophiuchus superbubble appears to be one-sided and does not extend below the Galactic plane (e.g., Pidopryhora et al. 2007), although the HI4PI data show a lot of anomalous structures below the Galactic plane (see also Ford et al. 2010; Su et al. 2021). We argue that the enhanced EHMCs at l ∼ 18°–22° (or R_GC ∼ 2.6–3.1 kpc) are not associated with the old Ophiuchus superbubble (e.g., an age of about 30 Myr; see Pidopryhora et al. 2007).

In principle, some EHMCs are probably related to the young star formation feedback (e.g., several Myr) near the Galactic plane. However, the extended direction of the cometary EHMCs (see Figure 3) and the coherent EHMC samples in Figure 6 show that the large and cometary EHMCs are likely related to dynamical processes toward regions of l ≲ 18°–22° (or toward the GC direction), in which the star-forming activities near the Galactic plane are not very intense except for the Central Molecular Zone (CMZ) region.

According to the above discussions, we propose that most of the EHMCs in R_GC ∼ 2.6–3.1 kpc are associated with the Milky Way nuclear wind. We also summarize the observation results as below:

(1) The dominant EHMCs are just located near the edges of the large-scale H i voids toward l ∼ 18°–22° or R_GC ∼ 2.6–3.1 kpc (see Figures 1, 2, and 6), indicating the association between them.

(2) The large EHMCs along the edges of the H i voids display unusual head-to-tail structures pointing away from the Galactic plane (or the direction away from the GC; see Figures 3 and 5).

(3) The observed velocity gradient of EHMC G021.548−3.414, together with its large velocity width (Figure 4), supports the idea that the MC is unstable and is partially destroyed by ambient dynamical processes. That is, the molecular gas is accelerated from the head to the tail owing to the blueshifted CO profile for the tail structure (see the bottom panel of Figure 4). The feature supports the entrainment scenario that the material is moving from the Galactic plane to the high-z regions.

(4) The IR emission of EHMC G021.548−3.414 (Figure 3) is bright at its long tail but is faint at the dense head, favoring the scenario of the molecular gas being ablated and heated from the cloud edges by a warm/hot wind.

These observational features can be naturally explained by the entrainment scenario that the cold gas is moving with the multiphase medium at the interface between the warm/hot nuclear wind of the Milky Way and the gaseous disk. The process also leads to the concentrated molecular gas (and mass) in narrow regions of l ∼ 18°–22° (or R_GC ∼ 2.6–3.1 kpc) and ∣z∣ ≳ 260 pc (see the cometary EHMCs along the edges of the H i voids in Figures 3 and 6, ∣z∣ ∼ 400 − 450 pc ≳ 3σ_z of the thick CO disk).

The H i voids, which are physically associated with the Fermi bubbles, are thus surrounded by enhanced CO emission at low latitudes of ∣b∣ ≳ 2°–5° and R_GC ≲ 2.6–3.1 kpc. This scenario also agrees well with the recent observation that the Milky Way nuclear wind can remove gas from the disk to the halo (e.g., Cashman et al. 2021) and can accelerate MC fragments into the high-z regions (e.g., Di Teodoro et al. 2020).

Finally, note that not all EHMCs in Table 1 are related to the Milky Way nuclear winds. Only samples of ID 05–31 in Table 1 are used to calculate the mass of the crater walls (see Figure 2). Among the EHMCs in the crater walls, over 60% of samples are resolved and nearly 30% of EHMCs (ID 13, 14, 15, 19, 23, 24, and 27) display cometary structures. These features are unusual in the narrow region of R_GC ∼ 2.6–3.1 kpc. And the dominant molecular mass in the CO crater walls is from the large-size and cometary EHMCs that are located along the edges of the H i voids (e.g., R_GC ∼ 2.6–3.1 kpc; see EHMCs in Figure 6 and the caption). Therefore, the possible contamination from the local star formation feedback near the Galactic plane has little effect on the subsequent estimation assuming that the cometary and large EHMCs along the crater walls are (very likely) of Milky Way nuclear wind origin.

