Quasars

Since their discovery, the nature of quasars has been one of the most
intriguing and baffling problems as evidenced by the following quotations: ”
the problem of understanding quasi-stellar objects is one of the most
important and fascinating tasks in all physics” – G.Burbidge and Hoyle.”The
quasar continues to rank both as one of the most baffling objects in the
universe and one most capable of inspiring heated argument” – Morrison.”The
redshift problem is one of the most critical problems in astronomy today” –
G. Burbidge. “Quasars still remain the profoundest mystery in the heavens” –
Hazard and Mitton.
The conventional interpretation of the spectral lines observed in quasars is
based on the redshift hypothesis. Three hypotheses have been advanced to
account for the supposed redshifts: 1. Cosmological hypothesis; the redshifts
are due to the expansion of the universe, 2. Gravitational hypothesis, 3
Local-Doppler hypothesis; in this hypothesis the redshifts are due to the
Doppler effect, but the quasars are relatively nearby and have nothing to do
with the expansion of the universe. Of these hypotheses, the first one is
the most publicized one.
One is led to attribute to quasars very many mysterious properties if one
assumes the redshift hypothesis to be correct. A patient analysis of the
data on quasars over the years has led to the conclusion that the real source
of the trouble is in the assumption that the spectra of quasars have
redshifts.

In the early 1960’s quasars were known as ‘radio stars’ because the method
used to discover the first quasars was based on coincidences between a strong
radio source and a point-like optical source. Since each radio source was
associated with a star it was originally thought that quasars were objects
within the galaxy hence the term ‘radio stars’. Quasars or quasi-stellar
radio source, from the method by which they where originally discovered: as
stellar optical counterparts to small regions of strong radio emission. With
increasing spatial resolution of radio telescopes the strong radio emission
often seemed to come from a pair of lobes surrounding many of these faint
star-like emission line objects.

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The initial method of selection was strong radio emission, and then later any
object with blue or ultraviolet excess was considered a good quasar
candidate. Very recent evidence from the near infrared portion of the
spectrum indicates that a large fraction of quasars may in fact be brighter
in the infrared than in other wavelength bands.

Answering these basic questions may summarize much of the information
regarding Quasar.

What is the definition of a quasar? When radio telescopes were first turned
on the heavens, point sources of radio waves were discovered (along with
spread-out regions of emission along our Milky Way). Astronomers using
ordinary visible-light telescopes turned toward these radio points and looked
to see what was there. In some cases a supernova remnant was found, in
others, a large star-birth region, in others a distant galaxy. But in some
places where point sources of radio waves were found, no visible source other
than a stellar-looking object was found (it looked like a point of a star).
These objects were called the quasi-stellar radio sources or quasars for
short.Later, it was found these sources could not be stars in our galaxy,
but must be very far away as far as any of the distant galaxies seen. We now
think these objects are the very bright centers of some distant galaxies,
where some sort of energetic action is occurring, most probably due to the
presence of a supermassive black hole at the center of that galaxy.
(Supermassive – made up from a mass of about a billion solar masses.)
What do quasars have to do with black holes? It is thought the infall of
matter into the Supermassive black hole can result in very hot regions where
huge energies are released, powering the quasar.

How big are quasars compared to galaxies? Well, the region of intense visible
emission is quite small compared to the rest of the galaxy that it is
imbedded in. The visible emission only occurs very near the center of the
galaxy. On the other hand, huge regions of radio emission, produced by the
quasar, can stretch out to large distances outside the galaxy.

Why do some quasars give off radio waves? The electrons near the center of
the quasar can be accelerated to speeds near the speed of light. In the
presence a magnetic field (which is present in these same regions), the
electrons move along helical paths (paths that look like a stretched out
slinky). As a result, they emit radio waves (it’s called synchrotron
radiation, since these waves are observed on Earth when physicists send high
energy electrons around in circles using magnetic fields, in particle
accelerators call synchrotrons).
How long do quasars last? It appears galaxies may only act, as quasars
during the early stages of their lives, but it would still be for times of
billions of years.
How long does it take for a quasar to form? Nobody really knows, since we
don’t exactly know how they form. However, it can’t take much longer than
something like a billion years.

Can anything develop from quasars after they die? Probably, the only thing
that would be left is the Suppressive black hole. In other words, the gas
near it would have been used up and so the quasar shuts off. But the
remaining stars in the galaxy, as a whole would, of course, still be there.

