January, 1987

John Kormendy of the Dominion Astrophysical Observatory, National Research Council of Canada (NRC) has found evidence that the Andromeda Galaxy contains a central black hole ten million times as massive as the Sun.

Like our Milky Way, the Andromeda Galaxy or M31 is a swirling mass of several billion stars. At a distance of 2 million light-years, it is one of our nearest neighbour galaxies.

Astronomers suspect that supermassive black holes might exist because of explosive activity seen at the centres of galaxies. Sometimes jets of energetic particles are seen shooting millions of light-years out into space. Other galaxies contain quasars, which look like stars but are brighter than galaxies made up of billions of stars. Such explosions require an extraordinarily powerful "engine". Astronomers believe that supermassive black holes (affectionately dubbed "monsters") are feeding on unwary stars and gas that pass too close to the centre. But clearcut evidence for monsters had not been found.

Kormendy's discovery was made possible by the excellent "seeing" conditions at NRC's Canada-France-Hawaii Telescope on Mauna Kea, Hawaii. This is located at 4200 m altitude where there is so little air that "seeing" (atmospheric turbulence) blurs the observations less than elsewhere. Measurements can then be made very close to the black hole; only there is its influence detectable.

The black-hole search is carried out by looking for large amounts of mass that cannot easily be explained by stars. Measurements are made of how rapidly a galaxy rotates. If there is a large mass at the centre, then stars have to move very quickly when they come close to it. Kormendy explains that "the velocities are so large near the centre of M31 that there must be three or four times as much mass hiding there as we see in stars. Ten million solar masses of matter are crowded into a volume less than 5 light-years in diameter. This diameter is about the same as the distance from the Sun to the next nearest star. A black hole is the most likely explanation."

Supermassive black holes in galaxy centres are a different breed from black holes discovered a few years ago by other DAO astronomers. These stellar black holes are the remains of single stars that exhausted their nuclear power sources and burnt out. Typically they are about 10 times as massive as the Sun. Supermassive black holes are remnants of the evolution of clusters of stars. They form with masses of thousands or millions of Suns.

Then they continue to grow whenever anything passes too close. This suggests how the engines powering activity in galaxy nuclei may work. "We think that gas continually falls into the black hole. Eventually it forms a superheated disk that rotates around the hole at almost the speed of light, radiating ferociously and burping out the jets that we see. The activity in galactic nuclei is really quite remarkable. Volumes as small as the Solar System can emit a trillion times as much energy as the Sun. It is no wonder that we were tempted to invent monsters. The question has been whether nature did likewise, or whether (as it so often does) nature knew a better way. Now evidence is accumulating that supermassive black holes really exist."

But despite its large central mass, the Andromeda Galaxy has one of the least active nuclei known. Ironically, the best case discovered so far may be a monster that is starving.

Released by the National Research Council of Canada, Herzberg Institute of Astrophysics