Black holes, quasars,
and active galaxies:
How Hubble found black holes
at the heart of all large galaxies
Black holes are objects so dense, and with so much mass, that even light cannot escape their gravity. It is in the study of supermassive black holes that Hubble
has made its biggest contribution.
It is impossible to observe black holes directly, and astronomers had no way to
test their theories until Hubble started its work. The high resolution of Hubble made
it possible to see the effects of the gravitational attraction of some of these objects on their surroundings. Hubble has also proved that supermassive black holes
are most likely present at the centres of most, if not all, large galaxies. This has important implications for the theories of galaxy formation and evolution.
Black holes have effects on their surroundings. These include powerful jets of
electrons that travel many thousands of light years from the centres of the galaxies. Matter falling towards a 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
This is an illustration of the supermassive
black hole located in the middle of the very
dense galaxy M60-UCD1. Because no light
can escape from the black hole, it appears
simply in silhouette against the starry background. The black hole’s intense gravitational
field warps the light of the background stars
to form ring-like images just outside the dark
edges of the black hole’s event horizon.
Credit: NASA, ESA, D. Coe, G. Bacon (STScI)
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 large and even small galaxies. Hubble also
managed not only to observe the jets created by black holes but also the glowing discs of material surrounding a supermassive black hole.
Furthermore, 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 is an ongoing area of research in astronomy.
Black holes and the quasar connection
Before Hubble, quasars were considered to be isolated star-like objects of a
mysterious nature. Hubble has observed several quasars and found that they all
reside at galactic centres. Today most scientists believe that supermassive black
holes at the galactic centres are the “engines” that power the quasars. They
also believe that quasars, radio galaxies and the centres of so-called active
galaxies just are 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.