快猫短视频

AT THE heart of every galaxy lies a dark despot. Unseen by its minions, it
holds sway over the billions of solar systems that stretch for thousands of
light years beyond. It was there before any of them were born, and it is already
shaping their future.

These tyrants are giant black holes, so heavy that they are called
鈥渟upermassive鈥� by astronomers. Ever since the existence of black holes was
predicted early this century, they have seemed like outlandish things, absurd
objects that offend all our ideas about how light and matter ought to behave.
And even when evidence for real black holes in the Universe began to trickle in,
they still seemed like curiosities, freaks. Intriguing and exciting, yes.
Important? Not really.

But now astronomers are beginning to suspect that black holes have stamped
their authority on the Universe. Earlier this year, researchers suggested that
giant black holes were the seeds from which every galaxy in the Universe grew.
Last week, more evidence emerged to support their claim. And over the past two
years it has become increasingly clear that black holes shape galaxies, and
might even create them. Without these strange by-products of Einstein鈥檚
imagination, we might none of us be here.

The first clue that supermassive black holes exist was the discovery several
decades ago of quasars鈥攅xtremely bright objects in the centres of distant
galaxies. Quasars can be hundreds of times as bright as their surrounding
galaxy, yet they are smaller than our Solar System. What could blast out so much
light and radiation from such a small space? Black holes, for one. Though they
are better known for swallowing light, they can also be a brilliant light
sources. Material being sucked in by a black hole forms a spiralling disc around
it, and as the disc churns the friction heats the gas until it is white hot.
Astronomers believe that this is what makes quasars shine.

Quasars are rare, however. Even in their heyday, billions of years ago, there
were hundreds of ordinary, unexciting galaxies for every one that harboured a
quasar. So it seemed that giant black holes were freaks of nature, of no
importance to common, everyday galaxies. 鈥淧eople thought they were something
gone wrong in the centre of a galaxy, like an illness,鈥� says Doug Richstone of
the University of Michigan.

So when black holes began to show up in the centres of more ordinary
galaxies, it was natural to assume that they must be burnt-out quasars.
Astronomers first caught a glimpse of such a dark mass in 1978 in a galaxy
called M87.

Then, in 1988, Richstone and his colleague Alan Dressler (now at the
Observatories of the Carnegie Institution of Washington, based in California)
looked at the spiral galaxy Andromeda and the small elliptical galaxy M32, both
neighbours of our own Milky Way. There was no particular reason to think that
these unremarkable galaxies harboured dead quasars, but because they are only a
few million light years away, it was possible to look deep into their cores. The
astronomers looked at the speeds of millions of stars in the centre of each
galaxy, and found they were moving remarkably quickly鈥攕o fast that they
ought to fly out of the centres of their galaxies. Unless, that is, there was a
giant black hole in each galaxy holding the stars in its grip.

Could finding black holes in both these neighbouring galaxies just be good
luck? Probably not. 鈥淲hen you trip over something on your own doorstep,鈥� says
Richstone, 鈥渋t鈥檚 not reasonable to think that it should be special.鈥� And sure
enough, in the past few years the Hubble Space Telescope has found evidence for
giant black holes in more than 20 nearby galaxies. One of them is our own: at
the centre of the Milky Way is a hole with a mass 3 million times that of the
Sun.

Indeed, it is beginning to look as though all galaxies have a hole at their
core. Rather than a rare and unfortunate illness, supermassive black holes could
be an integral part of every galaxy. If so, what part do they play in galactic
life? In January, at a meeting of the American Astronomical Society, Richstone
put forward a provocative idea: giant black holes might create galaxies in the
first place.

Richstone came to this conclusion after comparing the history of quasars and
of star formation in the Universe. He noticed that most quasars appeared perhaps
a billion years before most stars. This might mean that black holes control the
formation of galaxies, an idea that turns conventional astrophysics on its
head.

