Possibly the most remarkable finding has been that exceptionally energetic phenomena occur in the centers of a few percent of all galaxies. This is revealed by unusually strong emission across the electromagnetic spectrum of light, including radio waves, X-rays and gamma-rays. These "active galaxies" are believed to be powered by the appetite of massive black holes in their centers, which gobble up matter through their relentless gravitational pull and transfer mass to energy in the process. Has the existence of these black holes been proven? What are their masses? where did they come from? How common are black holes in galaxies? What are the details of the mechanisms that generate the observed activity? What is the relation between active and normal galaxies? With an ever increasing range of powerful telescopes, astronomers have recently started to unveil the answers to many of these intriguing questions.
The cataloging of nearby galaxies, or nebulae, as they were then known, started in the 18th century with the work of Messier. Until the 1920s it remained unanswered whether these were gaseous nebulae within our own galaxy, or separate "island universes" (i.e., galaxies like our own). Even at that time several cases had already been observed of what later came to be known as galaxy activity, including bright gaseous emission from the centers of some spiral galaxies and a luminous jet in galaxy number 87 of Messier's catalog (M87 in the constellation Virgo). Not surprisingly, these observations were viewed as mere curiosities in a time when the nature of galaxies themselves remained to be established.
As galaxies gradually came to be understood as collections of stars like our Milky Way, the peculiar properties of some galaxies gained interest. In the 1940s and 1950s research into active galaxies took off seriously. This started with Seyfert's spectroscopic work on the centers of spiral galaxies, and culminated with the beginning of radio astronomy. Pioneering work by Jansky and Reber had demonstrated that some astronomical objects emit radio waves. After improvements in radio observing techniques brought on by the Second World War, it became evident that some of the strongest astronomical radio sources were galaxies. These galaxies appeared normal on photographs made with visible light, and looked no different than galaxies without radio emission. The radio emission of the active galaxies was puzzling because it could not be attributed to a collection of stars and gas under normal conditions.
The mystery deepened with the discovery of quasars in the 1960s. These radio sources correspond to point-like objects on visible light photographs. Their light is unusually shifted towards redder wavelengths, which indicates that they are very distant. To be visible to us so far away, they must be very luminous. It was eventually realized that quasars are active galaxies that appear point-like because the active nucleus outshines all the stars in the galaxy. Only recently has it become possible to detect the underlying starlight in quasars. After their initial discovery it was found that quasars do not necessarily have to be strong radio sources; we know now that they can be either radio-loud or radio-quiet. Modern theories try to explain this apparent difference as a result of the three-dimensional orientation of the galaxy with respect to our line of sight.
X-rays, a highly energetic form of light, cannot penetrate our atmosphere (fortunately for life on Earth). The systematic exploration of X-ray emission from astronomical objects therefore had to await the development of satellites. X-ray astronomy came to maturity in the 1970s and 1980s. Active galaxies turned out to be among the brightest sources of X-ray emission in the sky. In fact, strong X-ray emission is probably the single most unifying characteristic of all active galaxies. Satellites have shown that some active galaxies are also strong emitters of gamma-rays, an even more highly energetic form of light.
In active galaxies, a region near the center produces enormous amounts of emission across the entire electromagnetic spectrum. The emission is often variable, from which it can be deduced that the emitting region must be very small. Nuclear fusion, the energy source known to power stars (and hydrogen bombs), is not efficient enough to explain the total energy output from active galaxies, and astronomers have been able to provide only one plausible alternative: accretion of matter onto a super-massive black hole. A black hole is an object that is so massive, yet so small, that a velocity greater than the speed of light is needed to escape from its gravitational pull. The theory of general relativity developed by Einstein determines that this is impossible. Hence, neither light nor matter can escape from a black hole. Matter near a black hole will fall into it, and will heat up while doing so. The energy that it emits as a result is responsible for the observed activity. To explain the observed energy output, the black holes in active galaxies must have masses of a million to a billion times that of the Sun.