Of course, observing a similar region at the other side of the crater (i.e., l ≲ −20°), where star formation activity is not very intense, would be a neat confirmation. However, the MWISP survey cannot cover that longitude range.

Multiphase outflows are a common feature in galaxies (e.g., Veilleux et al. 2020). For the Milky Way, multiwavelength observations have revealed many interesting large-scale structures related to the Galactic nuclear wind, i.e., the GC lobes in radio (e.g., Sofue & Handa 1984; Carretti et al. 2013), IR (e.g., Bland-Hawthorn & Cohen 2003), X-ray (e.g., Predehl et al. 2020), γ-ray (i.e., the Fermi bubbles in Su et al. 2010) emission and UV absorption (e.g., Zech et al. 2008; Fox et al. 2015; Savage et al. 2017; Karim et al. 2018; Ashley et al. 2020), showing the bipolar outflows on scales of a few degrees to tens of degrees.

New multiwavelength analyses also found chimney-like structures that are physically related to the intermittent activity near the GC (Ponti et al. 2019, 2021). Recently, the H i studies have revealed spatial (e.g., H i holes in Lockman & McClure-Griffiths 2016) and kinematic atomic gas features (e.g., anomalous high-velocity clouds extending up to the high-z regions; McClure-Griffiths et al. 2013; Di Teodoro et al. 2018; Lockman et al. 2020) associated with the Galactic wind. These studies support the existence of large-scale multiphase outflows in our Galaxy (i.e., the neutral gas at T ≲ 10² K, warm ionized gas T ∼ 10³–10⁴ K, and high-temperature gas at T ≳ 10⁶ K).

Based on the discussions in Section 3.4.1, we suggest that large amounts of molecular gas, which is concentrated in the edges of the H i voids associated with the Fermi bubbles, is entrained in large-scale multiphase outflows from the Galactic gaseous disk to the high-z regions.

The detected EHMCs could be an important mass reservoir of the cool outflows in the Milky Way. The molecular mass in the crater-wall structures of l ∼ 191–225 and ∣z∣ ≳ 260 pc (i.e., Area ∼ 2 × ΔR_GC × Δz = 2 × 220 pc width ×410 pc height; see Figure 2 and ID 05–31 in Table 1) is estimated to be ≳9.2 × 10³ M_⊙ by adopting the CO-to-H₂ conversion factor of X_CO = 2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹ (e.g., Dame et al. 2001; Bolatto et al. 2013b). In the meanwhile, many small clouds with weak CO emission (e.g., T_peak ≲ 1 K) may exist in the walls of the crater-like structures, but we cannot pick them out owing to the beam dilution and the limited sensitivity of the survey data. For a conservative estimate, the mean volume density of the molecular gas in the crater walls is ≳3 × 10⁻⁴ M_⊙ pc⁻³ (or ≳0.01 H cm⁻³) assuming a thickness of ∼200 pc along the LOS.

Due to confusion with the unrelated H i emission near the Galactic plane, the total mass of the atomic gas cannot be precisely measured in the same region of the EHMC concentrations. Based on the H i emission near the tangent points, however, the upper limit of the total mass of the atomic gas can be roughly estimated to be ≲1.3 × 10⁵ M_⊙ from I_Hi (v_LSR > v_tan), leading to the atomic gas density of ≲4 × 10⁻³ M_⊙ pc⁻³ (or ≲0.1 H cm⁻³). Note that the estimated H i mass contains the attribution from some unrelated gas structures with v_LSR < v_tan because of the broad line width of the H i emission. Therefore, the derived atomic gas mass (and the density) toward the tangent points is probably the upper limit based on the estimated value of I_Hi (v_LSR >v_tan).

