03.15
本文作者：小红猪小分队
Even in the furthest recesses of cosmic time we see supermassive black holes gorging on gas. But they shouldn’t be there
THEY are the great dark spots of the universe. Not merely black holes, but enormous black holes, billions of times the mass of our sun. They are everywhere we look, even looming distantly out of the light of the cosmic dawn.
Supermassive black holes have all the space-warping strangeness of their smaller kin, which we see dotted about our galaxy; they too swallow all matter, light and cries for help. But they hold another level of mystery. We know that small black holes with a mass a few times that of the sun are born when a large star’s heart collapses in a supernova, but nobody can explain the genesis of the giants.
We thought we could. We thought supermassive black holes simply grew slowly from very small seeds, bloating themselves on surrounding gas (see diagram). But recent observations show this cannot be the whole story. We need a new explanation for these ancient monsters.
Whatever did make supermassive black holes, it had to make an army. Observations of stars whirled around by powerful gravity suggest there is a huge black hole at the heart of almost all large galaxies, including our own.The Milky Way’s is 5 million times the mass of the sun
. Wander some 50 million light years away to the giant elliptical galaxy M87, and you will find a black hole of over 6 billion solar masses. Its event horizon – the boundary of no return for any passing object – is nearly five times as wide as the orbit of Neptune.
Further off we see even more dramatic signs of the supermassive. Extraordinarily active galaxies called quasars have a brilliant point of light at their heart that often outshines all the billions of surrounding stars together. Many spit out searing flares of X-rays and gamma rays and colossal jets of material at 99 per cent of the speed of light. These are the signs of a giant black hole’s voracious appetite. Gas spiralling inwards heats through friction and glows white-hot, generating magnetic fields that send the jets of matter hurtling outwards.
The conventional tale of supermassive black holes begins a few tens of millions of years after the big bang, as the very first stars formed from the densest clouds of primordial hydrogen and helium gas. These pioneers, the story goes, were several hundred times the mass of our sun. The core of such a star soon collapses to form a black hole of around 100 solar masses. As this seed gorges on gas it sinks towards the centre of its galaxy, eventually becoming the powerful heart of a quasar.
In 2000, however, NASA’s Chandra X-ray telescope spied a very distant and powerful quasar. We see SDSS J1030+0524 as it was just 900 million years after the big bang, and its power output must come from a hole of more than a billion solar masses.
How did this monster get so vast so fast? The more gas a black hole gulps, the more light and other radiation shines out. Eventually, the hole is starved by its own brilliant belching: the flood of light grows so fierce that it sweeps away any incoming gas, cutting off the food supply. Theory, backed by the behaviour of black holes nearby, says that a hole can double in size at most once every 30 million years.
To make the leap from 100 to a billion solar masses means doubling in mass 23 times, so in theory the black hole in SDSS J1030+0524 could have developed in around 700 million years. But that would require a gas supply perfectly tuned to the hole’s needs. A black hole’s surroundings are likely to be messy and changeable, so gorging so fast for so long is not so easy. “It is a problem to imagine they can accrete gas at all times,” says theorist Zoltan Haiman of Columbia University in New York.
Still, SDSS J1030+0524 might be a freakish example of a black hole managing to live the high life for the best part of a billion years. “One peculiar object you can always explain,” says astrophysicist Priya Natarajan of Yale University. But we have discovered scores more at comparable distances. “When there’s a population of these things, there has to be a natural way to make them,” says Natarajan.
Each discovery piles on the pressure. Last year, a team using the UK Infrared Telescope in Hawaii observed a quasar, ULAS J1120+0641, with a mass about 2 billion times that of the sun just 770 million years after the big bang. Theory dictates a minimum of about 750 million years to grow that large from a 100-solar-mass start.
The story of supersized holes growing from small seeds looks even less plausible after recent work on how the first stars formed. New simulations have followed the shrinking gas clouds that gave birth to them for a longer spell, and found that they tend to split up into smaller fragments than we thought, making stars no larger than about 50 solar masses. After exploding, these would make black holes of only about 10 solar masses – a disappointing size for would-be quasar seeds. “They are just not oomphy enough,” says Natarajan.
What’s more, common-or-garden stellar-mass black holes should pop up all over every young galaxy. Some of these holes would sink to a galaxy’s centre and seed more massive ones, so even fairly small galaxies should sport a fairly impressive central black hole by now. But that is not what we see. Last year Jenny Greene of Princeton University found that among smallish galaxies with a total mass of about a billion suns, only about half have a central hole.
There seems to be only one conclusion. We’re going to need a bigger seed (see diagram).
One possibility is that supermassive black holes began not with single stars, but many. “We know that early in the history of the universe, stars tended to form in bursts – regions that were spectacularly active,” says Fred Rasio of Northwestern University in Evanston, Illinois. In 2003, he ran simulations of ancient clusters where hundreds of bright young things were forming. The most massive tended to pile up near the centre, where they almost inevitably run into one another. “You form a thing – I don’t want to call it a star – with many thousands of solar masses,” he says. What happens next is very difficult to model. Rasio’s educated guess is that this object might well collapse to form a black hole of perhaps a few thousand solar masses.
