A dynamical model for gas flows, star formation and nuclear winds in galactic centres

Abstract

We present a dynamical model for gas transport, star formation, and winds in the nuclear regions of galaxies, focusing on the Milky Way's Central Molecular Zone (CMZ). In our model angular momentum and mass are transported by a combination of gravitational and bar-driven acoustic instabilities. In gravitationally-unstable regions the gas can form stars, and the resulting feedback drives both turbulence and a wind that ejects mass from the CMZ. We show that the CMZ is in a quasi-steady state where mass deposited at large radii by the bar is transported inward to a star-forming, ring-shaped region at ∼ 100 pc from the Galactic Centre, where the shear reaches a minimum. This ring undergoes episodic starbursts, with bursts lasting ∼ 5 − 10 Myr occurring at ∼ 20 − 40 Myr intervals. During quiescence the gas in the ring is not fully cleared, but is driven out of a self-gravitating state by the momentum injected by expanding supernova remnants. Starbursts also drive a wind off the star-forming ring, with a time-averaged mass flux comparable to the star formation rate. We show that our model agrees well with the observed properties of the CMZ, and places it near a star formation minimum within the evolutionary cycle. We argue that such cycles of bursty star formation and winds should be ubiquitous in the nuclei of barred spiral galaxies, and show that the resulting distribution of galactic nuclei on the Kennicutt-Schmidt relation is in good agreement with that observed in nearby galaxies.

The life cycle of the Central Molecular Zone – II. Distribution of atomic and molecular gas tracers

“…In Paper I, we showed that star formation mostly takes place within the dense ring-like structure, and that is characterised by oscillatory cycles of burst/quenching. Our simulations suggest that the current CMZ is near a minimum of a star formation cycle, in agreement with the findings of earlier analytic and one-dimensional dynamical models by Kruijssen et al (2014), Krumholz & Kruijssen (2015), and Krumholz et al (2017).…”

H2O Southern Galactic Plane Survey (HOPS): Paper III – properties of dense molecular gas across the inner Milky Way

“…Sofue 1995;Morris & Serabyn 1996;Bally et al 2010). In other scenarios, gas within the bar's inner Linblad resonance (∼1 kpc for the Milky Way) is driven inwards by angular momentum transport induced by acoustic instabilities (Montenegro, Yuan & Elmegreen 1999;Kruijssen et al 2014;Krumholz & Kruijssen 2015;Krumholz, Kruijssen & Crocker 2017). Approaching the galactic centre, the galaxy's rotation curve turns from flat to solid body and the inward transport slows down, leading to a buildup of gas.…”

“…As noted by , the H69α emission, which predominantly traces young H II regions, is much more uniformly distributed. This intriguing lack of correspondence between the large dense gas reservoir and star formation activity tracers in the CMZ, and the implications of this for our understanding of star formation in other environments, is currently under active investigation Kruijssen et al 2014;Krumholz & Kruijssen 2015;Krumholz et al 2017). …”

The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone

“…The complex interplay of energetic processes in the GC allows for different potential answers to this problem of low SFE, but none of these processes alone can explain the discrepancy (Kruijssen et al 2014). A general picture of episodic starbursts in gas rings in galactic centers introduced by Krumholz & Kruijssen (2015) and Krumholz et al (2017) might solve the SFE problem in the particular case of the CMZ and set the framework for another intriguing feature of the GC: a ring-like structure of dust and molecular gas. This structure (see Figure 1 for an overview) is projected onto an infinity (¥) shape that follows several arcs, the most prominent being the so-called "dust ridge" stretching from the massive molecular cloud G0.253+0.016 ("the Brick") to the star-forming region Sgr B2 (Lis et al 1999 , gas is more likely to be on the far side, i.e., behind SgrA * (Bally et al 2010;Molinari et al 2011).…”