The mean density of the cold gas estimated above is at least one order of magnitude larger than the hot gas density in the bubbles (e.g., the often-used value of ∼10⁻³ H cm⁻³ in Bland-Hawthorn & Cohen 2003; Crocker et al. 2015; Sarkar et al. 2015; Miller & Bregman 2016), suggesting that the hot gas in the nuclear wind is surrounded by dense and cold shells at the boundaries of the Fermi bubbles near the Galactic gaseous disk. Therefore, the cold gas at R_GC ∼ 3 kpc likely confines the hot wind near the gaseous disk at least on the height of ∣z∣ ∼ 600 pc (see Figures 1 and 2). That is what we observed based on the MWISP CO survey and the combination of the H i data.

Considering the bipolar-outflow structures for the Milky Way nuclear wind, we can estimate that the total mass of the molecular crater walls should be ≳1 × 10⁶ M_⊙ for regions of ∼2 × 2πR_GC × ΔR_GC × Δz = 3.4 × 10⁹ pc³ (i.e., the two bowl-like structures above and below the Galactic plane at R_GC ∼ 3 kpc and ∣z∣ ∼ 260–670 pc). It is interesting to note that the total molecular mass in the crater walls is well comparable to that of the H i gas in the Fermi bubbles (i.e., ∼10⁶ M_⊙ in Di Teodoro et al. 2018; Lockman et al. 2020). Additionally, some molecular gas could still survive in the inner of the Fermi bubbles (e.g., Di Teodoro et al. 2020), which will increase the total molecular mass of the cool outflows associated with the Milky Way nuclear wind.

Our results show that a large amount of neutral gas (the total atomic and molecular gas mass of ∼10⁷ M_⊙ at ∣z∣ ≳ 260 pc) is located in the crater walls, which surround the base of GC superbubbles at low latitudes above and below the Galactic plane. The high-z molecular gas, together with the related atomic gas and dust, constitutes the cool outflows associated with the Milky Way nuclear wind.

Figure 6 shows large-scale velocity distributions of the gas along the crater-wall structures (black dashed lines in the left panel). We find that the crater-wall structures display the systematic velocity gradient of ∼−0.03 km s⁻¹ pc⁻¹ along the LOS. That is, the observed velocity along the LOS decreases with increasing height of ∣z∣, for regions both above (e.g., EHMC IDs 5−13 in Table 1) and below (EHMC IDs 19−28 in Table 1) the Galactic plane. The lag is probably the result of the interaction between the entrained gas from the disk and the slowly rotating gas in the halo (e.g., Fraternali & Binney 2008; Melioli et al. 2009; Marasco et al. 2015; Lockman & McClure-Griffiths 2016).

Additionally, two substructures, which are identified from the coherent EHMCs with similar spatial and velocity features in the crater walls, display the positive velocity gradient with increasing height of ∣z∣ (see the black solid lines in the right panels of Figure 6). Here the coherent EHMCs mean that (1) the clouds have similar LSR velocities in a small region and (2) they have similar elongations along the crater walls (or the edges of the H i voids). Both the substructures traced by CO emission are located at ∼400–450 pc far from the Galactic plane (see, e.g., black contours in Figure 3).

The velocity gradients of the two substructures are ∼0.15 km s⁻¹ pc⁻¹ for the ∼20 pc long structure above the plane (see the black solid line in the top right panel of Figure 6 for EHMC IDs 13, 14, 15, and 17 from Table 1) and ∼0.16 km s⁻¹ pc⁻¹ for the ∼40 pc long one below the plane (see the black solid line in the bottom right panel of Figure 6 for EHMC IDs 23, 24, 26, and 27 from Table 1), respectively. The true velocity gradient is likely 3–6 times larger than the observed gradient along the LOS by considering the projection correction of the small inclination angle (e.g., ∇v_true = ∇v_obs/sin(i) for i ∼ 10°–20°; see Section 3.3).

