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How Does a Black Hole Eat Its Breakfast?

A large black hole

A large black hole located at the centre of an active galaxy. An accretion disk forms as matter falls inwards from the galaxy. The matter forms a spiral disc that is compressed and heated so that it begins emitting photons. The accretion disk becomes so hot that its radiation physically pushes matter away from the black hole, and accelerates gas into the jets that emerge from its poles.

By David Floyd

The bending of space–time by mass allows astronomers to peer deep into the universe, and they have begun to use this to probe one of the most enigmatic phenomena in the universe: the explosions of light surrounding black holes known as quasars.

Einstein postulated that all mass bends space–time, and that enough mass placed in a sufficiently small space will break it.

The bending of space–time is spectacularly demonstrated in gravitational lenses (Fig. 1), where a massive object such as a galaxy lies between us and a distant bright object, such as another galaxy. It is as if a glass of water has been placed in front of us, and we observe a number of magnified images of the room beyond. Gravitational lensing is the universe’s own natural telescope, and astronomers have long used the phenomenon to explore the details of both lensed and lensing objects.

The breaking point of Einstein’s law, along with all the other laws of physics, occurs when so much mass is accumulated in such a tiny region that nothing can prevent its gravitational collapse into an infinitely small point. This is a black hole, or a singularity in space–time: a point of infinite curvature. Here, the laws of physics are bent to their breaking point.

As a result, black holes remain a fascinating mystery. While we now have good evidence that extremely massive black holes exist at the centres of all galaxies, we cannot observe them directly: we can only observe their gravitational effects on the surrounding region.

Over the past decade astronomers have watched the centre of our own galaxy, the Milky Way, as the trajectories of dozens of stars are curved around some invisible point: they are in gravitational thrall to a minuscule yet extremely massive object lurking at the very centre of our galaxy. It is as though four million suns are packed into a volume the size of the solar system. In other galaxies, while we cannot directly observe the motions of stars we can observe the large-scale effects of a large and dark mass that accelerates the orbital velocities of any material close to the centre.

Sometimes, gas or dust within a galaxy makes its way down into the sphere of influence of the central black hole. If sufficient material falls close enough to the black hole, it triggers the most spectacular phenomenon in the universe: a quasar.

In a quasar, the infalling material is squeezed and accelerated by the enormous gravitational pull of the black hole. As a result, it heats up and it shines. The fundamental process is the same as when you fill a bicycle tyre with compressing air: the nozzle heats up due to the pressure of the air inside.

However, this material does not just get magically sucked into the black hole: as it draws nearer it is accelerated. Some material may indeed be on a trajectory that drops it straight into the hole, but this is like threading an extremely small needle. Most material will miss the hole slightly and be drawn instead into a vast, violently swirling disk of material that, slowly but surely, delivers the material through spiralling orbits into the black hole itself.

This process is known as accretion, and it is understood to provide the most efficient release of energy in the universe. We do not understand how it works, and the structure of an accretion disk has been impossible to measure: until very recently.

In an accretion disk, the mass energy (E=mc2) of infalling material is converted into light by the gravitational field of the black hole. The more compressed the material is, the hotter it gets and the brighter and “bluer” it shines. By “bluer”, astronomers mean shorter wavelengths – all the way down through ultraviolet to X-rays and gamma rays.

In theory, up to 42% of the rest mass energy of infalling material can be emitted as light. By comparison, just 0.7% of the rest mass energy of hydrogen is released in the most efficient nuclear fusion reaction that powers the stars.

As a result, quasars can be thought of as the particle accelerators of the universe, and the regions closest to the black hole experience conditions not experienced since the Big Bang – far beyond anything reproducible on Earth. Sufficient energy is generated by a quasar that it may actually affect the growth and evolution of the galaxy that hosts it.

By studying the light from quasars we can gain an insight into the very fundamental make-up of our universe, learn how matter and energy behave in the presence of strong gravitational fields, and possibly unlock one of the mysteries of the cosmos: how the galaxies themselves came into being.

A fundamental problem is that we do not understand how the black hole is fuelled in order to drive this enormous power house. While it might seem straightforward that matter should fall into a black hole, we do not expect the Earth to spiral into the Sun, and a black hole is in principle no different: it exerts a gravitational pull like any other object.

For matter to fall inwards, it must shed some of its angular momentum, and this is normally achieved through some sort of friction. In the extreme conditions around a black hole, we do not understand how such friction is produced, and until very recently we were unable to perform a direct measurement of the conditions close to the black hole. However, if we can directly measure the size of region that emits at each wavelength, we can begin to piece together the physics of how angular momentum and heat are distributed through the disk.

