Candidate super star cluster progenitor gas clouds possibly triggered by close passage to Sgr A*

Abstract: Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters are gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253+0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.

Overview of Candidate Super Star Cluster Progenitor Gas Clouds

The paper by Longmore et al. presents an exploration of potential super star cluster (SSC) progenitor gas clouds situated near the Galactic center (GC), focusing on their formation mechanics and initial conditions. The study predominantly investigates the inner few hundred parsecs of the Milky Way, commonly known as the Central Molecular Zone (CMZ), which is enriched with molecular gas and exhibits unique physical conditions for star formation. This research is rooted in identifying candidate gas clouds capable of evolving into young massive clusters (YMCs) and considers their relationship to the supermassive black hole, Sagittarius A* (Sgr A*).

Identification of YMC Progenitor Clouds

The study utilizes a robust methodology for identifying potential YMC precursor gas clouds within the CMZ. The authors employ a systematic analysis of enclosed mass distributions within specific projected radii from a column density map, paired with velocity information from molecular line data. This approach successfully identifies four candidate clouds (including $G0.253$+$0.016$) which satisfy the necessary conditions to possibly evolve into YMCs, characterized by extensive mass (>10⁴-⁵ M☉) and compact density (radius ~1 pc).

Implications for Star Cluster Formation

The paper emphasizes intriguing implications regarding the frequency and efficiency of YMC formation in the Galactic center as opposed to other regions within the Milky Way. Notably, the presence of starless YMC progenitor candidates in the CMZ, absent in the first Galactic quadrant, suggests higher YMC formation rates in the former. This disparity may be attributed to environmental influences unique to the GC that impact both the formation and destruction rates of these clusters. The authors propose that interaction with the Galactic gravitational potential during close passages to Sgr A* could instigate this process, offering a potential trigger mechanism for YMC formation. This mechanism, which involves tidal compression effects, may elucidate differences in protostellar densities and feedback effects critical to cluster formation.

Theoretical and Observational Considerations

The research prompts reconsideration of theories pertaining to cluster formation dynamics that incorporate gravitational influences at the GC. The potential for tidal forces to compress gas streams and instigate cluster formation presents a unique framework for understanding SSC birth in high-pressure environments. Observationally, confirming this theory could reshape interpretations of how galactic nuclei contribute to star formation rates and cluster longevity.

Future Directions

In advancing the study of SSC progenitor clouds, longitudinal tracking of identified candidates could yield insights into evolving star formation processes. Integrating numerical simulations to quantify tidal interaction impacts and detailed molecular line observations might enhance understanding of mass assembly physics. Given that current observations align with potential evolutionary sequences in cluster development, further verification of the proposed triggering mechanism holds significant promise for informing models of extreme star formation environments not only in the Milky Way but in extragalactic systems as well.

In conclusion, the work of Longmore et al. underscores the significance of gravitational dynamics near the Galactic center in potentially catalyzing the formation of some of the most densely packed stellar conglomerates. It provides a substantive foundation for future exploration into galactic nucleus-induced star formation mechanisms and enriches the theoretical landscape addressing the lifecycle of YMCs in extreme environments.