On the nature of the dark mass in the centre of the Milky Way

We discuss constraints on the properties and nature of the dark mass concentration at the core of the Milky Way. We present 0.15-arcsec astrometric K-band maps in five epochs beween 1992 and 1996. From these we derive imposed stellar proper motions within 3 arcsec of the compact radio source Sgr A* whose infrared counterpart may have been detected, for the first time, in a deep image in 1996 June. We also report lambda/Delta lambda similar to 35 speckle spectroscopy and show that several of the Sgr A* (infrared) cluster members are likely early-type stars of mass similar to 15 to 20 M.. All available checks, including a first comparison with high-resolution maps that are now becoming available from other groups, support our previous conclusion that there are several fast-moving stars (greater than or equal to 10(3) km s(-1)) in the immediate vicinity (0.01 pc) of Sgr A*. From the stellar radial and proper motion data, we infer that a dark mass of 2.61 (+/-0.15(stat))(+/-0.35(stat+sys)) x 10(6) M. must reside within about one light-week of the compact radio source. Its density must be 2.2 x 10(12) M. pc(-3) or greater. There is no stable configuration of normal stars, stellar remnants or substellar entities at that density. From an equipartition argument we infer that at least 5 per cent of the dark mass (greater than or equal to 10(5) M.) is associated with the compact radio source Sgr A* itself and is concentrated on a scale of less than 15 times the Schwarzschild radius of a 2.6 x 10(6)-M. black hole. The corresponding density is 3 x 10(20) M. pc(-3) or greater. If one accepts these arguments it is hard to escape the conclusion that there must be a massive black hole at the core of the Milky Way.