We argue that the velocity gradient of the large-scale coherent EHMCs probably results from cool outflows associated with the Milky Way nuclear wind. The velocity of cool outflows is ∼140–330 km s⁻¹ (i.e., v _w ∼ path × ∇v_true) assuming that the CO gas comes from the locations of 110–130 pc far from the Galactic plane at a constant acceleration (e.g., path ∼ 280–330 pc and ∇v_true ∼ 0.5 − 1 km s⁻¹ pc⁻¹). Here we tentatively assume that the entrained high-z gas is launched from the boundary of the thin CO disk to their current places (see, e.g., the cometary CO structures at b ∼ 1° or ∣z∣ ∼ 120 pc in the top panels of Figure 5). The estimated velocity of the cool outflows is roughly comparable to the value of ∼200–300 km s⁻¹ from the H i kinematic models (e.g., McClure-Griffiths et al. 2013; Di Teodoro et al. 2018; Lockman et al. 2020).

The cold gas in the multiphase outflows is mainly entrained along the walls of the hot gas cavity blown by the Milky Way nuclear wind. The scenario is also similar to the famous examples of nearby starburst galaxy M82 (e.g., Leroy et al. 2015; Yoshida et al. 2019; Krieger et al. 2021) and NGC 253 (e.g., Bolatto et al. 2013a; Meier et al. 2015; Walter et al. 2017; Krieger et al. 2019). Further observations and simulations are helpful in understanding the whole picture of the Galactic multiphase nuclear outflows/winds (e.g., Zhang & Guo 2020; Fang et al. 2020; Cecil et al. 2021; Mondal et al. 2022; Banda-Barragán et al. 2021; Pillepich et al. 2021; Tanner & Weaver 2022; Fielding & Bryan 2022; Yang et al. 2022).

In principle, the isolated and cool clouds (e.g., n ∼ 10–1000 cm⁻³ and T ≲ 10² K) will be eventually destroyed in the harsh environment (e.g., the high-velocity shock and/or the surrounding warm/hot winds, n ∼ 10⁻³–1 cm⁻³ and T ≳ 10⁴ K). The cloud-crushing time (e.g., Klein et al. 1994) can be defined as ${t}_{\mathrm{cc}}={\chi }^{\tfrac{1}{2}}\tfrac{2{r}_{\mathrm{cloud}}}{{v}_{{\rm{w}}}}$, where $\chi =\tfrac{{\rho }_{\mathrm{cloud}}}{{\rho }_{\mathrm{wind}}}$ is the density contrast between the cloud and the wind, r_cloud is the cloud radius, and v _w is the wind velocity.

By adopting χ = 1000 and v _w = 200 km s⁻¹ = v _{w 200} (see Section 3.4.1), we find that the crushing time of the high-z MCs is t_cc ∼ 0.9v _{w 200} ⁻¹ Myr for the typical EHMC radius of r_cloud=3 pc. The crushing time of the EHMCs is slightly small compared to the dynamical time of the gas flows (i.e., ${t}_{\mathrm{dyn}}=\tfrac{l}{{v}_{{\rm{w}}}}\gtrsim 1.3{v}_{{\rm{w}}200}^{-1}$ Myr, where the moving distance of gas flows is l ≳ 260 pc for the EHMCs). Note that changing χ = 1000 to χ = 100 will decrease the crushing time by a factor of ∼3. The smaller clouds (e.g., r_cloud≪ 1 pc) thus could not survive long in the high-velocity wind.

For a more realistic case, however, EHMCs with parsec scales likely survive for a long period of time (e.g., a few Myr for several times of t_cc) in the gas-rich environment by considering the radiative cooling, condensation of gas from warm clouds, and some other effects (see, e.g., McCourt et al. 2015; Armillotta et al. 2017; Gronke & Oh 2018; Banda-Barragán et al. 2019; Gronke & Oh 2020; Schneider et al. 2020; Sparre et al. 2020; Kanjilal et al. 2021; Girichidis et al. 2021; Farber & Gronke 2022; Gronke et al. 2022).

