ON THE NATURE OF THE DARK MASS IN THE CENTER OF THE MILKY-WAY

We discuss constraints on the properties and nature of the dark mass c oncentration at the core of the Milky Way. We present 0.15-arcsec astr ometric 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 lamb da/Delta lambda similar to 35 speckle spectroscopy and show that sever al of the Sgr A (infrared) cluster members are likely early-type star s of mass similar to 15 to 20 M.. All available checks, including a fi rst comparison with high-resolution maps that are now becoming availab le from other groups, support our previous conclusion that there are s everal 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 an d 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-wee k 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, stel lar remnants or substellar entities at that density. From an equiparti tion argument we infer that at least 5 per cent of the dark mass (grea ter than or equal to 10(5) M.) is associated with the compact radio so urce Sgr A itself and is concentrated on a scale of less than 15 time s the Schwarzschild radius of a 2.6 x 10(6)-M. black hole. The corresp onding density is 3 x 10(20) M. pc(-3) or greater. If one accepts thes e arguments it is hard to escape the conclusion that there must be a m assive black hole at the core of the Milky Way.