Stellar kinematics in the MW centre

The MW centre has a weak AGN in its centre, identified with the radio source Sagittarius A.

Over the past couple of years, Genzel and collaborators measured the kinematics of stars close to the MW centre from near-IR spectra^11.12 They also determined very accurate positions of the stars. By doing this over several consecutive years, they were able to measure not just the 3D proper motions of the stars, but also measure accelerations. In fact they were able to follow one star over nearly a whole orbit, when it swung passed the centre at velocities approaching 1000km s$^{-1}$.

So first of all, they demonstrated that there is indeed a very massive `dark' object in the centre of the MW. Using Kepler's laws, they were able to put very stringent limits on the mass of the central concentration, $M_{\rm DO}\approx 2.6\times 10^6\hbox{$M_\odot$}$, in a region smaller than 0.006, implying an enormous mass density higher than $2\times 10^{12}\hbox{$M_\odot$}/\pc ^3$. This leaves almost no room to escape the conclusion that the MDO is indeed a SMBH, at least in the MW.

The presence of the large mass is also supported by the presence of other very high velocity stars. From that, and from the fact that the putative centre, i.e. the radio source Sagittarius A itself does not seem to move, Genzel derived a lower limit to the density of $3\times 10^{20}\hbox{$M_\odot$}/\pc ^3$.

Incidentally, they also showed that the velocity dispersion remains nearly isotropic, and so unless the MW is peculiar, this then strengthens the case for SMBHs in other galaxies, from the arguments in the previous section.