As an interesting example, adopting the velocity gradient of ∼1 km s⁻¹ pc⁻¹ for EHMC G021.548−03.414 with the projection correction (Section 3.3), its dynamic time is estimated to be ${t}_{\mathrm{dyn}}={t}_{\mathrm{acc}}=2\times \tfrac{{\rm{\Delta }}\mathrm{length}}{{\rm{\Delta }}v}\sim 2.0$ Myr, which is comparable to its crushing time of ${t}_{\mathrm{cc}} \tilde 2.2{v}_{w200}^{-1}$ Myr for its effective radius of 7.2 pc (see Table 1). The long tail of the cloud, together with the revealed velocity gradient, indicates that the molecular gas is ablated by the surrounding high-velocity wind. The molecular gas of the cloud is entrained in the multiphase flows, in which the molecular gas will be transformed to the neutral atomic and/or warm ionized gas moving toward the high-z regions. We suggest that the EHMC is crushed and will be destroyed in future several Myr. Thus, the lifetime of the EHMC at ∣z∣ ∼ 450 pc far from the Galactic plane is probably ∼5–10 Myr, which is much longer than its crushing time.

Our CO observations reveal large reservoirs of cool gas surrounding the boundaries of the large-scale nuclear wind near the gaseous disk. That is, the abundant atomic gas is concentrated in the crater-wall structures that the dense EHMCs are embedded in (Sections 3.2 and 3.3). The EHMCs thus are not isolated objects situated in an empty space. The gas-rich environment with the local high density increases the survival ability of MCs in the crater walls.

For example, mass loading from the gaseous disk to the crater walls can flatten the density and temperature profiles on the boundary of the wind bubbles, creating multiphase flows in such regions. The multiphase gas-rich environment may cool fast to replenish the cold gas reservoir and then extend the lifetime of the high-z MCs. Additionally, the newly cooled gas from the surrounding high-velocity flows can carry momentum of the hot gas, leading to the observed entrainment scenario in the porous and mixed multiphase medium.

Nearly no EHMC is observed in the regions of R_GC ≲ 2.6 kpc (Figure 2). There are two plausible reasons for the feature. One is that high-z clouds are indeed destroyed by the hot winds, in which the wind velocity in the nuclear wind bubble is higher than that near the boundary of the bubble. The molecular gas of the clouds may be rapidly transformed to the warm/hot ionized gas (e.g., t_cc ≲ 0.2 Myr for the high-velocity wind with v _w = 1000 km s⁻¹ and T ≳ 10⁶ K). The ionized gas moves fast toward the high-z regions, in which little cool gas can survive in the hot flows. On the other hand, our EHMC samples are from the CO emission toward the tangent points. The EHMC samples only occupy a small volume of the bubble in a certain LOS. It thus decreases the detection rate of possibly survived high-z MCs with an origin size of tens of parsecs in the hot wind bubble.

Finally, we emphasize that the cloud, and even the ISM, is inherently complex in its structure rather than the isolated and homogenous distribution in temperature, velocity, and density. The mean volume density of the EHMCs is ∼20 H₂ cm⁻³, much below the CO critical density of 3000/τ_12CO H₂ cm⁻³ (see, e.g., Scoville 2013; Shirley 2015). This probably shows that the EHMCs consist of clumpy and multiphase medium, in which the highly structured molecular gas with a low volume filling factor is mixed with more diffuse gas (e.g., Falgarone et al. 1991, 1992; Falgarone & Phillips 1996; Snow & McCall 2006; Hacar et al. 2016).

The original material in the MCs is heated into the multiphase gas (i.e., ionized/atomic/molecular) and entrained as it mixed with the warm/hot wind. Once the cloud material is entrained, it may quickly cool back down to the molecular phase in the enhanced gas+dust environment. In such a scenario, many effects (e.g., conduction and cooling, magnetic field and cosmic rays, various instabilities) should be carefully taken into account for the interaction between the fractal MCs and the warm/hot wind. Detailed analysis of these effects is beyond the scope of this paper. More multiwavelength observations and simulations will be very helpful to clarify these issues.

