The Black Hole

As we zoom into the very core of the Galactic Center, our field of view shrinks to a mere 5 arcseconds (one thousandth of a degree) in size. At radio wavelengths, the most prominent feature of this tiny region is the point-like radio source called Sagittarius A* (pronounced "Sag A star"). Studies of the radio properties of Sgr A* reveal that it is a physically compact object approximately one Astronomical Unit (1 AU=1.5X10^8 km) in size, many times smaller than our solar system. However, as seen below, at near-infrared wavelengths, there is no clear sign of a single prominent source of emission like that seen in the radio. Therefore, the nature of the source of the radio emission remained a mystery for some time.

2.2 micron AO image of the central cluster of stars orbiting the supermassive black hole

In 1974, Sir Martin Rees proposed the idea that supermassive black holes could exist within the centers of active galactic nuclei or quasars. In that same year, Balick & Brown made the connection between their radio detection of Sgr A* and other known active galactic nuclei.

However, only in the past 20 years have we collected enough evidence through the observed motions of gas and stars to convince ourselves that something very massive lurks at the center of our galaxy. The first dynamical evidence came from the motions of the ionized gas streamers of the mini-spiral orbiting around Sgr A*. Using the velocities of the gas estimated from the Doppler shift of spectral lines, they were able to estimate that a mass of material six million solar masses must lie within 10 arcseconds of Sgr A*. This did not explicitly prove the existence of a black hole since that amount of matter could be accounted for by a high density of stars within such a large volume.

In the past eight years, recent high resolution near-infrared studies have observed a compact cluster of stars surrounding the radio position of Sgr A*. These stars have very large proper motions considering thier 8 kpc (24 million light years) distance from the Earth. The two main groups devoted to tracking these stars include Andrea Ghez and others at UCLA who have been using the 10-m Keck telescope on Mauna Kea, Hawaii and Genzel & Eckart who use the 3.5-m NTT telescope in La Silla, Chile. Both groups take advantage of the high spatial resolution and sensitivity of these large telescopes to track the positions of the stars within the cluster using near-infrared images collected once or twice a year.

Despite Keck's large diameter, air turbulence in the Earth's atmosphere blurs the image and therefore greatly reduces its detail and ability to distinguish between the stars in the tightly packed cluster. There are two ways to get around this problem: speckle interferometry and adaptive optics. Speckle interferometry involves collecting stacks of images with very short exposure times which effectively freezes the atmosphere resulting in a pattern of diffraction limited speckles. Then, in post processing, the high resolution information is recovered by shifting and adding onto the brightest speckle within the speckle pattern to produce a PSF with a diffraction limited core and seeing halo. Adaptive Optics (AO) uses a deformable mirror which mimics the shape of the incoming lightwave and corrects for the atmospheric turbulence before the data is recorded.

In both cases, very accurate stellar positions can be estimated in order to kept track of the motions of the stars in the compact central cluster which are zipping around Sgr A* at speeds up to 1400 km/s (3,000,000 mph)! Using Kepler's laws of motion, we use the orbital velocities and positions of the bright stars to estimate the mass that must be contained within their orbits. The resulting enclosed mass of 2.6X10^6 times the mass of the sun, combined with the minute size of Sgr A* constraint provided by the radio emission, suggests that the stars must be swiftly circling around a supermassive black hole. In fact, the large number of observations of the stars orbiting the black hole has allowed us to provide the first even detection of the accelerations of the stars in the central cluster.

Now that we believe there is a supermassive black hole at the center of our galaxy, we can attempt to understand its other mysterious properties including a lack of high energy emission like that seen in other active galactic nuclei. The emission properties of Sgr A* have long been a subject for discussion among many theorists who attempt to use a variety of different physical phenomena to model its spectrum from the X-ray to the radio.

Recent observations of a sample of nearby galaxies reveal that such supermassive black holes are not unique to the Milky Way. The formation of such a large black hole and how it affects the evolution of its host galaxy are not well understood, nor is the connection between the black hole in the Milky Way and those believed to exist in the cores of AGN's, which emit a huge amount of radiation from their nuclei at many different wavelengths. However, in the case of Sgr A*, there is a mysterious absence of the high energy emission (X-ray and ultra-violet radiation for example) often observed from AGN's. To further investigate how the brightness of Sgr A* changes with wavelength, Ghez and Morris are currently trying to measure the luminosity of Sgr A* in the near and mid-infrared. Their result will have implications for the type of physical processes occurring in and around the black hole.