An article published in the journal “Nature Astronomy” describes the discovery of the best candidate found so far for a type of black hole that’s been elusive for a long time. A team led by Dacheng Lin of the University of New Hampshire’s Space Science Center used observations from a number of telescopes to detect flares at various wavelengths emitted in the area near an intermediate-mass black hole while destroying a nearby star in what is called the tidal disruption event.
In recent decades it’s become normal to identify relatively small black holes, what remains after a supernova if there’s enough mass left, and supermassive black holes at the center of the galaxies, which can have masses of millions if not billions of times the Sun’s. According to a theory, there should also be intermediate-mass black holes, between one 100 and one 100.000 times the Sun’s, but after years of discussion, only a few candidates have recently been found that could provide proof of their existence.
It’s not clear how an intermediate mass black hole can be formed and there could be different processes. One possibility is that within a star cluster with massive stars close to each other some of them can merge. The problem is that the stars of the cluster absorb all the gas in the area so it’s almost impossible to detect the presence of a black hole thanks to the typical emissions that are present due to materials being swallowed after getting attracted from the surrounding area.
Dacheng Lin’s team tried to find the typical emissions of tidal disruption events, which is the destruction of a star caused by a black hole it approached too closely. In that case, there are flares with strong electromagnetic emissions. This was detected in events related to supermassive black holes at the center of various galaxies such as Arp 299B but the researchers tried to find them even in galaxies’ outskirts.
In the archives of the observations made by ESA’s XMM-Newton space telescope, the researchers found a candidate in a galaxy known only as 6dFGS gJ215022.2-055059 distant about 740 million light years from Earth. These data collected between 2006 and 2009 during a galaxy survey and thanks to them it was possible to find others in the archives of two NASA space telescopes: Chandra, collected between 2006 and 2016, and Swift, in 2014.
In the next phase of the research, images of the galaxy 6dFGS gJ215022.2-055059 taken from other telescopes were used to check for other emissions at the candidate at other wavelengths, including those of visible light. They found two images from 2005 showing flares in the area where the candidate was located. Putting together all the data they determined that probably the tidal disruption event occurred in 2003 generating a remarkable flare whose intensity has decreased in the following decade.
The distribution of the photons emitted during that event depends on the size of the black hole. The consequence is that the observations found in the various archives provided enough data to estimate the mass of the one identified in about 50,000 times the Sun’s. It’s within the range for intermediate-mass black holes therefore if the estimate is correct the candidate is of the type that so far was so elusive.
The image (Optical: NASA/ESA/Hubble/STScI; X-ray: NASA/CXC/UNH/D. Lin et al.) shows the galaxy 6dFGS gJ215022.2-055059 in white-yellow at the center seen by the Hubble space telescope combined with observations of the intermediate-mass black hole candidate at the outskirts of the galaxy in white-purple to its lower left obtained by the Chandra space observatory.
Intermediate-mass black holes may be common but rarely active and therefore difficult to detect. Natalie Webb, co-author of this research and director of the XMM-Newton Survey Science Center at the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France, pointed out that the catalog of X-ray sources created thanks to the observations of the XMM-Newton space telescope is the largest of that type and could hide more traces of that kind of black holes. Studying them is important because they’re considered the “seeds” of supermassive black holes, which have a considerable influence on their host galaxies, starting with that on star formation.