Extended Data Fig. 4: Empirical verification of the escape velocity.

Panels a and b show histograms of the observed 2D velocity distribution in the very centre (0″ < r < 3″; c) and in an outer ring (3″ < r < 10″; d). While the lower velocities are well described by a 2D Maxwell-Boltzmann distribution with σ_1D = 20 km s⁻¹ (marked with a solid black line, the dashed black lines refer to alternative distributions with σ_1D = 17 km s⁻¹ and σ_1D = 23 km s⁻¹), there are clearly notable differences at higher velocities. Those become especially visible in the cumulative normalized histogram shown in d and the zoom-in in e: while the distribution between (0″ < r < 3″, blue line) shows an excess of fast-moving stars, the distribution at larger radii (3″ < r < 10″, orange line) shows a clear deficit of stars at velocities larger than the escape velocity, making the used escape velocity threshold very plausible. Even though the sample is ten times larger, there is only a single star with a velocity significantly larger than v_esc. This star has a 2D velocity of 75.8 km s⁻¹ and is at a radius of r = 9.5″. It is consistent with being a Milky Way foreground star.