Already at the end of the 18th century, Michell and Laplace had hypothesized the existence of objects from which light could not escape. Schwarzschild found the corresponding solution to Einstein's equations of general relativity, just before he died during the First World War. Nonetheless, black holes were long viewed more as a mathematical curiosity rather than a physical reality. This changed in the 1960s when they became the favored explanation for active galaxies. The term "black hole" was coined by Wheeler in 1968, and has since made a lasting impression. Nonetheless, for many years it remained to be proven that active galaxies do indeed contain black holes.
The most direct way to detect the presence of a black hole in a galaxy is through its gravitational pull. Matter far from the black hole will move primarily under the gravitational influence of the stars in the galaxy, which make up most of the mass. However, close to the center of the galaxy, the gravity from the black hole dominates. Stars and gas in that region move much faster than they would if there were no black hole. Black holes can therefore be detected by observing rapidly moving stars or gas near the center of a galaxy, provided that the region where the black hole gravity dominates can be resolved by the observations. The spatial resolution for optical observations from Earth is limited by turbulence in our atmosphere. The Hubble Space Telescope circumvents this limitation and delivers observations that are up to ten times sharper than from the ground. It has therefore provided a breakthrough in the detection of black holes in galaxy centers. There are now approximately 20 galaxies for which the presence of a black hole has been established. The mass of each black hole is roughly proportional to the mass of the galaxy itself, and makes up approximately half a percent of the total galaxy mass.
The activity in a galaxy ceases when the black hole runs out of "fuel", or when this fuel stops being efficiently transformed into radiation. Hence, a normal galaxy may have had an active phase in the past, and if so would still have a massive black hole lurking in its center. To test this idea, astronomers have used the Hubble telescope not only to study the centers of active galaxies but also of normal galaxies. Indeed, black holes are found in normal galaxies as well and may even exist in the centers of all galaxies. In our own Milky Way galaxy, ground-based observations detected rapidly moving stars. These are believed to move under the influence of the gravitational pull of a black hole with 3 million times the mass of our Sun. Black holes have also been detected in Messier 31, the neighboring giant spiral galaxy in Andromeda (just visible to the naked eye), as well as in its smaller companion Messier 32. Neither of these galaxies is active.
The light from very distant quasars has taken billions of years to reach us and provides information from a time when the universe was young. Observations show that quasars were much more numerous in the past than they are now. The number of quasars peaked when the universe was about ten per cent of its current age, about one billion years after the Big Bang. The number of quasars in the early universe and their energy output provide information on the total mass that must have accumulated into black holes through quasar activity. The results are consistent with the black hole masses that are being found with the Hubble telescope in nearby galaxies. Hence, it is clear what happened to the population of luminous quasars that lit up the universe when it was young -- these quasars have turned into the normal galaxies that we see around us today.
Although we are beginning to understand the evolutionary
relation between normal and active galaxies, many questions remain
unanswered. How do the black holes in galaxies form? Why do bigger galaxies
grow bigger black holes? Do the black holes form before or after the
galaxies? In 2005, NASA plans to launch its Next Generation Space
Telescope, which will focus on the formation and evolution of galaxies and
will provide more insight into these issues. NASA's new X-ray satellite
AXAF/Chandra is expected to provide improved information on the energy
generation processes in active galaxies. X-ray observations of emission
from iron atoms close to black holes may determine for the first time
whether the black holes are rotating or not. The spin of a black hole
plays a role in the production of the jets that are often seen in radio
observations. Also, galaxies sometimes collide and merge, and if they
contain black holes, these will merge as well. This causes massive ripples,
called "gravitational waves", in the space-time fabric of the universe.
These may be observed for the first time with the LIGO observatory, which
is currently under construction, or the LISA satellites, which the European
Space Agency will launch more than a decade from now. Either way, there is
little doubt that the future will have many more exciting discoveries in
store on the subjects of black holes and active galaxies!