The Galactic Center
More than a quarter century ago, it was suggested that galaxies such as our own Milky Way may harbor massive, though possibly dormant, central black holes. Definitive proof for or against the existence of a massive central black hole lies in the assessment of the distribution of matter in the central few parsecs of the Galaxy. Assuming that gravity is the dominant force, the motion of the stars and gas in the vicinity of the putative black hole offers a robust method for accomplishing this task, with the objects located closest to the Galactic Center providing the strongest constraints on the black hole hypothesis.
Images of the core of our Galaxy such as those here of the inner 6" ´ 6" (upper) and 1" ´ 1" (lower) are now available with unprecedented resolution from the W. M. Keck 10-meter telescope. Tracking the motion of some ~100 stars in these and similar images implies a central mass of 2.6±0.2 ´ 106 M¤ interior to a radius of ~0.01 pc (1 pc = 3.3 light years). The inferred central mass density is greater than for any stellar cluster and leads to the conclusion that our Galaxy harbors a massive central black hole. If galaxies as inactive as our own support a central black hole, then perhaps such objects can be found at the centers of all galaxies.