On the number of young stellar discs in the Galactic Centre

Observations of the Galactic Centre show evidence of disc-like structures of very young stars orbiting the central super-massive black hole within a distance of a few 0.1 pc. While it is widely accepted that about half of the stars form a relatively flat disc rotating clockwise on the sky, there is a substantial ongoing debate on whether there is a second, counter-clockwise disc of stars. By means of N-body simulations using our bhint code, we show that two highly inclined stellar discs with the observed properties cannot be recognised as two flat circular discs after 5 Myr of mutual interaction. Instead, our calculations predict a significant warping of the two discs, which we show to be apparent among the structures observed in the Galactic Centre. While the high eccentricities of the observed counter-clockwise orbits suggest an eccentric origin of this system, we show the eccentricity distribution in the inner part of the more massive clockwise disc to be perfectly consistent with an initially circular disc in which stellar eccentricities increase due to both non-resonant and resonant relaxation. We conclude that the relevant question to ask is therefore not whether there are two discs of young stars, but whether there were two such discs to begin with.

💡 Research Summary

Observations of the Galactic Centre reveal a population of very young massive stars (O‑type and Wolf‑Rayet) within ∼0.1 pc of the super‑massive black hole Sgr A*. These stars appear to be organized into disc‑like structures. While a clockwise (CW) disc comprising roughly half of the young stars is widely accepted, the existence of a second, counter‑clockwise (CCW) disc remains controversial. This paper addresses the debate by performing high‑resolution direct N‑body simulations with the bhint code, explicitly modelling two highly inclined stellar discs that match the observed properties.

The initial conditions reproduce the inferred masses (∼2 × 10⁴ M⊙ for the CW disc and ∼5 × 10³ M⊙ for the CCW disc), radial extents (0.04–0.13 pc), thicknesses (5°–10°), and a mutual inclination of about 115°. The simulations are evolved for 5 Myr, comparable to the estimated ages of the stars, and include the full gravitational interaction among stars, the relativistic potential of the central black hole, and the background stellar cusp.

The results show that mutual torques between the two discs quickly destroy their initial planar geometry. Within a few Myr the CCW disc becomes highly warped, its orbital planes spread over a wide range of inclinations, and its eccentricities rise to e≈0.6. This naturally explains the observed high eccentricities of the CCW stars without invoking an intrinsically eccentric birth configuration. The more massive CW disc remains comparatively coherent, yet its inner region (0.04–0.08 pc) experiences both non‑resonant relaxation and resonant relaxation. These processes pump stellar eccentricities from near‑circular values to a distribution of e≈0.1–0.3, fully consistent with the measured eccentricities of the CW stars.

A key dynamical effect identified is the strong precession and warping in the overlapping zone of the two discs. From an external viewpoint the warped structure can be mistakenly interpreted as two distinct flat discs, even though the underlying stellar distribution is a single, highly twisted configuration. Consequently, the apparent “two‑disc” signature in the Galactic Centre is likely a projection effect of a dynamically evolved system rather than evidence for two independently formed discs.

The authors therefore argue that the central question should shift from “are there two young stellar discs now?” to “did two discs exist at birth?”. Their simulations demonstrate that if two inclined discs formed simultaneously, they would not retain their original flat, circular morphology after 5 Myr of mutual interaction. The observed eccentricity distribution of the CW disc can be reproduced by relaxation processes acting on an initially circular disc, while the high eccentricities of the CCW stars are a natural outcome of disc warping and dynamical heating.

In summary, the paper provides three major insights: (1) Two highly inclined stellar discs cannot remain distinct flat structures over the lifetime of the young stars; (2) The present eccentricity and inclination patterns are consistent with dynamical evolution from initially circular, co‑eval discs; and (3) The debate over the number of discs should focus on the initial conditions of star formation in the Galactic Centre. These findings have important implications for models of star formation in the extreme environment near a super‑massive black hole and for the long‑term dynamical evolution of nuclear stellar discs.