Another interesting fact about black holes... we can't seem to find any medium-sized ones. We've located many small ones, and many supermassive ones. But no intermediate sized black holes. Which seems really strange...
Missing Seeds: Mysterious Enigma of Supermassive Black Holes
In the vast garden of the universe, the heaviest black holes grew from seeds. Nourished by the gas and dust they consumed, or by merging with other dense objects, these seeds grew in size and heft to form the centers of galaxies, such as our own Milky Way. But unlike in the realm of plants, the seeds of giant black holes must have been black holes, too. And no one has ever found these seeds — yet.
One idea is that supermassive black holes — the equivalent of hundreds of thousands to billions of Suns in mass — grew from a population of smaller black holes that has never been seen. This elusive group, the “intermediate-mass black holes,” would weigh in somewhere between 100 and 100,000 Suns. Among the hundreds of black holes found so far, there have been plenty of relatively small ones, but none for sure in the intermediate mass-range “desert.”
Scientists are working with powerful space telescopes from NASA, as well as other observatories, to track down far-flung objects that fit the description of these exotic entities. They have found dozens of possible candidates, and are working toward confirming them as black holes. But even if they do, that opens up a whole new mystery: How did intermediate-mass black holes form?
“What is fascinating, and why people have spent so much time trying to find these intermediate-mass black holes, is because it sheds light on processes that happened in the early universe— what were the masses of relic black holes, or new formation mechanisms for black holes that we haven’t thought of yet,” said Fiona Harrison, professor of physics at Caltech in Pasadena, California, and principal investigator for NASA’s NuSTAR mission.
The smallest black holes are called “stellar mass,” with between 1 and 100 times the mass of the Sun. They form when stars explode in violent processes called supernovae.
Supermassive black holes, on the other hand, are the central anchors of large galaxies – for example, our Sun and all other stars in the Milky Way orbit a black hole called Sagittarius A* that weighs about 4.1 million solar masses. An even heavier black hole — at a whopping 6.5 billion solar masses — serves as the centerpiece for the galaxy Messier 87 (M87). M87’s supermassive black hole appears in the famous image from the Event Horizon Telescope, showing a black hole and its “shadow” for the very first time.
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The density of matter needed to create a black hole is mind-boggling.
To make a black hole 50 times the mass of the Sun, you would have to pack the equivalent of 50 Suns into a ball less than 200 miles (300 kilometers) across. But in the case of M87’s centerpiece, it is as though 6.5 billion Suns were compressed into a ball wider than the orbit of Pluto. In both cases, the density is so high that the original material must collapse into a singularity— a rip in the fabric of space-time.
Key to the mystery of black holes’ origins is the physical limit on how fast they can grow. Even the giant monsters at the centers of galaxies have limitations on their feeding frenzies, because a certain amount of material is pushed back by the high-energy radiation coming from hot particles accelerated near the event horizon. Just by eating surrounding material, a low-mass black hole might only be able to double its mass in 30 million years, for example.
“If you start from a mass of 50 solar masses, you simply cannot grow it to 1 billion solar masses over 1 billion years,” said Igor Chilingarian, an astrophysicist at the Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, and Moscow State University. But, “as we know, there are supermassive black holes that exist less than 1 billion years after the formation of the universe.”
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Intermediate-mass black hole hunters eagerly await the launch of NASA’s James Webb Space Telescope, which will peer back to the dawn of the first galaxies. Webb will help astronomers figure out which came first — the galaxy or its central black hole — and how that black hole might have been put together. In combination with X-ray observations, Webb’s infrared data will be important for identifying some of the most ancient black hole candidates.
Another new tool launched in July by the Russian space agency Roscosmos is called Spectrum X-Gamma, a spacecraft that will scan the sky in X-rays, and carries an instrument with mirrors developed and built with NASA Marshall Space Flight Center, Huntsville, Alabama. Gravitational-wave information flowing from the LIGO-Virgo collaboration will also aid in the search, as will the European Space Agency’s planned Laser Interferometer Space Antenna (LISA) mission.