A massive black hole

There has been considerable speculation on the nature of the central object. It has been suggested that it might be a few million M0 black hole. In establishing the existence of such a black hole, there are two observational steps that must be made. First, we must accurately determine the mass of the object. The best way to do this is by its gravitational influence on nearby surrounding stars or gas. Second, we must show that the object is smaller than the Schwarzschild radius for that mass.

You can get an idea of how difficult this is. By equation (8.10), the Schwarzschild radius for a 106 M0 black hole is 3 X 106 km, which is about 10-7 pc, and would subtend an angle of about 10~6 arc sec.

One interesting approach to this problem was started in 1995 by Andrea Ghez (Fig. 16.18) of UCLA, and co-workers, using the Keck 10 m telescope (described in Chapter 4). Their goal was to measure the proper motions of stars in the direction of SgrA*, the source in the center of the galaxy. Ghez's group observed in the near infrared (2.2 ^m), which provided good angular resolution, but still had less extinction to the galactic center than they would have had in the visible. To obtain the best possible resolution, they took a number of short exposures, so there was little atmospheric smudging in each image. In adding the images together, they shifted the image to remove changes in atmospheric refraction from image to image. The final images had a resolution of about 0.5 arc sec. If the diffraction pattern is very clean (and it is for their system) then it is possible to measure the position of each star to much greater accu racy, in their case to within 0.002 arc sec. When we have previously talked about using velocities to measure the mass of a system, in using the vir-ial theorem for example, we used radial velocities as measured from Doppler shifts. We could have used proper motions, but they are generally too small to measure over a period of a few years. Velocities near a massive black hole would be large

Fig 16.18.

Andrea Ghez. [Andrea Ghez, UCLA]

Fig 16.18.

Andrea Ghez. [Andrea Ghez, UCLA]

enough to produce measurable proper motions. Measuring Doppler shift for these stars would be hard, because they required such short exposures to achieve good angular resolution. Their results are summarized in Fig. 16.19.

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