Closing in on Supermassive Black Holes in ActiveGalaxies
Supermassive black holes have long been thought to exist in the centers of galaxies, providing the power behind active galactic nuclei (AGN) via the release of gravitational energy sustained by an accretion disk. Although direct evidence for supermassive black holes has remained elusive, the indirect evidence is strong. Recent observations indicate a significant amount of gas around the cores of galaxies. Not only do we witness "galactic cannibalism", where material is acquired as the result of a merger, but we see large, dusty disks that often obscure the galactic nucleus. Using the motions of this gas to probe the vicinity of supermassive black holes has been a significant achievement in astronomy.
The hard X-ray emission of AGN is thought to arise from flares in a hot corona associated with the accretion disk. These X-rays often illuminate the underlying disk, and the subsequent reflection imprints atomic features into the spectrum, most notably the fluorescent iron K line at 6.4 keV. Reflection can also arise from the large-scale gas disk or torus. X-ray observations of bright, nearby AGN have revealed the Fe K line to range in shape from narrow, indicating its origin far from the black hole, to broad and asymmetric, resulting from high velocities and strong gravity within 10 Schwarzschild radii of the black hole. Such observations provide convincing evidence for supermassive black holes, and are our first step toward understanding how matter behaves under conditions of extreme gravity.