5. Quasars: the real heavyweights!

"Well, the thing about a black hole - its main distinguishing feature - is it's black. And the thing about space - your basic space colour - is it's black. So how are you suposed to see them?."

-- Holly (Red Dwarf)

The figure below 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) because it is 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. The 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 enourmous 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 stricking "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 bahaves like a massive gryoscope. If the supermassive black hole was spinning it could stay spinning around the same axis for millions of years.

These three images are all of one quasar, M87. The upper left image shows a radio map, revelaing the beautiful radio jets and plumes. The upper right image shows an optical Hubble Space Telescope image of M87, the jet is clear here along with the bright nucleus. To the 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 chapter). These all point towards there being a truely 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 blackhole, 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 (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 investiage the black hole region we need to turn to X-ray observations.

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 impart on the emerging X-ray spectrum characteristic signatures of 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 and is thought to originate from the direct vicinity of the black hole.

The plot shows the strength of the emission from iron in the nucleus of an AGN called MCG-6-30-15. Iron emits at a specific frequency in X-rays, about 6.4 keV in the plot on the left. So the iron emission should appear as a sharp spike at 6.4 keV. But as seen in the plot the emission is stretched out to the lower frequencies (towards the left of the plot). This is because of the effect of the central black hole.

Because iron atoms emit X-rays at a fixed frequency they behaving 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 slows time down, and to us (at a safe distance) the iron appears to be ticking slower... so we see it at a lower frequency.

The detailed shape of the iron line is a function of 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.

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.