It’s a nice idea. It would be nicer still if we could find similar middleweight black holes in star clusters today. A few promising objects called ultraluminous X-ray sources (ULXs) have been found in nearby galaxies. They are seemingly bright enough to be based around biggish holes. But in 2011, observations of one ULX in our neighbouring galaxy Andromeda showed that it has the same characteristic spectrum and behaviour as small black holes in our galaxy, with around 10 solar masses. The other ULXs may be this small too.
In any case, even a seed hole of a 1000 solar masses would have to double its mass 20 times over to become a gigasun giant. Almost constant gorging would still be needed to explain an object like ULAS J1120+0641.
Perhaps, then, we need to think a little bigger still. If a small black hole springs from the collapsing heart of a star, might a big one come from the collapsing heart of a galaxy?
This was originally suggested as an outside possibility in 1978 by Martin Rees at the University of Cambridge. It sounds seductively simple, but it is not easy to cram so much matter into a galaxy’s heart. The first hurdle is spin. Even the earliest protogalaxies rotated a little, tweaked by their neighbours’ gravity. As they contracted, their gas whirled faster, like air drawn into a tornado. Eventually, the rotation balanced gravity, producing a spinning disk of gas with little material within the innermost few hundred light years.
Rasio and Abraham Loeb of Harvard University showed in the 1990s how this barrier might be overcome. If a protogalaxy is slow-spinning and dense, its core can become unstable. Excess gas collects into rotating, elongated bars, which act like gravitational gear-wheels transferring the rotation outwards. The heart of the protogalaxy can then collapse into a much denser knot.
The next stage is uncertain, but according to calculations performed in 2006 by Rees, his then colleague Marta Volonteri and Mitchell Begelman of the University of Colorado at Boulder, one possibility is a monstrous “quasistar”: a dense cocoon of gas a few hundred million kilometres across surrounding a small central black hole. The cocoon’s great weight would force matter into the hole, incubating it to a million solar masses in just a few tens of millions of years. Such a heavy seed could square the theory with observations: it would need to double in size just 10 times to make a billion-sun black hole. Even without a carefully balanced gas supply that could happen within the allotted time frame of about 700 million years.
Problems remain. During its initial collapse, the gas is liable to split up into little blobs that will coalesce and ignite into stars, denying material to the quasistar. Some of these stars will go supernova, blasting away fresh gas supplies and halting the black hole’s growth. There are ways around this: ultraviolet radiation from nearby starbursts could heat the blobs and stop them coalescing, or turbulence might prevent fragmentation. To many astronomers, however, these remedies seem too contrived. “Direct collapse needs a lot of fine-tuning,” says Rasio.
Do these failures mean something altogether stranger is going on? Perhaps the most radical suggestion is that giant black holes were forged directly in the fires of the big bang, during tumultuous moments known as phase transitions when matter and radiation suddenly rearranged themselves. About one microsecond after the beginning of time, for example, quarks were coming together to form protons and neutrons. This process could have been uneven, producing sharp spikes in density that turned into black holes of around one solar mass.
That is too small to make our seeds, but Sergei Rubin at the Center for CosmoParticle Physics in Moscow, Russia, has suggested that these holes might cluster together and swiftly merge into one giant. Another promising phase transition happens when the universe is about 10 seconds old, when a haze of electrons and positrons destroy one another to leave space filled instead with gamma ray photons. At this point, holes of up to 100,000 solar masses might form spontaneously.
These mammoths of the universal dawn would suck in hot gas around them and shine brightly in X-rays, leaving marks on the cosmic microwave background radiation. Searches have so far found nothing, but do not rule out a few rare giant primordial holes, enough to form the seeds of early quasars.
Even so, most astronomers trying to fathom the origin of supermassive holes think that primordial seeds are an unnecessarily exotic option. “There is no compelling argument for why they should have formed,” says Begelman. “You need some weird cosmology.”
Perhaps more palatable is the notion of dark stars. In 2007, Douglas Spolyar, then at the University of California, Santa Cruz, and colleagues suggested that some of the very first stars might have been powered by dark matter. Dark matter particles would settle into the stellar core, where they could collide with and annihilate one another, providing a gentler heat than the nuclear burning of hydrogen and helium-powered giants. With no harsh radiation to blow away incoming gas, dark stars could keep growing almost indefinitely, finally collapsing to form a seed black hole of up to 100,000 solar masses.
Point of no return
This idea is very testable. Dark stars would emit enough infrared light to be seen by the heir of Hubble, the James Webb Space Telescope, which is scheduled for launch in 2018. It might also be able to detect quasistars.
If it sees nothing, we will need an even more sophisticated piece of kit to spy on the origin of supermassive black holes. The Laser Interferometer Space Antenna is designed to detect gravitational waves, travelling space warps that relativity says should be produced copiously by colliding black holes. LISA has spent a long time trying to get off the drawing board, but if it finally blasts into space it could detect the gravitational waves from merging holes across the universe. “Then it should be easy to tell whether the seeds are small or massive,” says Volonteri.
Meanwhile, astronomers will be searching for quasars yet deeper in space and time. What happens if they keep finding them ever earlier in the history of the universe? Eventually, they will reach a point a couple of hundred million years after the big bang when their black centres would not have had time to grow even from a million-sun seed. Then none of these stories would work. We would truly be in the dark about why the universe is full of holes.
This article appeared in print under the headline “Monster mix”
Stephen Battersby is a New Scientist consultant based in London
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