Simulations of the Milky Way's central molecular zone -- II. Star formation

Abstract: The Milky Way's central molecular zone (CMZ) has emerged in recent years as a unique laboratory for the study of star formation. Here we use the simulations presented in Tress et al. 2020 to investigate star formation in the CMZ. These simulations resolve the structure of the interstellar medium at sub-parsec resolution while also including the large-scale flow in which the CMZ is embedded. Our main findings are as follows. (1) While most of the star formation happens in the CMZ ring at $R\gtrsim100 {\, \rm pc}$, a significant amount also occurs closer to SgrA* at $R \lesssim 10{\, \rm pc}$. (2) Most of the star formation in the CMZ happens downstream of the apocentres, consistent with the "pearls-on-a-string" scenario, and in contrast to the notion that an absolute evolutionary timeline of star formation is triggered by pericentre passage. (3) Within the timescale of our simulations ($\sim100$ Myr), the depletion time of the CMZ is constant within a factor of $\sim2$. This suggests that variations in the star formation rate are primarily driven by variations in the mass of the CMZ, caused for example by AGN feedback or externally-induced changes in the bar-driven inflow rate, and not by variations in the depletion time. (4) We study the trajectories of newly born stars in our simulations. We find several examples that have age and 3D velocity compatible with those of the Arches and Quintuplet clusters. Our simulations suggest that these prominent clusters originated near the collision sites where the bar-driven inflow accretes onto the CMZ, at symmetrical locations with respect to the Galactic centre, and that they have already decoupled from the gas in which they were born.

• The paper demonstrates star formation patterns along elliptical rings and near SgrA*, supporting a pearls-on-a-string scenario.
• It employs sub-parsec hydrodynamical simulations that integrate gas self-gravity, star formation, and supernova feedback with live chemical networks.
• Findings indicate that variations in CMZ mass, rather than changes in depletion time, primarily drive fluctuations in star formation rates.

Star Formation in the Central Molecular Zone of the Milky Way

The study conducted by Sormani et al. leverages advanced hydrodynamical simulations to gain insights into star formation within the Central Molecular Zone (CMZ) of the Milky Way. This region, characterized by its extreme environmental conditions, provides a unique natural laboratory for probing the complexities of star formation processes, especially under physical conditions that differ markedly from the Galactic disk.

The research utilizes hydrodynamical simulations that achieve sub-parsec resolution to capture intricate details of the interstellar medium (ISM), while also considering the influences of large-scale flows driven by the Galactic bar. The simulations successfully encompass essential physical processes such as gas self-gravity, star formation, and supernova feedback, calculated using a combination of live chemical networks and sophisticated models for star formation and stellar feedback.

Key Findings

1. Spatial and Temporal Star Formation Distribution: The CMZ primarily hosts star formation along its elliptical ring at radii greater than 100 parsecs from the Galactic center. Significantly, the simulations reveal a substantial amount of star formation activity also occurring closer to SgrA*. This star formation predominantly trails the apocenters of the orbits, aligning with the "pearls-on-a-string" scenario rather than the pericentric passage notion (often suggested where star formation is triggered as gas clouds reach the closest point to the Galactic center).
2. Depletion Time and Star Formation Rates: Over the simulation's 100 Myr timescale, the depletion time—a measure of how rapidly gas is converted into stars—remains relatively constant for the CMZ. This indicates that variations in the star formation rate are more closely tied to changes in the CMZ’s mass, rather than fluctuations in depletion time. Factors such as active galactic nucleus (AGN) feedback or variations in the inflow rate due to the Galactic bar's dynamics potentially affect the CMZ mass and consequently the star formation activity.
3. Star Trajectories and Clusters Formation: The trajectories of newly formed stars suggest that clusters such as the Arches and Quintuplet likely formed near collision sites. These sites are where the inflow of material along the bar-driven streams impacts the CMZ. These clusters are symmetric in relation to the Galactic center and have decoupled from their natal clouds, emphasizing a distinct kinematic history from the surrounding gas.
4. Comparison with Observed Galactic Center Phenomena: The simulations provide a coherent framework to interpret the dynamics of star formation in the CMZ, enhancing our understanding of the peculiar behaviors observed in dense stellar clusters and star-forming regions within the Milky Way’s center.

Implications and Future Prospects

The implications of this research extend to the broader understanding of star formation under extreme conditions, which is relevant for interpreting star formation in other galaxy centers. The results suggest that the CMZ can be characterized both by the ongoing star formation distributed along its main ring and by sporadic yet significant events closer to the Galactic center.

Looking ahead, the study paves the way for further exploration into the CMZ's role as a test-bed for star formation theories. The research underscores the need for observational programs focused on resolving the dense gas and young stellar populations in the CMZ, potentially utilizing next-generation telescopes capable of delivering high-resolution data.

Moreover, speculative advancements could include incorporating detailed magnetohydrodynamics (MHD) to capture the effects of magnetic fields on the star formation processes comprehensively. This complexity could further elucidate the physical mechanisms underpinning the observed deviations from dense gas-star formation scaling relations noted in the CMZ.

In conclusion, the paper provides a valuable contribution to the field of astrophysics, offering a detailed and sophisticated view of star formation mechanisms within the volatile environment of the Milky Way's CMZ. The insights gained here form a critical bridge linking empirical observations with theoretical models of extreme star formation physics.