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Quasars
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In the 1960s it was observed that certain objects emitting radio waves but
thought to
be stars had very unusual optical
spectra. It was finally realized that the reason the
spectra were so unusual is that the lines were Doppler shifted by a very large
amount, corresponding to velocities away from us that were significant fractions of
the speed of light. The reason that it took some time to come to this conclusion
is that, because these objects were thought to be relatively nearby stars,
no one had any reason to
believe they should be receding from us at such velocities.
Quasars and QSOs
These objects were named Quasistellar Radio Sources
(meaning "star-like radio sources")
which was soon contracted to quasars. Later, it was found that many
similar objects did not emit radio waves. These were termed Quasistellar
Objects or QSOs. Now, all of these are often termed quasars
(Only about 1% of the quasars discovered to date have detectable radio emission).
Here are some Hubble Space Telescope
quasar images, and the following figure shows the quasar 3C273, which was the
first quasar discovered and is also the quasar with the greatest apparent
brightness. It will be
discussed further below.
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The quasar 3C273. Left image shows the quasar and the jet. Right image
superposes on this contours of radio frequency intensity. The sharp radial lines
from the quasar are optical spike artifacts because of its brightness
(Source).
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Quasars Are Related to Active Galaxies
The quasars were deemed to be strange new phenomena, and initially there was
considerable speculation that new laws of physics might have to be invented to
account for the amount of energy that they produced. However, subsequent
research has shown that the quasars are closely related to the active galaxies that
have been studied at closer distances.
We now believe quasars and active
galaxies to be related phenomena, and that their energy output can be explained
using the theory of general relativity. In that sense, the quasars are certainly
strange, but perhaps are not completely new phenomena.
Quasar Redshifts Imply Enormous Distance and Energy Output
The quasars
have very large
redshifts, indicating by the
Hubble law
that they are at great
distances. The fact that they are visible at such distances implies that they
emit enormous amounts of energy and are certainly not stars.
The following image
from the
Sloan Digital Sky Survey
shows the three most distant quasars known. The quasars are the faint red
smudges near the head of each arrow. Their redshift parameters are
4.75, 4.90, and 5.00 respectively, which places them at distances of about 15 billion light years
(Ref).
The Energy Source of Quasars is Extremely Compact
Quasars are extremely luminous at all wavelengths
and exhibit variability on timescales as little as hours, indicating that their
enormous energy output originates in a very
compact source.
Here are some light curves
at different wavelengths
illustrating the variability in intensity of some quasars and other active
galaxies. Here is an
explanation of these light curves. In all cases, the timescale for
variability of the light from an active galaxy sets an upper limit on the size of
the compact energy source that powers the active galaxy. These limits are
typically the size of the Solar System or smaller.
Some quasars emit radio frequency, but
most (99%) are radio quiet. Careful observation shows faint
jets coming from some quasars. The above images of the quasar 3C273 illustrate
both a jet in the optical image on the left and radio frequency emission associated
with the jet on the right.
Here are some
spectra of quasars and
other active galaxies - see the following
description.
Relationship of Quasars and Active Galaxies
The quasars are thought to be powered by supermassive
rotating black holes at their centers.
Because they are the
most luminous objects known in the universe,
they are
the objects that have been observed at the greatest distances
from us. The most distant are
so far away that the light we see coming from them was produced when
the Universe was only one tenth of its present age.
The present belief is that
quasars are actually closely related to active galaxies such as Seyfert Galaxies or BL Lac objects
in that they are very active galaxies with bright nuclei
powered by enormous rotating black holes.
However, because the quasars are at such large distances, it is difficult to see
anything other than the bright nucleus of the active galaxy in their case. As we
have noted above, modern observations have begun to detect around some
quasars jets and evidence for the surrounding faint nebulosity of a galaxy-like
object.
Evolution of Quasars
The standard theory is that quasars turn on when there is matter to feed their
supermassive black hole engines at the center and turn off when there is no longer
fuel for the black hole. Recent Hubble Space Telescope observations indicate
that quasars can occur in galaxies that are interacting with each other. This
suggests the possibility that quasars that have turned off because they have
consumed the fuel available in the original galaxy may turn back on if the galaxy
hosting the quasar interacts with another galaxy in such a way to make more matter
available to the black hole. Here is a recent
survey of quasar host galaxies that sheds light on
this issue.
Abundance of Quasars in the Early Universe
Looking at large distances in the Universe is equivalent to
looking back in time because of the
finite speed of light. Thus,
the observation of quasars at large distances and their scarcity nearby implies
that they were much more common in the early Universe than they are now, as
illustrated in the adjacent figure (see the
Source
for a further discussion of the figure).
This is
one piece of evidence that argues against the
steady state theory
of the Universe but would be consistent with the
big bang theory.
We shall discuss this further below.
Hungry Black Holes
Notice that the greater abundance of quasars
early in the Universe would be consistent with the mechanism discussed above
whereby a quasar shuts off when its black hole engine has consumed the fuel
available in the host galaxy. We would expect that generally in the early
Universe there may have been more mass easily accessible to the black hole than
later, after much of it had been consumed. Perhaps later quasars are more
dependent on interactions between galaxies to disturb mass distributions and
cause galaxies to begin to feed
the hungry black hole.
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