Simulations of the Milky Way's central molecular zone -- I. Gas dynamics

Abstract: We use hydrodynamical simulations to study the Milky Way's central molecular zone (CMZ). The simulations include a non-equilibrium chemical network, the gas self-gravity, star formation and supernova feedback. We resolve the structure of the interstellar medium at sub-parsec resolution while also capturing the interaction between the CMZ and the bar-driven large-scale flow out to $R\sim 5\kpc$. Our main findings are as follows: (1) The distinction between inner ($R\lesssim120$~pc) and outer ($120\lesssim R\lesssim450$~pc) CMZ that is sometimes proposed in the literature is unnecessary. Instead, the CMZ is best described as single structure, namely a star-forming ring with outer radius $R\simeq 200$~pc which includes the 1.3$^\circ$ complex and which is directly interacting with the dust lanes that mediate the bar-driven inflow. (2) This accretion can induce a significant tilt of the CMZ out of the plane. A tilted CMZ might provide an alternative explanation to the $\infty$-shaped structure identified in Herschel data by Molinari et al. 2011. (3) The bar in our simulation efficiently drives an inflow from the Galactic disc ($R\simeq 3$~kpc) down to the CMZ ($R\simeq200$~pc) of the order of $1\rm\,M_\odot\,yr^{-1}$, consistent with observational determinations. (4) Supernova feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of $\sim0.03\,\rm M_\odot\,yr^{-1}$. (5) We give a new interpretation for the 3D placement of the 20 and 50 km s$^{-1}$ clouds, according to which they are close ($R\lesssim30$~pc) to the Galactic centre, but are also connected to the larger-scale streams at $R\gtrsim100$~pc.

• The paper establishes a unified CMZ model, showing it as a single star-forming ring (~200 pc radius) rather than a split inner and outer zone.
• The simulations demonstrate that the Galactic bar drives gas inflow at about 1 M☉/yr, matching key observational data on bar-induced dynamics.
• The study also reveals that supernova feedback contributes an additional inflow (~0.03 M☉/yr), affecting the structure and evolution of the circumnuclear disk.

Analysis of Gas Dynamics in the Milky Way's Central Molecular Zone

This paper by Tress et al. investigates the dynamics of gas within the Milky Way's Central Molecular Zone (CMZ) through hydrodynamical simulations. These simulations combine a non-equilibrium chemical network with considerations of gas self-gravity, star formation, and supernova feedback. The paper distinguishes itself by reaching sub-parsec resolution in the simulations, thereby enabling detailed observation of the complex gas flows associated with the CMZ.

Key Findings

1. Unified CMZ Model: The study challenges the commonly proposed dichotomy of an inner and an outer CMZ. Instead, the simulations reveal the CMZ as a singular structure—a star-forming ring with an approximate outer radius of 200 pc. This structure directly interacts with the dust lanes, which are crucial for mediating the inflow driven by the Galactic bar.
2. Bar-Driven Inflow: The Galactic bar is shown to effectively channel material from the disk at approximately 3 kpc into the CMZ at a rate of about 1 M☉ yr^−1. This aligns with observational data suggesting significant bar-driven gas migration towards the galactic center.
3. Supernova-Induced Inflow: Contrary to the bar mechanism, supernova feedback within the CMZ facilitates further inflow towards the circumnuclear disk at a lesser rate of approximately 0.03 M☉ yr^−1. This contribution is significant to the dynamics within the CMZ, potentially influencing star formation activities.
4. Revisiting the 3D Structure: The simulations provide a novel interpretation of the placement of the 20 and 50 km s^−1 clouds. These clouds are posited to exist relatively near the Galactic center, within 30 parsecs, and are connected to larger-scale gas streams originating from distances greater than 100 parsecs.
5. Tilt of the CMZ: The simulations suggest that the accretion process can cause the CMZ to possess a slight tilt out of the galactic plane. This feature might offer an alternative explanation for the observed infinity-shaped structure identified in previous studies, such as the work by Molinari et al. (2011).

Implications and Future Directions

The insights from this paper enhance the understanding of gas dynamics and star formation in a barred spiral galaxy’s central region. Importantly, they suggest that the process in our galaxy can serve as an analogue for star-forming rings in external galaxies. Furthermore, the detection and analysis of significant inflow mechanisms—including those driven by stellar feedback—provide critical pathways for understanding fuel delivery to central galactic regions, with implications for circumnuclear disk formation and subsequent central activity.

The paper’s methodological approach provides a solid framework for future numerical simulations. It emphasizes the importance of accounting for both large-scale galactic dynamics and small-scale processes in modeling intricate galactic environments. Future work could expand these simulations by integrating magnetic fields, which are known to have dynamic effects at such scales, or by incorporating radiation feedback from young stellar objects to understand their roles alongside supernova feedback.

In conclusion, this study provides valuable detail to the discourse on CMZ dynamics and prompts further exploration of its mechanisms and consequences in galactic evolution.