5. Quasars: the real heavyweights!
The figure shows two Hubble Space Telescope images of
galaxies. On the right is NGC 3277 and on the left is NGC 5548.
There is something very bright in the centre (nucleus) of NGC 5548 but
not in NGC 3277. Most galaxies do not have particularly bright nuclei
(no brighter than expected from the stars there) so the
supermassive black holes in their nuclei seem to be dormant. NGC 5548 on
the other hand has an "active galactic nucleus" (AGN).
It's central regions are
much brighter than can be explained just by stars. These AGN
occur in something like 1 per cent of all galaxies.
5.1 Quasars
There are thousands of AGN known, and there are myriad different types
that astronomers refer to. Quasars are the most luminous of
all the AGN. They can outshine galaxies of 100 million stars, making
them the most luminous continuously emitting objects in the Universe!
This extraordinary luminosity means that quasars can been seen at
enormous distances (billions of light years).
The image to the left shows the Very Large
Array of 27 radio telescopes on the Plains of San
Agustin fifty miles west of Socorro (New Mexico, USA).
Radio telescopes like the VLA reveal striking "jets" and
"plumes" of radio emission shooting out either side of many
quasars. Some examples of
radio jets are shown below.
These jets can extend over more than 10 million light years, often in
almost straight lines. Obviously there's something very powerful in
the centres of quasars that remains stable for at least 10
million years. The best candidate for this power source is (...you
guessed it!...) a supermassive black hole. A spinning black hole
behaves like a massive gyroscope. If the supermassive black hole was
spinning it could stay spinning around the same axis for millions
of years.
The three images below are all of one quasar called M87. The far left image
shows a radio map, taken with the VLA, revealing the beautiful radio jets and plumes. The
centre image shows an optical Hubble Space Telescope image of
M87, the jet is clear along with the bright nucleus. To the far left
is another Hubble Space Telescope result. This shows the velocity of
gas around the nucleus of M87 (compare with the image of M84 in the
previous section). These all
point towards there being a truly
enormous black hole in M87, with a mass of a billion Suns! A black
hole this massive would be 3 billion km across - it is about the size
of the orbit of Saturn around the Sun. This supermassive black hole, as
massive a billion Suns, is smaller than our solar system.
5.2 The "central engine" of quasars and
AGN
To the right is an artists impression of a black hole in a
quasars. The black hole is in the centre of the accretion disc.
You can watch a movie of flying around the central engine of an AGN
here: http://ast.leeds.ac.uk/staff/rjrw/agnpov/agnpov.mpg
(13 Mb)
The majority of AGN do not possess huge radio jets, but all AGN known
produce X-rays. And, as mentioned in chapter 2, the X-ray emission
comes from the immediate environment of the black hole. The optical or
radio emission comes from much farther out. So, if we want to
investigate the black hole region we need to turn to X-ray observations.
The X-ray iron line.
The X-ray spectrum can reveal the strong gravity of the black
hole. X-rays are produced in the innermost regions of the central
engine, close to the black hole,
in conditions of temperature and gravity that cannot be reproduced in
a laboratory on Earth. Near the black hole the combination of a strong magnetic
field and a dense accretion disc creates a soup of fast moving
particles that collide with photons of light to generate X-rays (this
is called inverse-Compton scattering). As the X-rays leave the central
regions they interact with matter in the nucleus (like the accretion
disc itself) and are redshifted as they escape black hole's
gravity. These effects distort the emerging X-ray spectrum
in ways that depend on the conditions deep inside the central engine. Such X-ray
signatures have provided further evidence
for the existence of black hole/accretion discs in AGN.
The iron line shown to the left is one of the best diagnostics of the
inner regions of AGN because it is the strongest emission line
seen in X-rays.
This plot was made from a long observation of an AGN called
MCG-6-30-15 with the XMM-Newton observatory.
It shows a part of the spectrum where iron atoms emit very strongly at
a very specific energy. In the laboratory this iron emission
would be a very narrow "spike" in the spectrum, usually called and
emission line, at an energy of 6.40 keV.
But as seen in the plot the emission is stretched out to
lower energies (towards the left of the plot). This is because
of the effect of the central black hole.
The iron line in AGN is thought to originate from the direct vicinity of the black
hole. The exact way the line is distorted - i.e. bent away from its
normal straight "spike" profile - depends on the conditions near the
black hole.
Because iron atoms emit X-rays at a fixed energy, this corresponds to
a fixed frequency of radiation. The iron atoms behave like
a little clocks. It's the frequency of this clock that we
measure. When the iron is very close to the black hole the distortion
in spacetime is so strong it actually slows time down, and to us (at a safe distance) the iron
appears to be ticking slower, at a lower frequency. It therefore emits
radiation at a lower frequency (lower energy X-rays).
Effects of black hole spin. (Image courtesy of NASA)
The detailed shape of the iron line depends on the spin rate of
the black hole. The swirl of spacetime around a rotating black hole
allows the accretion disc to extend closer, where the gravitational
effects are stronger. Emission from around a rapidly rotating black
hole therefore produces a more distorted line profile. Using the
latest X-ray telescopes, such as NASA's Chandra and ESA's XMM-Newton
we are trying able to measure the spin-rate of these black holes for
the first time.
X-ray flickering.
One of the most interesting facts about the X-ray emission from AGN
and quasars is that it is variable. They "flicker" in X-ray
brightness. The plot to the right shows light curves of two
AGN. These simply show how bright the AGN was in X-rays at different
times.
The X-ray brightness is "flickering" very erratically. There's
doesn't appear to be any simple pattern to this X-ray variability at
all. However, these light curves resemble very closely those from
some X-ray binaries. The only difference is that the timescales are
different by a factor of about a million. This is because the black
holes in X-ray binaries are a million times smaller than those in
AGN.
Maintained by Simon Vaughan
(sav2 at star. le. ac. uk)
Last updated: 16/9/2003
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