Science
Nugget—Unveiling a Supermassive Black Hole at the Center
of Our Galaxy
Astronomers are presently closing in on proof that one of the most enigmatic objects in our Universe - a supermassive black hole - lurks at the heart of our own galaxy, the Milky Way. Over the last decade, near-infrared diffraction-limited imaging with 10 meter class telescopes has revealed that several of the stars near the Galactic center (GC) are moving on elliptical trajectories with velocities as large as 12,000 km/sec, a few percent the speed of light (see Figure 1). These motions can only be explained if the stars are orbiting a central dark mass 4 x 106 times more massive than the Sun, confined within a volume only 45 AU on a side. These measurements provide the most definitive evidence for the existence of a supermassive black hole (BH), not only at the center of our galaxy, but more broadly of any galaxy in our Universe (Ghez et al. 2003, 2005; Schodel et al. 2002, 2003). Because the center of the Milky Way is 100 times closer than the next closest galaxy, the GC presents a unique opportunity to study a supermassive BH and its environs in much more detail than is possible in any other galaxy.
With the improved angular resolution of the Thirty Meter Telescope (TMT), astronomers will be able to track the orbital motions of many (~ 100) more stars at the GC (see Figure 2). Furthermore, the orbital motion of each of these stars will be measured with astrometric precisions 10 times finer than currently possible. As a result, the shape of the gravitational potential through which the stars move will be mapped out in exquisite detail (Figure 3). For an astrometric precision of ~ 100 µas, a conservative estimate of the capabilities of the TMT, General Relativistic effects, which show up as deviations from pure Keplerian (elliptical) motion, can be measured even for single orbits of known stars. At a precision of ~ 10 µas, the TMT can measure how quickly the BH is spinning by looking for the "dragging" of stars that pass through the swirling space-time near the BH. Such a measurement has far-reaching implications, from constraining the formation process of the BH to testing alternate theories of gravity. In addition to detecting such General Relativistic effects, with an astrometric precision of ~ 100 µas the TMT can detect slight perturbations to the orbital motions induced by the extended matter distribution surrounding the BH. This extended matter may be dominated by a cusp of exotic dark matter remaining from the Galaxy's formation process. Scattering of stars by encounters with solar mass BHs, which are expected to inhabit the GC in great number, would also be detectable. This could help constrain the currently uncertain mass distribution of solar mass BHs.
The wealth of information gained from a decade of GC imaging at high angular resolution has also yielded numerous puzzles related to the stars themselves and the accretion physics of the central BH. The unparalleled imaging capabilities of the TMT can directly address many of the most outstanding questions. In particular: (1) how did the monitored stars, whose spectral features suggest that they are young (< 10 Myr), come to reside in a region so close to the supermassive BH and thus so inhospitable to star formation? (2) Why is the emission from the accreting central BH, as measured by multiwavelength imaging of SgrA*, so dim compared to that of massive BHs at the center of other galaxies, and what is the origin of the large flaring behavior, seen most readily in the infrared? These questions relate to the more global issues of galaxy formation and massive BH growth and their complicated interplay with star formation in dense galactic nuclei. |
