Is there a black hole at the center of the Galaxy?

The center of our Galaxy, as seen in the radio.
Credit: Farhad Zadeh, VLA, NRAO, APOD

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). At radio wavelengths, the brightest feature of this region is the point-like radio source Sagittarius A* (pronounced "Sag A star"). This source is a compact object, and approximately one Astronomical Unit (1 AU is about 93 million miles) in size, which is much smaller than our solar system (Neptune is 2.8 billion miles from the Sun). At near-infrared wavelengths, this point source in the radio is not clearly seen. Astronomers have seen pulsation near the radio position of Sgr A* in the near-infrared, which they attribute to this radio source flaring.

Three-color Laser Guide Star Movie of the Galactic Center.

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 and Brown made the conenction between their radio detection of Sgr A* and other known active galactic nuclei

The mini-spiral in the GC. It is centered on Sgr A*.

In the past 20 years, astronomers have 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 Sgr A*. Using the velocities of the gas estimated from the Doppler shift of spectral lines, astronomers estimated that a mass of 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.

The twin Keck 10 meter telescopes. Credit: Swinburne University of Technology

Since 1995, high-resolution near-infrared studies have observed a compact cluster of early-type stars surrounding the radio position of Sgr A*. These stars have very large proper motions (they are moving across the sky very quickly) considering their 24 million light year distance from the Earth. The two main groups devoted to tracking these stars include Andrea Ghez and others at UCLA, who use the 10-m Keck telescope on Mauna Kea, Hawaii, and Reinhard Genzel and Andreas Eckart who use the 8-m VLT telescopes in 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 the large diameters of the Keck and VLT telescopes, air turbulence in the Earth's atmosphere blurs the images taken at the telescopes. The atmosphere has a lot of molecules that are colliding into each other and getting heated up. On really hot days, we can see the heat waves coming up off the ground, or if you look at a flame, you can see the heat influencing the air around it. This is what happens in our atmosphere. In order to correct for it, astronmers are now using Adaptive Optics (AO) systems, which increases the sensitivity of observations. AO systems use a deformable mirror that mimics the shape of the incoming lightwave and corrects for the atmospheric turbulence before the data is recorded.

In order to see the individual stars in the GC, adaptive optics is necssary.

The orbits of the stars in the Galactic Center from 1995 - 2011.

Very accurate stellar positions can be estimated in order to keep track of the motions of the stars in the compact central cluster, which are zipping around Sgr A* at speeds up to 3 million miles per hour! Using Kepler's laws of motion, the orbital velocities and the positions of the bright stars an be used to estimate the mass that must be contained within their orbits. The resulting enclosed mass is 4.6 ± 0.7 X 10^6 solar masses--4.6 million times the mass of our Sun! This large mass combined with the minute size of Sgr A* in radio emission suggests taht the stars must be swiftly circling around a supermassive black hole.

An active galactic nucleus (Cen A). There is a lot of activity at long wavelengths and short,
energetic wavelengths. This is completely different than our galactic nucleus, Sgr A*.
Sgr A* may have been more energetic in the past. Credit: NASA

Recent observations 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. In the case of Sgr A*, there is a mysterious absence of the high energy emission (X-rays and UV radiation) often observed from active galactic nuclei.