ESA Science & Technology05-Jul-2005 10:17:18

Beyond The Milky Way

Black Holes, Quasars, and Active Galaxies

In the 1950s and 1960s astronomers had found objects, such as quasars and radio sources, whose energy output was so immense that it could not be explained by traditional sources of energy such as that produced by normal stars. It was suggested that their vast energy output could best be explained if massive black holes were at the centres of these objects.

Prior to the launch of Hubble a handful of black-hole candidates had been studied, but the limitations of ground-based astronomy were such that irrefutable evidence for their existence could not be obtained. Black holes themselves, by definition, cannot be observed, since no light can escape from them. However, astronomers can study the effects of black holes on their surroundings. These include powerful jets of electrons that travel huge distances, many thousands of light years from the centres of the galaxies. Matter falling towards the black hole can also be seen emitting bright light and, if the speed of this falling matter can be measured, it is possible to determine the mass of the black hole itself. This is not an easy task and it requires the extraordinary capabilities of Hubble to carry out these sophisticated measurements.

Hubble observations have been fundamental in the study of the jets and discs of matter around a number of black holes. Accurate measurements of the masses have been possible for the first time. Hubble has found black holes 3 billion times as massive as our Sun at the centre of some galaxies.

While this might have been expected, Hubble has surprised everyone by providing strong evidence that black holes exist at the centres of all galaxies. Furthermore, as it appears that larger galaxies are the hosts of larger black holes, there must be some mechanism that links the formation of the galaxy to that of its black hole and vice versa. This has profound implications for theories of galaxy formation and evolution and will certainly be the subject of considerable additional research with Hubble during the next decade.


In the 1980s observations made with different ground-based telescopes showed that some quasars were surrounded by fuzzy light. It was suspected that the quasars reside in galaxies and that the fuzzy patches of light could be those host galaxies.

Hubble's high-resolution Faint Object Camera images showed with clarity that this is indeed the case. More importantly the hosts of quasars appear to be galaxies of all types, contrary to earlier predictions that favoured the idea that quasars were to be found only in elliptical galaxies. This is important since the light from quasars is believed to be produced by black holes at the centres of their host galaxies. Astronomers can now show that this is indeed the case and that quasar host galaxies are the same types of galaxies found in our neighbourhood.

This realisation also leads to the question of why most of the nearby galaxies, including our own Milky Way have 'dormant' black holes, namely black holes which are inactive at this time. This will be the subject of new studies with Hubble.

Unified model

Today most astronomers believe that quasars, radio galaxies and the centres of so-called active galaxies are just different views of more or less the same phenomenon: a black hole with energetic jets beaming out from two sides. When the beam is directed towards us we see the bright lighthouse of a quasar. When the orientation of the system is different we observe it as an active galaxy or a radio galaxy. This 'unified model' has gained considerable support through a number of Hubble observational programmes. The simplistic early ideas have, however, been replaced by a more complex view of this phenomenon - a view that will continue to evolve in the years to come.

Duccio Macchetto
ESA astronomer, Head of the Science Policies Division, STScI

"Hubble provided strong evidence that all galaxies contain black holes millions or billions of times heavier than our Sun. This has quite dramatically changed our view of galaxies. I am convinced that over the next 10 years Hubble will find that black holes play a much more important role in the formation and evolution of galaxies than we believe today. Who knows, it may even influence our picture of the whole structure of the Universe...?"

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