Supermassive black holes – the fathers of galaxies
06 January 2010
by
Marcus Chown
ONCE
upon a time, a vast cloud of cold gas was floating in the void of
space, a patch of inert blackness against the even deeper blackness
behind. Then, as if from nowhere, a thin jet of matter streaked towards
it at ultra-high speed. It slammed into the cloud, compressing its
matter and triggering a firestorm of star formation. What had once been
a dormant gas cloud was now a full-blown galaxy.
Is this how a galaxy is born? David Elbaz's team of
astrophysicists is convinced of it.
Their idea that galaxies were zapped into existence affects our story
of how the universe unfolded and puts supermassive black holes, objects
that were once considered esoteric cosmic curiosities, at the very
heart of the picture. Supermassive black holes power objects called
quasars that are capable of unleashing jets of matter at very high
speeds, and it's these jets that Elbaz believes trigger galaxy
formation.
If
he is right, our accepted notion of galaxy formation will be turned on
its head. It also has a startling implication for our ultimate origins.
"It may be that none of us would be here but for the supermassive black
hole whose jet created the proto-galaxy that in time became our own
Milky Way," says Elbaz, at the French Atomic Energy Commission (CEA) in Saclay.
Elbaz
and his team didn't plan to rewrite cosmic history. They set out to
study the link between supermassive black holes and galaxies, a
connection that has baffled researchers trying to understand galaxy
formation. The problem first came to light, literally, in the
mid-1960s, when quasars were discovered. These very bright, compact
objects can release up to 100 times more light than a galaxy like the
Milky Way. Such prodigious light output from so small an object can
only be explained if this energy is springing from the brilliant core
of a galaxy containing a supermassive black hole: the black hole's
gravity pulls in surrounding gas and dust, which swirls into a disc and
heats up to release enough radiation to outshine everything else.
Back
then, many researchers thought that supermassive black holes were only
found in quasars, and that as quasars are very rare in the universe,
they were inconsequential cosmic anomalies. That view changed, however,
with the discovery that at the heart of most, if not all, galaxies
there lurks a supermassive black hole. They had remained hidden because
in most galaxies, including our own, the "central engine" is starved of
fuel and therefore lies dormant.
What
is so surprising is the extent to which supermassive black holes
influence their parent galaxies. Despite their name, supermassive black
holes are very compact objects so you wouldn't expect one to hold much
sway over its parent galaxy. Take the Milky Way, for example. We know
it is a dense disc of stars and gas 150,000 light years wide, whereas
the black hole lurking at its heart would fit inside the orbit of
Mercury.
Naked quasar
Yet
nearby galaxies all seem to follow an elegant but unexplained
relationship: all seem to have central bulges of stars about 700 times
as massive as the black holes at their hearts. This relationship
between galaxies and supermassive black holes suggests that the growth
of one influenced the other. Further evidence of a link comes from the
fact that star formation and quasar activity both peaked around the
same time, 8 to 10 billion years ago.
So
how could something so small affect a galaxy? Elbaz and his colleagues
knew one way a supermassive black hole could punch above its weight:
when it squirts out matter in two oppositely directed, thread-like
jets. This is because the jets of matter can travel within a whisker of
the speed of light for the first few light years, allowing them to
break out of the surrounding galaxy and travel millions of light years
into intergalactic space.
To
find out more about the influence of these jets, Elbaz's team began
studying an unusual quasar called HE0450-2958 and its jets. Located 5
billion light years away, HE0450-2958 is the only known "naked quasar"
- a supermassive black hole without a surrounding galaxy (Nature, vol 437, p
381).
Some
astronomers have speculated that HE0450-2958 does have a surrounding
galaxy, but that it is hidden from view by large amounts of dust. As
dust glows at infrared wavelengths, Elbaz's team used the infrared
instruments on the Very Large Telescope in Chile to investigate. That's when
they made a startling discovery.
As
they looked more closely, they noticed that the quasar's jets stabbed
like a laser beam into a galaxy 23,000 light years away. That galaxy is
rich in bright, young stars and is forming them at a rate equivalent to
350 suns per year, a hundred times more than you would expect for
galaxies in that area. Elbaz reckons this frantic pace of star
formation is no coincidence. "We think this indicates that the quasar
jet actually gave birth to the galaxy," he says.
This
is a radical shift from the standard view of galaxy formation, in which
galaxies came first and supermassive black holes follow (see diagram). Elbaz
and his colleagues think the opposite is true: supermassive black holes trigger
galaxy formation.
In
the case of HE0450-2958, they think it started off as a supermassive
black hole that sucked in gas from intergalactic space until it became
a quasar. It kept growing until a critical moment about 200 million
years ago when its jets switched on. One of the jets slammed into a gas
cloud 23,000 light years away, sending shock waves through the gas.
This triggered star formation, resulting in the galaxy we now see.
The
team knew how controversial their idea would be. So before they
announced their theory, they checked there was no other explanation for
the association between the quasar and the galaxy that its jet points
towards. First they considered the idea that HE0450-2958 had been
kicked out of the galaxy. Simulations have shown that when two galaxies
merge, their central black holes can ricochet off each other, with one
ejected into intergalactic space.
