The episodic and multiscale Galactic Centre

Within the central few hundred parsecs of the Milky Way, extending from longitude l = −1° to 1.5°, lies the Central Molecular Zone of the Galactic Centre. This extraordinary region is defined by a diverse variety of ISM features in numerous stages of evolution. Molecular cloud H₂ volume densities range from 10^3-8 cm⁻³ with an average of 10⁴ cm⁻³, two orders of magnitude above that of the galactic disk. The CMZ contains ∼3-5 × 10⁷ M_☉ of molecular gas, corresponding to around 5-10% of the content of the entire galaxy, and a similar fraction of its infrared luminosity. Gas temperatures, pressures and turbulent mach numbers are also significantly raised here, providing one of the more extreme environments for star formation within our observational reach.

We have hence been provided with a unique laboratory for probing the effects of these environments on the interplay between the ISM and star formation, and high resolution observations of both individual features and the large-scale structure of the CMZ can improve our understanding of the formation and evolution of this region, which we can then apply to similar regions in nearby galaxies.

This review will address historical and recent advancements in our observational and theoretical interpretations of the morphologies, dynamics and processes occurring in the ISM and massive stellar populations in the central few hundred parsecs. It will demonstrate how, across various spatial scales, episodic cycles of star formation, matter transport and feedback can be identified and potentially linked to observed features. The evolutionary states of molecular clouds, star forming regions and stellar clusters can be linked to their positions along orbits spanning the CMZ, and may be regulated by episodic processes such as material inflow or feedback. The concentric series of expanding bubbles and fronts visible in various electromagnetic bands can be related to echoes of past activity in the central cluster and Sgr A*. The ensemble of stellar ages and populations in the highly inhospitable environment of the central few parsecs points towards a series of accretion and starburst events.

The range of timescales and spatial scales involved in the aforementioned processes raises the possibility of a nested series of episodic cycles occurring concurrently, in which shorter timescale cycles regulate longer ones. The resulting complex and highly time-variable picture can help to explain many of the currently observed characteristics of the Galactic Centre, such as its deficient star forming efficiency, and can be applied to our understanding of the evolution of the galaxy as a whole.