In the crater-wall regions (i.e., ∣z∣ ∼ 260–670 pc and R_GC ∼ 3 kpc), the total mass of the molecular gas is ≳1 × 10⁶ M_⊙. For the highest EHMC at ∣z∣ ∼ 620 pc, its dynamical time is estimated to be ${t}_{\mathrm{dyn}}=\tfrac{620\ \mathrm{pc}}{{v}_{{\rm{w}}200}}\,\lesssim \,3.0{v}_{{\rm{w}}200}^{-1}$ Myr. The mass-loading rate from the inner gaseous disk of R_GC ≲ 3 kpc to the high-z regions is thus ≳0.3v _{w 200} M_⊙ yr⁻¹.

If we take into account the disturbed MCs in the region close to the Galactic plane (e.g., 110 pc ≲ ∣z∣ ≲ 260 pc), the true mass-loading rate may be increased by a factor of ∼10 by assuming a Gaussian distribution (e.g., the total mass is ∼10 times that in ∣z∣ ≳ 260 pc regions for the thick CO disk of σ_z ∼ 110–120 pc; see Su et al. 2021). By considering the neutral atomic gas coexisting with the molecular gas, the mass-loading rate of the cool outflows may be slightly larger than the estimated value of ∼3v _{w 200} M_⊙ yr⁻¹. Although large uncertainty remains and more observations are needed, the rough estimate shows that the outflow rate at the order of ∼2–4 M_⊙ yr⁻¹ is possible according to the large-scale enhanced CO emission at R_GC ∼ 3 kpc.

The energy source of the Milky Way nuclear wind is still being debated, i.e., intermittent activities from Sgr A* for AGN-like model versus integrated effects of the stellar feedback from the CMZ for the starburst model. The total kinetic energy of the cool-gas outflows can be estimated as ${E}_{{\rm{K}}}=0.5{M}_{\mathrm{gas}}{v}_{{\rm{w}}}^{2}\gtrsim 4\times {10}^{54}{v}_{{\rm{w}}200}^{2}$ erg for the total molecular gas mass of ≳1 × 10⁷ M_⊙ at ∣z∣ ≳ 110 pc (e.g., ∼10 times that in ∣z∣ ≳ 260 pc regions). The required kinetic power to the pushed molecular gas is then $\sim 4\times {10}^{40}{v}_{{\rm{w}}200}^{3}$ erg s⁻¹ for the assumed dynamical time of $\sim 3.0{v}_{{\rm{w}}200}^{-1}$ Myr. Whatever the exact origin of the Milky Way nuclear wind, our estimates show that the cool-gas outflows can be easily powered by the energetic processes near the GC (e.g., the total energy of 10⁵⁶–10⁵⁷ erg or the total power of 10⁴²–10⁴⁴ erg s⁻¹ for the Fermi bubbles).

On the other hand, low-density gas can be easily accelerated in the hot wind environment. And the hot ionized gas is probably the dominant reservoir of energy of the Fermi bubbles (e.g., the inferred temperature of ≳2 × 10⁶ K, the low density of ∼10⁻³ cm⁻³, and the typical velocity of ∼500–1000 km s⁻¹ for the hot gas; Miller & Bregman 2016; Bordoloi et al. 2017). The warm and hot ionized gas of the nuclear wind plays an important role in driving the cool-gas outflows near the gaseous disk. Accordingly, the ablated gas from the high-structured MCs in the gaseous disk joins the moving flow, modifying the velocity, temperature, and density distribution of the multiphase medium.

