Supermassive black holes (SMBHs) are found at the core of almost every galaxy. They weigh millions and sometimes billions of times the mass of our Sun and play a major role in shaping their host galaxy. How they form, however, remains a mystery.
The "direct collapse black hole" theory suggests they don’t have to go through a star phase to become black holes. A big enough cloud of gas could collapse on itself and form a large black hole. “Could,” however, doesn’t mean that it did happen, but researchers now report some new and interesting evidence to support the theory.
As published in the Monthly Notices of the Royal Astronomical Society, a team from the University of Cardiff has spotted one of the smallest black holes at the center of a galaxy. It is still between 200,000 and 1 million times the mass of the Sun, but compared to the average, it is not that “supermassive.”
The black hole was found at the center of a galaxy called Mirach’s Ghost. Mirach is the second-brightest star in the constellation of Andromeda, and the star and the galaxy appear close to each other in our line of sight. The brightness of the star makes it difficult to observe this galaxy, hence its peculiar name.
"The SMBH in Mirach's Ghost appears to have a mass within the range predicted by 'direct collapse' models," lead author Dr Tim Davis from Cardiff University's School of Physics and Astronomy, said in a statement. "We know it is currently active and swallowing gas, so some of the more extreme 'direct collapse' models that only make very massive SMBHs cannot be true.”
This work does not confirm that this direct collapse is the right theory but provides some important insights and a more stringent framework. The alternative to the direct collapse is that these supermassive black holes were “seeded” by the black holes formed from massive stars, although this model is not ideal to explain how supermassive black holes grew so big so quickly.
"SMBHs have also been found in very distant galaxies as they appeared just a few hundred million years after the big bang," added co-author Dr Marc Sarzi, from the Armagh Observatory & Planetarium. "This suggests that at least some SMBHs could have grown very massive in a very short time, which is hard to explain according to models for the formation and evolution of galaxies."
The observations were possible thanks to the Atacama Large Millimeter/submillimeter Array (ALMA). Mirach’s Ghost is 10 million light-years from Earth and ALMA was able to observe gas clouds as small as 1.5 light-years across. This is the most detailed map of gas in another galaxy, which allowed researchers to estimate the mass of this peculiar black hole.
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