Original | Lee Mohon
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Supermassive black holes found in relatively small galaxies could help astronomers unravel the mystery of how supermassive black holes form.
Using NASA's Chandra X-ray Observatory, the researchers found a black hole in the gas and dust of the Mrk 462 galaxy with a mass about 200,000 times that of the Sun.
With only a few hundred million stars, Mrk 462 is a dwarf galaxy. In contrast, our Milky Way galaxy has hundreds of billions of stars. This is the first time a hidden supermassive black hole has been found in a dwarf galaxy.
"The black hole on Mrk 462 is the smallest of the supermassive black holes," said Jack Parker of Dartmouth College in New Hampshire, who led the study with colleague Ryan Hickox, also from Dartmouth. "As we all know, such black holes are difficult to find."
In larger galaxies, astronomers often discover black holes by looking for the rapid motion of stars at the center of the galaxy. However, dwarf galaxies are too small and too dark for most instruments to detect at this time. Another technique is to look for features of a growing black hole, such as when it falls toward the black hole, where the glowing gas of X-rays is heated to millions of degrees.
For the study, the researchers used the Chandra X-ray Observatory to observe eight dwarf galaxies that had previously shown signs of black hole growth from optical data collected by the Sloan Digital Sky Survey (SDSS). Of those 8 black holes, only mrk462 showed the X-ray signature of a growing black hole.
Compared with low-energy X-rays, the intensity of high-energy X-rays is unusually large. Compared to other wavelength data, the Mrk 462 black hole is heavily obscured by gas.
Hickox said: "Because hidden black holes are harder to detect than exposed black holes, finding this example could mean that there are more dwarf galaxies with similar black holes." "This is important because it can help solve a major problem in astrophysics: How did black holes become so large in the early universe?"
Previous research has shown that when the universe is less than 1 billion years old (only a fraction of the current cosmic age), a black hole can reach 1 billion times the mass of the sun. One view is that these big guys are created when massive stars collapse into black holes, which are only about 100 times the mass of the sun. Theoretically, however, it is difficult to explain how they quickly increased in weight to the same sizes seen in the early universe.
Another explanation is that the early universe was formed by black holes with tens of thousands of solar masses — possibly formed by the collapse of huge clouds of gas and dust.
In dwarf galaxies with supermassive black holes, most examples support the idea that the seeds of small black holes formed by early stars grew at an alarming rate, forming objects billions of times the mass of the Sun in the early universe. A few examples support the alternative view that black holes have the mass of thousands of suns at the beginning of their birth.
These expectations apply because the conditions necessary for a direct collapse from a massive nebula to a medium-sized black hole should be rare, so we don't think that most dwarf galaxies will contain supermassive black holes. Stellar-mass black holes, on the other hand, are expected to appear in every galaxy.
"We can't draw strong conclusions from one example, but this result should encourage a broader search for those hidden black holes in dwarf galaxies," Parker said. "It's exciting that we might learn something new from it."
original:
https://www.nasa.gov/mission_pages/chandra/images/mini-monster-black-hole-could-hold-clues-to-giant-s-growth.html