Most astronomers assume that each galaxy creates its own black hole. Here鈥檚
how it might work: a very massive star ends its life in a supernova explosion,
leaving behind a middling-sized black hole that weighs just a few times as much
as our Sun. It feeds on a rich supply of gas near the galactic centre, and grows
like a pearl in an oyster. It might also swallow stars and merge with small
holes from other supernovae. But for giant black holes to form this way takes a
long time. If they existed before most galaxies, this can鈥檛 be right.

Smooth start

Instead, according to Gordon Garmire of Pennsylvania State University, giant
black holes might have formed at the dawn of time, and been the seeds around
which galaxies grew. But why would galaxies need such seeds? The reason is that
the early Universe was very smooth. The leftover glow from the big
bang鈥攖he so-called cosmic microwave background鈥攕hows that the
density of the early Universe didn鈥檛 vary from place to place by more than about
1 part in 100 000.

To produce our Universe of galaxies and voids, these small density
fluctuations must have been amplified enormously. And the amplification must
have happened very quickly, as the first galaxies existed a mere billion years
after the big bang. 鈥淭hat鈥檚 not much time for the Universe to go from smooth to
rough,鈥� says Garmire. So he suggests that giant black holes might have acted as
gravitational seeds, pulling a mantle of matter around themselves that went on
to form stars. In other words, a galaxy.

In that case the supermassive holes must have formed very early indeed, not
long after the big bang. It鈥檚 an astonishing idea. Stephen Hawking has suggested
that tiny black holes weighing only as much as a mountain might have formed in
the first instant of the Universe (see p 24). But Garmire is talking about holes
a trillion trillion times bigger.

And yet black holes of about the right size could fall out of a cosmological
theory called supernatural inflation, developed a few years ago by Alan Guth of
the Massachusetts Institute of Technology and his colleagues Lisa Randall and
Marin Soljacic. According to the theory, quantum fluctuations in the first
fraction of a second after the big bang naturally produce the gentle bumps and
dips we see in the microwave background. But they would also have made some
small regions of space especially dense. Within a few years, these regions would
have been compressed by gravity and collapsed to form black holes. The masses of
these black holes aren鈥檛 tightly constrained by the theory, but Guth says they
would be some fraction of the mass of a galaxy, 鈥減erhaps a tenth, perhaps a
thousandth鈥�. If they are towards the bottom of this range, and if enough are
produced, these could be the black holes that power quasars. And they could be
Garmire鈥檚 gravitational seeds. 鈥淥ur theory is on a backburner,鈥� says Guth, 鈥渂ut
we may want to get back onto it.鈥�

However, not everyone is convinced by these arguments. Despite Richstone鈥檚
graph, many astronomers don鈥檛 even believe that quasars came before galaxies.
鈥淯ncertainties in these histories are very great because of the obscuring
effects of dust,鈥� says Martin Rees of Cambridge University. The star-formation
history used by Richstone is based on optical observations, and if early
galaxies were much dustier than their modern counterparts, as many theorists
believe, then their optical emissions could all be absorbed by dust. An
instrument called SCUBA, on Britain鈥檚 James Clark Maxwell Telescope in Hawaii,
appears to confirm this possibility. It looks at infrared light emitted by dust,
which reveals signs of many more early galaxies than the optical observations
do.

But although Richstone admits that there is room for doubt, he is sticking to
his guns. He thinks that dustiness would affect both quasars and stars in a
similar way, so that comparing their histories would still give the same answer.
鈥淚f you ask me, do quasars really come before galaxies鈥攚ell, I鈥檇 bet a
hundred dollars on it at three to one. I just wouldn鈥檛 bet anything I couldn鈥檛
afford to lose.鈥� And he has his supporters. 鈥淩ichstone is very probably right,鈥�
says Virginia Trimble of the University of California at Irvine.

Bright sparks

NASA鈥檚 new Chandra X-ray Observatory has lent him some support, too. This
week in Nature(vol 404 p 459), a group of astronomers published
observations by Chandra that finally explain the cosmic X-ray background
discovered in 1962. This high-energy glow turns out to be made up of little
sparks from about a hundred million X-ray sources. Among them are ordinary
quasars, as well as many unfamiliar objects that could be quasars obscured by
dust. There鈥檚 also a third type of source that the Chandra team call 鈥渙ptically
faint objects鈥�.

