On the physical mechanisms governing the cloud lifecycle in the Central Molecular Zone of the Milky Way

TL;DR

This paper applies an analytic theory to understand the cloud lifecycle in the Milky Way's Central Molecular Zone, revealing how gravitational collapse and galactic shear influence cloud evolution and star formation potential.

Contribution

It introduces a model combining multiple physical processes to predict molecular cloud lifetimes and their dependence on galactic radius in the Galactic center.

Findings

Clouds near 45-120 pc are mainly governed by gravitational collapse.

Most clouds elsewhere are dispersed by galactic shear within 3-9 Myr.

Tidal compression may trigger star formation in the 100-pc stream.

Abstract

We apply an analytic theory for environmentally-dependent molecular cloud lifetimes to the Central Molecular Zone of the Milky Way. Within this theory, the cloud lifetime in the Galactic centre is obtained by combining the time-scales for gravitational instability, galactic shear, epicyclic perturbations and cloud-cloud collisions. We find that at galactocentric radii $\sim 45$-$120$ pc, corresponding to the location of the '100-pc stream', cloud evolution is primarily dominated by gravitational collapse, with median cloud lifetimes between 1.4 and 3.9 Myr. At all other galactocentric radii, galactic shear dominates the cloud lifecycle, and we predict that molecular clouds are dispersed on time-scales between 3 and 9 Myr, without a significant degree of star formation. Along the outer edge of the 100-pc stream, between radii of 100 and 120 pc, the time-scales for epicyclic perturbations and gravitational free-fall are similar. This similarity of time-scales lends support to the hypothesis that, depending on the orbital geometry and timing of the orbital phase, cloud collapse and star formation in the 100-pc stream may be triggered by a tidal compression at pericentre. Based on the derived time-scales, this should happen in approximately 20 per cent of all accretion events onto the 100-pc stream.