What controls star formation in the central 500 pc of the Galaxy?

Abstract: The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of >10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scale height, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The HI-H$_2$ phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse - once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments.

• The paper investigates the significantly lower-than-expected star formation rate in the Milky Way's central 500 pc despite abundant dense gas, exploring potential physical mechanisms.
• Key findings suggest elevated turbulent pressure in the Central Molecular Zone increases the critical density required for star formation, challenging universal density thresholds.
• The study proposes an episodic star formation cycle where gas accumulates until self-gravity initiates rapid star formation, potentially applicable to other galactic nuclei.

Overview of "What Controls Star Formation in the Central 500 pc of the Galaxy?"

This paper by J.M. Diederik Kruijssen et al. investigates the underwhelming star formation rate (SFR) observed in the Central Molecular Zone (CMZ) of the Milky Way, an area replete with dense gas. Despite the substantial gas mass, the SFR in this region is lower than expected by a factor of $\geq10$, contrasting sharply with predictions from established star formation theories. The study examines potential physical mechanisms that could suppress star formation at both global and local scales, and proposes a coherent cycle of star formation in the CMZ.

Key Findings

1. Global Mechanisms:
   • The CMZ's low SFR is partially reconcilable with episodic star formation driven by secular instabilities or variations in gas inflow along the Galactic bar.
   • The CMZ is found to be marginally Toomre-stable when including both gas and stars, indicating a suppressed condensation rate of self-gravitating clouds.
2. Local Mechanisms:
   • An elevated turbulent pressure in the CMZ likely increases the critical density for star formation. This suggests that a universal density threshold does not apply to these conditions.
   • Various factors such as the H{\sc i} to H$_2$ phase transition, tidal fields, magnetic fields, and feedback mechanisms do not singularly account for the suppressed SFR.

Implications of the Study

• Starburst Cycle: The study proposes a cycle where gas in the CMZ is accumulated over extended periods through secular instabilities until it becomes self-gravitating, at which point star formation is rapidly initiated. This process naturally leads to episodic star formation activity.
• Theoretical Framework: The findings challenge universal star formation laws and suggest that local conditions, such as the increased critical density due to turbulent pressure, significantly alter star formation dynamics in this galactic environment.
• Extragalactic Comparisons: The episodic star formation model may extend to other galactic nuclei, possibly shedding light on varying SFRs observed in similar regions of other galaxies.

Future Directions and Developments

For future developments, the study suggests direct observations and comparisons of galactic nuclei in external galaxies using advanced facilities like the Atacama Large Millimeter Array (ALMA). These observations are crucial for addressing the paper's hypotheses on density thresholds and turbulence-driven star formation cycles. An empirical evaluation of mass inflow and outflow rates in galactic centers may also provide insights into the observed variability of SFRs. The interplay between turbulence, feedback, and gas inflow dynamics remains a pivotal area of study for advancing our understanding of star formation in the extreme environments of galactic centers.

The paper notably underscores the importance of accounting for environmental diversity in star formation theories, challenging the notion of universality within astrophysical mass accumulation and stellar life cycle processes. A broad comparative approach with extragalactic systems will be essential for generalizing the proposed star formation mechanisms beyond the Milky Way.