The inner cavity of the circumnuclear disc

TL;DR

This study uses simulations to challenge existing ideas about the Galactic Centre's circumnuclear disc, showing magnetic fields stabilize the inner rim and that unstable clumps do not imply the disc's transience.

Contribution

It demonstrates that magnetic fields prevent rapid inward migration of the disc's inner rim and that unstable clumps are not necessarily indicative of a transient disc.

Findings

Inner cavity fills quickly without magnetic fields

Magnetic fields stabilize the inner rim

Unstable clumps do not imply disc transience

Abstract

The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and second, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. Furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. Thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. The next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here.