However,
Elbaz points out that it would be an odd coincidence if the quasar was
ejected in the same direction as one of its jets. What's more, to
escape a galaxy as big as the one in question - which is about the mass
of our Milky Way - it would have to be kicked out at about 500
kilometres per second. Not only is there no evidence of a galaxy merger
within the past few hundred million years, but the velocity of the
quasar is only about 200 kilometres per second, making it too slow to
be a galactic escapee.
In
fact, the slow speed of the quasar means that, over time, it will fall
into the galaxy it apparently created. "Rather than the supermassive
black hole growing in the centre of a galaxy after the galaxy has
formed, a supermassive black hole creates a galaxy from outside, then
later takes its place at the heart of the galaxy," says Elbaz.
Astrophysicist
Joseph Silk of the University of Oxford is impressed with the work.
"It's definitely a new and important advance," he says. Others agree.
"HE0450-2958 certainly fits the scenario for quasar-induced galaxy
formation and that's what makes it so exciting," says Kevin Schawinski
of Yale University. However, he cautions that it is only one example
from recent cosmic history. "The epoch of massive galaxy formation and
quasar activity occurs at early times," he says, "so it will be
interesting to see if other systems like HE0450-2958 will be found in
the early universe."
For
others, it's a leap too far. David Merritt at the Rochester Institute
of Technology in New York state has made detailed studies of
HE0450-2958. "Elbaz's team does a good job
of showing that the quasar is affecting conditions in the companion
galaxy," he says. "However, it is a leap from there to the idea that
the quasar is basically responsible for the companion galaxy."
Undeterred,
Elbaz and his team have already begun looking for other, earlier
examples. The beauty of their scenario is that it does not rely on
finding more naked quasars like HE0450-2958. "By virtue of their jets,
supermassive black holes have the power to transform gas clouds into
galaxies whether they are naked or deep in the heart of a galaxy," he
says.
He
thinks it's even possible for a single quasar to trigger the formation
of not just one, but many galaxies. Some quasars have jets that sweep
around the heavens like a lighthouse beam - this is thought to happen
when another supermassive black hole is in the process of merging with
the quasar. As the jets sweep around they could awaken one sleeping gas
cloud after another, says Elbaz. That would certainly explain why
normal galaxies are often seen clustered near quasars.
It's even possible for a single quasar
to trigger the formation of not just one, but many galaxies
Astronomers, most notably Halton Arp at the Max Planck
Institute for Astrophysics in Garching, Germany, and Geoffrey Burbidge
of the University of California, San Diego, have claimed that this
clustering is evidence that galaxies give birth to quasars, then eject
them. "We're suggesting the exact opposite," says Elbaz. "It's quasars
that give birth to galaxies."
One
intriguing possibility is that galaxy formation may not even require
the presence of a sleeping gas cloud to be brought to life by a quasar
jet. That's because such jets can eject up to 100 million solar masses
of matter per year and stay switched on for a few hundred million
years. This means that, in their lifetime, they may eject enough
material to build a large galaxy from scratch.
Look to the clouds
So
how can the theory of quasar-induced galaxy formation be proved? Elbaz
says we should be looking for objects at an earlier stage of evolution
than HE0450-2958 and its nearby galaxy, in particular quasars that are
offset from clouds of cold molecular gas - the stage prior to star
formation.
In
fact, Elbaz thinks some such systems have already been found.
"Astronomers have been puzzled to find quasars offset by clouds of
carbon monoxide," he says. "In our scenario, there is no puzzle. It
makes perfect sense."
Further
support for Elbaz's theory comes in the form of evidence showing that
supermassive black holes had reached their maximum mass early on in
cosmic history. This suggests that there was an epoch preceding galaxy
formation when supermassive black holes grew and ruled the universe -
the opposite to what the standard view posits. "This supports our
idea," he says.
Of
course, if supermassive black holes did form first and then gave birth
to galaxies, the $64,000 question is: where did the supermassive black
holes come from? "This is the one missing jigsaw piece," admits Elbaz.
While
no one knows for sure, there are some ideas on the table. Observations
of quasars show that monster black holes weighing 10 billion solar
masses formed within a billion years of the big bang. For years,
various teams have been puzzling over how they could have grown into
such behemoths so quickly.
One
idea is that they grew from the much smaller black holes which form
when a star reaches the end of its life and collapses. In a superdense
cluster of stars, several of these black holes might merge to create a
huge one that continues to grow by feeding on gas. But critics of this
idea point out that there was simply not enough time in the first
billion years after the big bang for stellar-mass black holes to merge
into something big enough.
An
alternative idea involves the formation of single, supermassive stars,
a scenario first envisaged by the astronomers Fred Hoyle and Willy
Fowler in 1963. If a star like this ever formed, it would be so massive
that the heat generated by nuclear burning at its core would not be
enough to oppose the gravity trying to crush it. The whole thing would
collapse at once, creating a supermassive black hole.
Mitchell
Begelman at the University of Colorado in Boulder has studied this
scenario in detail and believes that something even stranger happened (New
Scientist, 16 May 2008, p 30).
According to his calculations, the seeds of supermassive black holes
formed inside the supermassive stars, growing at a faster rate than
they could support in the void of space. Eventually, the outer layers
of these curious stars explode to reveal the black hole hidden within.
To test the idea, we'll have to wait for the next generation of
telescopes.
Hoyle famously showed that we are all made of stardust. We may
soon want to add that we all come from black holes, too.
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