Why are quasars so interesting to study? They are only seen far away. Since
the light from quasars were emitted billions of years ago and we are just
seeing them today we realize that they are all very old. There are no nearby
quasars, so there are no young quasars; quasars are not made during our era
of the universe, only during an ancient era.
This also implies the universe was a different place in the past. It also
says the galaxies we see around us now may have been quasars in the distant
past; even our Milky Way galaxy may have been a quasar-like galaxy long ago,
now not much material falls into the large black hole at the Milky Way’s
center, so the radiation output from the center is not as great as it used to
be. How difficult are quasars to study? Not all that difficult, if you have
a huge telescope such as the Hubble Space Telescope.

The Hubble Space Telescope has shown that quasars live in a remarkable
variety of galaxies, many of which are violently colliding. This complicated
picture suggests there may be a variety of mechanisms some quite subtle for
turning on quasars, the universes most energetic objects. Hubble researchers
are also intrigued by the fact that the quasars studied do not appear to have
obviously damaged the galaxies in which they live. This could mean that
quasars are relatively short lived phenomena which many galaxies, including
the Milky Way, experienced long ago. John Bahcall of the Institute for
Advanced Study, Princeton, NJ, emphasizes that Hubble’s clarity opens a
complicated picture. The basic assumption was that there was only one kind of
host galaxy, or catastrophic event, which feeds a quasar. In reality we do
not have a simple picture, we have a mess. Mike Disney, University of Wales
College, Cardiff, U.K., who is a leader of the European team, says, “People
had suspected that collisions might be an important mechanism for feeding
black holes and generating the vast amounts of energy emitted by quasars.
Now we know they are and we didn’t know that before Hubble. Although a number
of images obtained through the use of the Hubble Telescope show collisions
between pairs of galaxies, which could trigger the birth of quasars, some
pictures reveal apparently normal, undisturbed galaxies possessing quasars.
The beauty and clarity of the Hubble images, as well as the diversity of
quasar environments amazed investigators.

Discovered only 33 years ago, quasars are among the most baffling objects in
the universe because of their small size and prodigious energy output.
Quasars are not much bigger than Earth’s solar system but pour out 100 to
1,000 times as much light as an entire galaxy containing a hundred billion
stars. A super massive black hole, gobbling up stars, gas and dust, is
theorized to be the engine powering a quasar. Most astronomers agree an
active black hole is the only credible possibility that explains how quasars
can be so compact, variable and powerful. Nevertheless, conclusive evidence
has been elusive because quasars are so bright they mask any details of the
environment where they live.

These problems could not be solved without the Hubble Telescope. Observations
by the European team, using the Wide Field Planetary Camera (WFPC2) in high
resolution mode, reveal that quasars appear to be born in environments where
two galaxies are interacting violently and probably colliding. This had long
been suspected as a mechanism for igniting a quasar, but no one knew whether
the idea was really right before the Hubble. In nearly every quasar we look
at we clearly see one galaxy apparently swallowing another . Disney selected
three quasars known to be strong infrared emitters, suggesting that they
might be in spiral galaxies, which typically contain an enormous amount of
gas and dust. When we image them with Hubble we see the most colossal
smashups, where two giant spiral galaxies like our own Milky Way have crashed
head on into one another and flung off pieces violently in all directions.
Some of those bits seem to have finished up in the nucleus of one of the
spirals where there is probably a giant black hole feeding on it. Both teams
agree that Hubble images do show conclusively:
1.That most quasars lie at the cores of luminous galaxies, both spiral and
elliptical.
Though underlying galaxies were suggested in ground-based quasar
observations, astronomers had to wait for Hubble’s capabilities to show the
host galaxies clearly enough for astronomers to begin to classify their
shapes.

2. Interactions between galaxies, either through direct collisions or near
encounters, can be important in turning on a quasar, by dumping fuel onto a
black hole. However some quasars look unperturbed, so there may be other,
more subtle mechanisms for feeding the black hole. Some of the galaxies we
observed don’t appear to know they have a quasar in their core.

3. Quasars that are radio quiet are often in elliptical galaxies, not always
in spiral galaxies, as previously believed.
Advanced instruments planned for Hubble should also help pin down more
details. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), to
be installed in 1997, and the Advanced Camera, to be installed in 1999, will
have coronagraphic devices which will block out the glare of a quasar,
allowing astronomers to see closer into a galaxy’s nucleus. By viewing
galactic structures in infrared light , the NICMOS should be able to provide
important new details about the host galaxies of quasars. The continued study
of quasars and the information that it will provide us with may help us to
develop a better understanding of space and how we fit in to this great
puzzle.


Category: Miscellaneous

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