The Galactic Center

While the proximity of our Galactic Center (only 8kpc away) is obviously a big advantage for any study of the SMBH residing there, it is a difficult region to study for other reasons. The substantial quantities of dust in the plane of the Galactic disk (and hence between us and the Galactic Center) extinguish the optical light from the Galactic Center by a factor of $10^{10}$, rendering it basically invisible. Progress must be made by utilizing wavelengths that can more readily penetrate the Galactic dust -- in particular, radio, infra-red, hard X-rays and $\gamma$-rays.

In recent years, dramatic progress has been made by high spatial resolution studies of the near infra-red emission from bright stars in the central-most regions of the Galaxy. By achieving diffraction-limited near infra-red (K-band) images of the Galactic Center (with resolutions of 60 milli-arcsecs) over the time span of several years, one can directly observe the motions of rapidly-moving stars within the central stellar cluster that inhabits the Galactic Center. One finds that the velocities of stars within the central regions of this cluster drops as the square-root of the distance from the Galactic Center, implying a gravitationally dominant mass of $2.6\times 10^6\hbox{$\rm\thinspace M_{\odot}$}$ confined to the central $10^{-6}{\rm\thinspace pc}^3$ [14,15]. A supermassive black hole is the only long-lived object of this mass and compactness -- a compact cluster of stellar-mass objects would undergo dynamical collapse on a time scale of $10^7{\rm\thinspace yr}$ or less [19]. More recently, these proper motion studies have also detected acceleration of several stars in the Galactic Center region[17,18]. The supermassive black hole hypothesis survives the powerful consistency check allowed by a vector analysis of these accelerations. Any remaining doubt seems to have been removed by recently reported observations that show a star passing within 17 light hours of the putative SMBH [16]. The star remains on a Keplerian orbit, even at the peri-center of the orbit where it achieves a velocity exceeding $5000\hbox{${\rm\thinspace km}{\rm\thinspace s}^{-1}\,$}$. This raises the implied mass-density to more than $10^{17}\hbox{$\rm\thinspace M_{\odot}$}\,{\rm pc}^{-3}$ -- if this were a cluster of compact stellar remnants, it would evaporate or collapse on a timescale of less than $10^5{\rm\thinspace yr}$.