The thinner disk of the atomic and molecular gas within the region of R_GC ≲ 3 kpc can be explained by the effect of the large-scale Milky Way nuclear wind. Assuming the hot wind velocity of ∼500–1000 km s⁻¹ and the mass-lose rate of ∼10–20 M_⊙ yr⁻¹ near the Galactic inner gaseous disk, the disk will lose ∼6 × 10⁷ M_⊙ in a period of ∼3–6 Myr (e.g., from the CMZ to the R_GC ∼ 3 kpc regions), which is about 30% of the molecular gas within the R_GC ≲ 3 kpc region (e.g., the total molecular mass of ∼2 × 10⁸ M_⊙; see Heyer & Dame 2015; Nakanishi & Sofue 2016). Therefore, a bulk of the molecular gas can be removed from the gaseous disk within R_GC ≲ 3 kpc, especially for the region of ∣z∣ ≳ 50–100 pc.

The total mass of the removed molecular gas in the inner disk is roughly comparable to the value of ∼7 × 10⁷ M_⊙ for the whole 3 kpc arm (e.g., refer to H₂ masses per unit length of ∼4 × 10⁶ M_⊙ kpc⁻¹ in Dame & Thaddeus 2008). We propose that the 3 kpc arm is probably related to the Milky Way nuclear wind. It is true that only a small fraction of molecular gas (≳1 × 10⁶ M_⊙) is accelerated to the ∣z∣ ≳ 260 pc region, while the dominant molecular gas seems to be accumulated at the disk (e.g., the 3 kpc arm; see Dame & Thaddeus 2008; Reid et al. 2019; Sofue & Kataoka 2021). Both of the crater walls traced by EHMCs and the 3 kpc arm are naturally located at the similar Galactocentric distance of R_GC ∼ 3 kpc, where the high-velocity hot wind is almost stopped and/or is confined by the cold gas near the gaseous disk.

Due to angular momentum conservation, gas inflows generally accompany gas outflows. The current star formation rate in the CMZ is ≲0.1 M_⊙ yr⁻¹ (e.g., Yusef-Zadeh et al. 2009; Longmore et al. 2013), which will exhaust the CMZ gas in a few × 10⁸ Myr without other gas supply. The continuous gas inflow from the inner disk may provide additional gas supply for the star formation in the CMZ (e.g., the inflow rate at the order of ∼1–4 M_⊙ yr⁻¹; see Armillotta et al. 2019; Tress et al. 2020; Hatchfield et al. 2021).

The estimated mass outflow rate based on the CO data is roughly comparable to the gas inflow rate from the simulations at the same order of magnitude of ∼2–4 M_⊙ yr⁻¹. Generally, the gas is transported from the inner gaseous disk at R_GC ∼ 3 kpc to the CMZ by inflows, while the angular momentum of gas is taken away by outflows from the inner disk. The removed gas in the inner disk may fall back to the disk (e.g., the fountain models; Shapiro & Field 1976; Bregman 1980; Houck & Bregman 1990; Spitoni et al. 2008; Melioli et al. 2009; Marasco et al. 2015; Fraternali 2017) and/or may be accumulated in the 3 kpc arm and the high-z crater walls. In this regard, the total molecular gas mass in the CMZ should be roughly comparable to that in the 3 kpc arm.

Briefly, the existence of the outflows and the inflows is probably the common feature in the inner region of the Milky Way (or other barred spiral galaxies). The dynamical processes indicate that there is a delicate balance between gas outflows and inflows toward the inner region of the Milky Way. Highly variable inflow rate from a recent epoch may lead to the episodic accretion onto the CMZ and intermittent activity from Sgr A* (see, e.g., recent observations in Ponti et al. 2019, 2021). The following enhanced outflows will then terminate star formation near the GC and/or restrain nuclear activity due to decreasing the gas supply.

Finally, Figure 7 shows a schematic view of the observation results, i.e., the large-scale H i voids related to the Fermi bubbles/X-ray bubbles in the inner Galaxy, the CO crater walls surrounding the edges of the H i voids above and below the Galactic plane, the thinner gaseous disk within R_GC ≲ 3 kpc, the expanding 3 kpc arm at the base of the enhanced EHMCs, and the entrained MCs with cometary structures pointing away from the Galactic plane.

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