The trouble is that the shining hot material in a quasar is compressed into a region roughly the size of our own solar system. While vast in everyday terms, this is dwarfed both by the size of the galaxy and, overwhelmingly, by the distance between us and the quasar.

For example, compared with a galaxy in which a typical quasar lives, the quasar itself is roughly the size of your head compared with planet Earth. Imagine viewing the Earth from deep space and being blinded by a single head-sized object!

Even more mind-boggling are the distances to these objects. As viewed from Earth, a typical quasar appears about a billion times smaller than the full Moon.

For this reason, quasars have posed an insurmountable observational challenge since their discovery in the 1960s. Our understanding of this, the most powerful phenomenon in the cosmos, has been almost entirely theoretical, with no means of directly testing competing theories that attempt to explain how all this energy is released.

One answer lies in gravitational lensing. About 100 quasars have so far been discovered that have a massive galaxy lying about halfway between us and them, thus acting as a gravitational lens. In a typical lens we observe two or four magnified images of the quasar arranged around the image of the intervening galaxy (Fig. 2). Sometimes we also see a crescent of light that is the lensed image of the host galaxy of the quasar.

We can use the geometry of the system to determine the mass of the intervening galaxy. In a normal lens we expect each pair of images to have roughly the same magnification, and thus to appear the same brightness. However, in a handful of systems we find that one of the images is relatively demagnified. This happens most pronouncedly in lenses with a pair of images that are very close to each other, and in such cases we have strong reason to believe that the images should have the same brightness.

For a long time, the nature of these anomalies was a matter of some debate. One obvious possibility is that there is some intervening matter that lies along one of the lines of sight to the quasar – perhaps a cloud of dust on one side of the lensing galaxy. Another possibility is that the dimming is due to “imperfections” in the lens galaxy due to its structure – a bit like having a magnifying lens made out of the rippled or bubbly glass that is sometimes used in bathroom windows. If there is structure in the lens at just the right size we will see a different magnification for background sources of different sizes.

Think about it: if someone holds a candle outside your bathroom window and move it around, you will see it magnified sometimes and just dimly at others as it moves across the peaks and troughs in the structure of the bubbled glass. A large object like the Moon would show very little variation in its overall brightness. This process is called microlensing, and it produces small changes in the lensing magnification if the lens’ imperfections (or “microlenses”) are of roughly the same size as the object that is being lensed.

It turns out that this is exactly the sort of tool that we can use to probe the structure of a quasar. All theories predict that the hottest material is emitted from the smallest region at the centre of the disk. Larger and larger regions should emit progressively redder and redder light.

But the degree by which this changes depends on the physical mechanism that drives the emission. If we could detect a change in the magnification of a quasar image with wavelength, we could begin to explore the physics of accretion.

When we look at an “anomalous” lensed quasar we find that the dim image does indeed get dimmer as we look at bluer light. Qualitatively we would expect the same effect from dust, but the change is too slow to be explained this way.

Instead we are observing gravitational microlensing by the individual stars in the lens galaxy. The amount by which the dimming changes with colour therefore tells us about the size of the region of material that is shining at each colour.

We have recently performed detailed studies of two quasars, MG 0414+0534 and SDSS J0924+0219 (Fig. 3). In both cases the change in magnification is too little to be explained by dust, but is too much to be explained by the long-standing theory of accretion. In one case we are detecting material down to the inner light-week of the quasar, and in the other it is the inner 11 light-days. That is just 1000 times larger than the event horizon of the black hole itself, and rather smaller than our own solar system. This is the first time any method has probed so close to a supermassive black hole.

While we are still restricted to a very small number of objects, a picture is emerging that the change in temperature with radius from the black hole is too steep to be explained by any kind of friction. This suggests that magnetic fields are required for the black hole to continue to feed itself. Without them, the black hole would essentially have no power to draw material inwards, and the quasar would gradually fizzle out due to lack of fuel.

The technique paves the way for more detailed spectroscopic studies that will tell us how black holes are able to convert gravitational energy into electromagnetic radiation in the most efficient way observed in the universe, and will help us to understand how galaxies themselves are formed.

What is really exciting about this technique is that we can probe these extreme objects at all, and these early results are just a taste of what’s to come. We have shown that the standard picture of how a black hole eats is not complete – it needs some “cutlery”, probably in the form of a magnetic field.

We can now begin to explore in detail exactly what a black hole eats!

David Floyd is an Australian Astronomical Observatory “Magellan Fellow” and researcher at the University of Melbourne.