But how are they born? In and , Yale University astrophysicist Priyamvada Natarajan, working with Giuseppe Lodato now at the University of Milan in Italy , published a series of papers explaining how dense primordial gas clouds essentially protogalaxies in the early universe could have collapsed to form seed black holes with masses of 1, to , Suns.
Normally, such clouds would have fragmented during the collapse process to form a multitude of massive stars instead of a single black hole. But under certain rare conditions, a few clouds could have collapsed to form extremely massive black holes. The key to this process is how gas clouds cool. Most of these clouds in the early universe contained a high abundance of molecular hydrogen H2 , which consists of two hydrogen atoms bound together.
Natarajan and Lodato found that such clouds will cool quickly, which causes them to fragment into numerous clumps that will each go on to form a star. But if a small primordial cloud lies close to a much larger protogalaxy that is rapidly forming stars, those stars relentlessly zap the cloud with ultraviolet radiation. This massive input of energy breaks the chemical bonds that bind molecular hydrogen together, converting the cloud into one of almost pure atomic hydrogen H , which is less efficient at radiating away energy.
This process bypasses the formation of traditional stars, although it could form a very short-lived supermassive star. Such a massive black hole would have quickly merged with the nearby galaxy, where it could have bulked up very rapidly on stars and gas.
Natarajan and Lodato originally intended this model to explain ultra-massive black holes — those with masses of 1 billion Suns or more — at slightly lower redshifts.
But it also offers a promising solution to the timing problem for the redshift This explains why high-redshift quasars are uncommon. Luckily, these monster black holes are very rare, so you can accommodate what is seen so far, easily, without a problem. The DCBH model seems to work in a computer simulation. But did such black holes actually form in the real universe? We shall soon find out. Natarajan says her model can be directly tested in the near future.
But a DCBH would outweigh the total mass of stars in its small host galaxy by up to 50 times, Natarajan says, creating a bizarre object known as an obese black hole galaxy. Obese black hole galaxies would be relatively bright at infrared wavelengths, with a characteristic spectrum. And whether it detects these objects will test their model, Natarajan says.
Theorists have come up with other ideas about how the universe could birth the massive seeds of supermassive black holes. First, massive stars live out their life cycles, creating many smaller black holes.
There, they can merge to form a single black hole with 10, to , solar masses. This process progresses extremely quickly, taking just 50 million to million years. What if some black holes in the early universe were able to accrete matter at super-Eddington rates for prolonged periods of time? In this scenario, a black hole starting off with a few hundred solar masses could have bulked up relatively quickly into a billion-solar-mass behemoth.
Hennawi, his Santa Barbara colleague Frederick Davies, and other collaborators have recently performed observations suggesting this might have been the case.
Using the Magellan and Gemini telescopes, they have measured the amount of light from several redshift-7 quasars by observing how their radiation has ionized the intergalactic gas between the quasar and us. Theorists have a difficult time simulating how black holes accrete material. The underlying physics is exceedingly complex, especially for supermassive black holes. But one thing is clear: The presence or absence of magnetic fields, and how they arrange themselves around a black hole, plays a critical role in controlling the rate of the accretion flow.
Magnetic fields thus determine how efficiently a gram of matter radiates light as it spirals toward a black hole. A magnetic field with twists and turns will gum up the flow, causing it to heat up and emit powerful radiation that could stem the infall of material. But, says Natarajan, if the magnetic field takes a direct path and neatly threads an accretion disk, simulations suggest it can whisk material to the black hole, feeding it much faster than the Eddington rate.
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E-mail the story How do supermassive black holes get so big? Your friend's email. Your email. But collisions won't happen indefinitely because the universe is big and because it's expanding, and so it's very unlikely that any sort of black hole runaway effect will occur. The late physicist Stephen Hawking proposed that while black holes get bigger by eating material, they also slowly shrink because they are losing tiny amounts of energy called "Hawking radiation.
Hawking radiation occurs because empty space, or the vacuum, is not really empty. It is actually a sea of particles continually popping into and out of existence.
Hawking showed that if a pair of such particles is created near a black hole, there is a chance that one of them will be pulled into the black hole before it is destroyed. In this event, its partner will escape into space.
The energy for this comes from the black hole, so the black hole slowly loses energy, and mass, by this process. Eventually, in theory, black holes will evaporate through Hawking radiation. But it would take much longer than the entire age of the universe for most black holes we know about to significantly evaporate. Black holes, even the ones around a few times the mass of the Sun, will be around for a really, really long time!
Want to visit a black hole? Galaxy NGC is shown in visible light and X-rays in this composite image. The X-ray light is coming from an active supermassive black hole, also known as a quasar, in the center of the galaxy. This supermassive black hole has been extensively studied due to its relatively close proximity to our galaxy.
Scientists obtained the first image of a black hole, seen here, using Event Horizon Telescope observations of the center of the galaxy M The image shows a bright ring formed as light bends due to the intense gravity around a black hole that is 6. Image credit: Event Horizon Telescope Collaboration. This animation illustrates the activity surrounding a black hole.
While the matter that has passed the black hole's event horizon can't be seen, material swirling outside this threshold is accelerated to millions of degrees and radiates in X-rays. This illustration shows a glowing stream of material from a star disrupted as it was being devoured by a supermassive black hole. The black hole is surrounded by a ring of dust.
When a star passes close enough to be swallowed by a black hole, the stellar material is stretched and compressed as it is pulled in, releasing an enormous amount of energy.
This entirely new class of black holes, would dwarf the supermassives. These "stupendously large black holes" would start at a trillion solar masses 10 times bigger than the current largest known black hole and could possibly be even bigger. Understandably, these monsters among monsters would be rare. It's hard for our universe to make large things, because you need to glue a bunch of material together and get it to settle down and stay put, which matter doesn't really like to do.
Still, it's theoretically possible for these beasts to exist. And if we find them, it would help explain how many types of black holes form. Related: The 12 strangest objects in the universe. The first black hole's appeared when the universe was very young, less than a billion years old. Over the eons, they merged and fed and grew to become supermassive black holes, and possibly the stupendously large black holes. But there's a limit to how quickly they can grow. To grow by mergers, they actually have to encounter and swallow other black holes.
So if there aren't a lot of other black holes around, mergers aren't going to happen very frequently, and that won't be a viable avenue to greatness.
On the other hand, black holes can also grow by feeding on material. But as material falls toward the event horizon considered the point of no return of a black hole, it compresses and heats up. That releases radiation, which pours out of the central regions near a black hole and prevents new gas from falling into the black hole. The complex physics of falling into a black hole then sets an upper limit to how quickly black holes can feed.
The largest known black holes are a challenge to current astrophysical knowledge. It's hard to concoct the scenario of enough mergers and enough gas feeding to grow a tiny baby black hole in the early universe into the monsters lurking in galactic cores. To find a stupendously large black hole would force us to consider new avenues for how black holes are born.
Perhaps the first, and largest, black holes didn't come from the deaths of massive stars.
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