As well as the stellar mass black holes lurking in our galaxy there
are much more massive black holes further out in the universe,
supermassive ones millions of times more massive than those we've
discussed to far. The closest supermassive black hole lives
in the centre of our own galaxy, the Milky Way.
The Milky Way is a pretty normal spiral galaxy, our solar system is about 26,000 light years from its centre (so it's not going to be a threat to us!). The image to the right shows a nice spiral galaxy probably much like our own. We cannot get a view of the Milky Way like this for the simple reason that we are embedded in it! (see the cartoon below)
4.1 Peering into the Milky Way
This is an image taken in the direction the the galactic centre. The
big dark patches across the image are due to dust lying in the
plane of the galaxy. This makes it rather difficult to see all the way
to the Galactic centre in optical light. However, we can peer through
the dust using other wavelengths of light (infrared and X-rays can
get through dust).
This image below shows multiple views of our Galaxy taken at different wavelengths, from radio though optical light to X-rays and gamma-rays Each wavelength reveals something different about the galaxy we live in. The big question is "what's in the centre of it all?"
Radio (0.4 GHz)
Radio (21 cm)
Radio (2.7 GHz)
Radio (Molecular
Hydrogen; 115 GHz)
Infrared
Near Infrared
Optical
X-rays
Gamma rays
The image to the right shows a radio map near the centre region. There is a strong radio source, Sgr A* (called "Sagittarius A-star"), right at the dynamical centre of the Galaxy. It was discovered in 1974 by Bruce Balick and Bob Brown and the nature of Sgr A* has been a long-standing puzzle for astronomers.
4.2 Watching the Galactic centre
In recent years there has been major progress in the quest to find out out
whether there is a supermassive black hole in the centre of the Milky Way.
Two groups in particular, one based in UCLA
(California, USA) and one based in MPE
(Garching, Germany), have been monitoring the central regions.
Their results show there is something very massive, compact and dark in the galactic centre region. What they have done it take detailed images of the central region over a period of years and watch to see what moves. Any star in orbit close to the black hole will be moving fast enough to detect the movement if monitored for a few years.
From the ground the turbulent upper atmosphere can seriously blur astronomical images. In order to take such detailed images of the Galactic centre region new techniques were used, including "speckle interferometry." The animation on the left shows some real data from the MPE team, the wobbling is due to atmospheric effects. In order to reduce this, many quick exposures are taken in succession. These are hardly burred at all but are too short to get a good image. Many of these short exposure images can be combined in a computer in such a way that the blurring cancels out, producing a nice exposure with minimal blur.
The image of the right shows the result: six years worth of action in the Galactic centre in only a few seconds. But it's clear that stars are moving! These stars are 26,000 light years away and we can actually watch them move. The position of Sgr A* is marked with a "+" in the image. It is obvious that the stars closer to Sgr A* are moving fastest and seem to be moving in orbits. Not just that, but their (apparent) speed and distance from the centre can be used to estimate the mass of the unseen, dark object they are orbiting.
The plot on the left shows the mass of the dark central object as
deduced from each of the moving stars (big dots). All of the dots line
up very nicely with the prediction (solid line) for a central mass of
2.61 million solar masses. So we know there is a dark, compact,
supermassive object in the centre of our galaxy. This is very
strong evidence to suggest Sgr A* is a black hole.
The left-hand image shows a detailed image of the Galactic centre
region by the UCLA group. And the image of the right shows their
measured (dots) and predicted (ellipses) orbits of the stars.
The predicted orbits were calculated assuming
the stars are orbiting a 2.6 million solar mass black hole.
One interesting thing to note about this black hole is that it is very dark. Unlike the ones in X-ray binaries that we've discussed this supermassivle black hole seems dormant, as if it's hardly accreting ("feeding").
4.3 What about other galaxies?
There is strong evidence to suggest the centre of our Galaxy harbours
a supermassive black hole. It seems sensible to ask if the same is true of other galaxies.
The image to the left shows the
results of a Hubble Space Telescope observation of a nearby galaxy
called M84. The left-hand panel shows an image of the galaxy, with a
dust-lane running through it (the dark streak). The blue rectangle
marks where the
spectrograph was placed. A spectrograph separates light by frequency,
rather like a prism will separate the colours of white light. The
resulting spectrum is like a "cosmic bar code" and reveals the motions
of the gas in the inner regions of M84.
The result of analysing the spectrum is shown in the right panel. In this plot the up-down direction marks the direction along the length of the blue rectangle. The left-right direction shows the amount of redshift/blueshift. As discussed earlier this gives away the motion of the gas - it is a little bit like using a police speed gun on M84. The result is very interesting: just above the centre of the galaxy (just above the middle of the plot) there's a strong blueshift - meaning the gas is moving towards us fastest. And just below the centre (below the middle of the plot) is a strong redshift - the gas is moving away from us. The velocities involved (about 880,000 mph!) strongly suggest this gas is orbiting something compact with a mass of around 300 million solar masses. There's not much else it could be except a supermassive black hole.
This seems to be almost the norm. Most big galaxies seem to contain supermassive, compact, dark objects in their centres, almost certainly black holes.
There is a big question remaining: what happens when these supermassive black holes start feeding...?
Maintained by Simon Vaughan
(sav2 at star. le. ac. uk)
Last updated: 18/9/2003
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