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.
Quasars
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...?"
Gravitational Lensing
Light does not always travel in straight lines. Einstein predicted in his Theory of General Relativity that massive objects will deform the fabric of space itself. When light passes one of these objects, such as a cluster of galaxies, its path is changed slightly. This effect, called gravitational lensing, is only visible in rare cases and only the best telescopes can observe the related phenomena.
Hubble's sensitivity and high resolution allow it to see faint and distant gravitational lenses that cannot be detected with ground-based telescopes whose images are blurred by the Earth's atmosphere. The gravitational lensing results in multiple images of the original galaxy each with a characteristically distorted banana-like shape.
Hubble was the first telescope to resolve details within these multiple banana-shaped arcs. Its sharp vision can reveal the shape and internal structure of the lensed background galaxies directly and in this way one can easily match the different arcs coming from the same background galaxy by eye.
Since the amount of lensing depends on the total mass of the cluster, gravitational lensing can be used to 'weigh' clusters. This has considerably improved our understanding of the distribution of the 'hidden' dark matter in galaxy clusters and hence in the Universe as a whole.
Richard Ellis
Astronomer, University of Cambridge and California Institute of Technology
"When we first observed the galaxy cluster Abell 2218 with Hubble in 1995 we mainly aimed at studying the cluster and its galaxies. But we got a surprise. The images showed dozens and dozens of gravitationally lensed arcs. When we showed these ultrasharp images to our colleagues they could immediately see the importance of using gravitational lensing as a cosmological tool."
The Deep Fields
One of the main scientific justifications for building Hubble was to measure the size and age of the Universe and test theories about its origin. Images of faint galaxies give 'fossil' clues as to how the Universe looked in the remote past and how it may have evolved with time. The Deep Fields gave astronomers the first really clear look back to the time when galaxies were forming.
The idea for the Hubble Deep Fields originated in results from the first deep images taken after the repair in 1993. These images showed many galaxies, which were often quite unlike those we see in the local Universe and could not otherwise be studied using conventional ground-based telescopes.
The first Deep Field, the Hubble Deep Field North (HDF-N), was observed over 10 consecutive days during Christmas 1995. The resulting image consisted of 342 separate exposures, with a total exposure time of more than 100 hours, compared with typical Hubble exposures of a few hours. The observed region of sky in Ursa Major was carefully selected to be as empty as possible so that Hubble would look far beyond the stars of our own Milky Way and out past nearby galaxies.
The results were astonishing! Almost 3000 galaxies were seen in the image. Scientists analysed the image statistically and found that the HDF had seen back to the very young Universe where the bulk of the galaxies had not, as yet, had time to form stars. Or, as the popular press dramatically reported, 'Hubble sees back to Big Bang'...
These very remote galaxies also seemed to be smaller and more irregular than those nearby. This was taken as a clear indication that galaxies form by gravitational coalescence of smaller parts.
In 1996 it was decided to observe a second Deep Field, the Hubble Deep Field South (HDF-S), to assess whether the HDF-N was indeed a special area and thus not representative of the Universe as a whole. This time the field also contained a quasar, which was used as a cosmological lighthouse and provided valuable information about the matter between the quasar and the Earth.
After the Hubble observations of HDF-N and -S, other ground- and space-based instruments targeted the same patches of sky for long periods. Some of the most interesting results seem to emerge from these fruitful synergies between instruments of different sizes, in different environments and with sensitivity to different wavelengths.
Stefano Cristiani
Space Telescope-European Coordinating Facility (ST-ECF)
"In my view the Hubble Deep Fields are some of the images that have made the greatest impact on observational cosmology so far. These impressive dips into the depths of space and time have allowed astronomers to glimpse the first steps of galaxy formation more than 10 billion years ago and are without doubt some of the great legacies of the Hubble Space Telescope."
