Measuring the near-infrared radiation from plasma in the vicinity of the black hole is essential for constraining and refining the existing theoretical models on how matter is accreted by the black hole in the center of the Milky Way and why the radiation processes in the accreted gases are so surprisingly inefficient.

Near-infrared observations of the dynamics of stars in the central light year of our home galaxy, the Milky Way, have shown beyond doubt in the last decade—and above all in the past two years—that there is a black hole of about 3.5 million solar masses located at the very center of the Milky Way. Black holes accrete (capture and "swallow") interstellar gas from their surroundings. Before the gas vanishes beyond the event horizon of the black hole, it is converted into extremely hot plasma that radiates energy from radio to X-ray wavelengths. This process is usually very efficient and therefore black holes are—contrary to their names—among the brightest objects in the universe. The central black hole of the Milky Way, Sagittarius A* (Sgr A*), however, has been found to be surprisingly faint across the electromagnetic spectrum. This has puzzled astronomers for decades. By now it has become clear that there are two main reasons for the faintness of Sgr A*:

1. It only accretes a minute amount of the actually available interstellar gas in its surroundings.
2. At such low accretion rates, the process of converting heat into electromagnetic radiation becomes highly inefficient.

Models that intend to theoretically describe these processes near Sgr A* are usually complex and have to rely on the observed quantities. Before 2003, radiation from Sgr A* had only been known at radio and X-ray wavelengths, i.e., at two extremes of the electromagnetic spectrum. With our discovery of near-infrared radiation from plasma near the black hole—i.e., between radio and X-ray wavelengths—it will become possible to better understand what is going on near the black hole. A second impact of our discovery is the short time scales on which the radiation from the black hole varies. It shows that the light must be emitted in a very small region, close to the event horizon of the black hole. Periodicities found in the variations may also help in measuring the angular momentum of the black hole.

Reinhard Genzel has been involved in Galactic Center research for decades. Rainer Schödel has become involved in this research as a Ph.D. student of Reinhard Genzel.