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3.4 Our Galaxy: MBH appeq (2.9 ± 0.4) x 106 Msun

Our Galaxy has long been known to contain the exceedingly compact radio source Sgr A*. Interferometry gives its diameter as 63 rs by less than 17 rs, where rs = 0.06 AU = 8.6 x 1011 cm is the Schwarzschild radius of a 2.9 x 106 Msun BH. It is easy to be impressed by the small size. But as an AGN, Sgr A* is feeble: its radio luminosity is only 1034 erg s-1 appeq 100.4 Lsun. The infrared and high-energy luminosities are higher, but there is no compelling need for a BH on energetic grounds. To find out whether the Galaxy contains a BH, we need dynamical evidence.

Getting it has not been easy. Our Galactic disk, which we see in the sky as the Milky Way, contains enough dust to block all but ~ 10-14 of the optical light from the Galactic center. Measurements of the region around Sgr A* had to await the development of infrared detectors. Much of the infrared radiation is in turn absorbed by the Earth's atmosphere, but there is a useful transmission window at 2.2 µm wavelength. Here the extinction toward the Galactic center is a factor of ~ 20. This is large but manageable. Early infrared measurements showed a rotation velocity of V appeq 100 km s-1 and a small rise in velocity dispersion to ~ 120 km s-1 at the center. These were best fit with a BH of mass MBH ~ 106 Msun, but the evidence was not very strong. Since then, a series of spectacular technical advances have made it possible to probe closer and closer to the center. As a result, the strongest case for a BH in any galaxy is now our own.

Figure 4

Figure 4. Images of the star cluster surrounding Sgr A* (green cross) at the epochs indicated. The arrows in the left frame show approximately where the stars have moved in the right frame. Star S1 has a total proper motion of ~ 1600 km s-1. [This figure is updated from Eckart & Genzel 1997, M.N.R.A.S., 284, 576 and was kindly provided by A. Eckart.]

Most remarkably, two independent groups led by Reinhard Genzel and Andrea Ghez have used speckle imaging to measure proper motions - the velocity components perpendicular to the line of sight - in a cluster of stars at radii r ltapprox 0".5 appeq 0.02 pc from Sgr A* (Figure 4). When combined with complementary measurements at larger radii, the result is that the one-dimensional velocity dispersion increases smoothly to 420 ± 60 km s-1 at r appeq 0.01 pc. Stars at this radius revolve around the Galactic center in a human lifetime! The mass M(r) inside radius r is shown in Figure 5. Outside a few pc, the mass distribution is dominated by stars, but as r -> 0, M(r) flattens to a constant, MBH = (2.9 ± 0.4) x 106 Msun. Velocity anisotropy is not an uncertainty; it is measured directly and found to be small. The largest dark cluster that is consistent with these data would have a central density of 4 x 1012 Msun pc-3. This is inconsistent with astrophysical constraints (Section 5). Therefore, if the dark object is not a BH, the alternative would have to be comparably exotic. It is prudent to note that rigorous proof of a BH requires that we spatially resolve relativistic velocities near the Schwarzschild radius. This is not yet feasible. But the case for a BH in our own Galaxy is now very compelling.

Figure 5

Figure 5. Mass distribution implied by proper motion and radial velocity measurements (blue points and curve). Long dashes (green) show the mass distribution of stars if the infrared mass-to-light ratio is 2. The red curve represents the stars plus a point mass MBH = 2.9 x 106 Msun. Short green dashes provide an estimate of how non-pointlike the dark mass could be: its chi2 value is 1 sigma worse than the solid curve. This dark cluster has a core radius of 0.0042 pc and a central density of 4 x 1012 Msun pc-3. [This figure is updapted from Genzel et al. 1997, M.N.R.A.S., 291, 219 and was kindly provided by R. Genzel.]

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