The optically faint objects might be very distant quasars whose ordinary
light has been absorbed by intergalactic gas so that only their X-rays get
through, suggests Richard Mushotzky of NASA鈥檚 Goddard Space Flight Center in
Maryland, the head of the group. These would be quasars in their glorious youth,
at a time before most galaxies formed.

But even if black holes don鈥檛 turn out to be so venerable, they could still
have had profound effects on galaxies. About two years ago, Richstone and
Dressler teamed up with John Magorrian of the University of Toronto, John
Kormendy of the University of Texas and eight other astronomers to pull together
all the known information about black holes in nearby galaxies.

They compared the mass of each hole with the mass of the blobby part of each
galaxy. Galaxies come in two broad types, spirals and ellipticals. A spiral,
such as the Milky Way, looks like a fried egg, with a flat white disk full of
gas and new stars and a yellowish yolk of old stars in the middle. The yolk is
called a bulge, or spheroid. An elliptical galaxy is just a big blob of old
stars鈥攁 separated yolk.

When Magorrian and co. plotted the mass of each black hole against the mass
of its bulge, they found that the points fell on a straight line. From little
spiral bulges to giant elliptical galaxies, the central black hole always has a
mass about 1 per cent that of its surrounding bulge. 鈥淭here seems to be an
intimate link between the two masses,鈥� says Richstone.

But why should the sizes of the bulge and the central black hole be so
closely connected? In 1998, Rees and Joe Silk, then both at Cambridge
University, suggested how this might happen. The radiation from a young quasar
should drive a wind of charged particles out into the surrounding galaxy. As the
black hole swallows more and more material and grows steadily larger, the quasar
should get brighter and the wind stronger. Eventually, the wind will be so
strong that it overcomes the gravity of the galaxy and blows all the gas away.
With its gas supply cut off, the hole will stop growing, and so will the galaxy.
Silk and Rees worked out that the hole would have to grow to around the size
given by Magorrian鈥檚 mass relation before it calls a halt to growth.

Before this happens, the young quasar could have other influences. 鈥淚n the
first 100 million years of its life,鈥� says Richstone, 鈥渢he quasar can dominate
the energy output of its galaxy.鈥� And, by a rather complicated route, all that
radiation could help trigger star formation.

Clouds of gas have to cool down before they can collapse to make stars. But
the gas in early galaxies would be mostly hydrogen, and hydrogen atoms tend to
hang on to their thermal energy. On the other hand, hydrogen molecules, made of
two hydrogen atoms, are good emitters of heat. A blast of radiation from a
quasar could turn some hydrogen atoms into ions, which would then react to form
molecules and radiate their energy, speeding cloud collapse and star
formation.

Quasars might stir up their galaxies, too. They squirt out powerful jets of
high-speed matter, which could sweep around the galaxy, generating shockwaves
that compress the surrounding gas. That compression could also encourage stars
to form.

Finally, a big black hole could change its galaxy鈥檚 shape. James Binney of
Oxford University worked out back in the 1970s that most elliptical galaxies
ought to be an odd shape, with one long axis, one short axis, and one of
intermediate length. They would look a bit like a watermelon seed, or a squashed
rugby ball. But later observations showed that most of them are more
symmetrical鈥攆lattened spheres, like a chocolate M&M.

In 1996, David Merritt and Tema Fridman at Rutgers University in New Jersey
came up with an explanation. In a seed-shaped galaxy, stars travel on elongated
orbits that plunge very close to the centre, like comets in our Solar System.
Merritt and Fridman calculated that a big central black hole in the centre
disturbs these orbits, making them unstable. So such a galaxy would soon settle
into a more stable flattened sphere. 鈥淚t鈥檚 incredible that this tiny pip in the
middle forces the whole enormous object to change shape,鈥� says Richstone.

Indeed, it鈥檚 incredible that a black hole could have any of these
powers鈥攖hat something smaller than our Solar System could control a vast
island universe of billions of stars. But then, think of Alexander, Attila,
Napoleon . . . Human dictators have a famous tendency to be diminutive. Perhaps
it is a universal law.