Supermassive Black Hole Discovered in a Tiny Galaxy

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Stock illustration of a supermassive black hole. iStock

Scientists have found a supermassive black hole (SBH) at the center of a tiny galaxy, according to a study published in the Monthly Notices of the Royal Astronomical Society.

The galaxy, called Fornax UCD3, belongs to a recently discovered, rare and unusual class of stellar systems known as ultracompact dwarfs (UCDs), which are populated by older stars.

"UCDs are rather compact stellar systems with typical radii of about 100 parsecs (around 326 light-years), while their masses are up to 100 million solar masses," Anton Afanasiev, first author of the study from the Faculty of Physics at MSU, told Newsweek. "As a result their stars exist 'densely packed,' with interstellar distances way shorter than in our galaxy."

By comparison, the radius of the Milky Way is about 50,000 light years, while its mass is thought to be hundreds of billions of times that of our Sun.

But despite Fornax's status as a dwarf, the scientists found that the mass of the black hole at its center was equivalent to about 3.5 million suns—roughly the same as the black hole that lies at the heart of our own galaxy, known as Sagittarius A*.

The black hole at the center of Fornax, which is only the fourth ever to be found in a UCD, corresponds to around 4 percent of the galaxy's total mass. In "normal" galaxies, this ratio is significantly lower, around 0.3 percent.

To identify the black hole, the team used data collected by SINFONI, an infrared-detecting instrument at the European Southern Observatory's Very Large Telescope (VLT) facility in the Chilean Atacama Desert. The patterns in this data could only be explained by the presence of a massive central black hole, according to the researchers.

Although there are only a handful of known examples, the existence of black holes at the center of UCDs supports the tidal origin hypothesis of these galaxies.

This hypothesis states that an average-sized galaxy passed a larger and more massive one at some point in its evolution, losing the majority of its stars as a result of tidal forces. The remaining dense nucleus becomes what is referred as an ultracompact dwarf.

"We believe that presence of a black hole comprising such a fraction of galaxy total mass is clear evidence that UCDs are remnants of tidal stripping process," Afanasiev said.

"This hypothesis is backed by the fact that all UCDs hosting black holes are found in the central parts of galaxy clusters, where tidal stripping events are much more likely and the fraction of stars that are being "stolen" by a bigger galaxy is higher. But to confidently state this we need to see some more black holes—at least 5—in UCDs following proposed regularity."

Supermassive black holes are the largest type of black hole, in the order of hundreds of thousands to billions of solar masses. It is commonly accepted among scientists that an SBH lies at the center of nearly every galaxy, although this claim is impossible to state with certainty.

There is no common agreement yet among scientists whether UCDs are galaxies or not (Afanasiev thinks they more likely are than not). But either way, it is probable that most still have an SBH at their center.

"Speaking of supermassive black holes, there is a research proving a non-detection of massive black holes in the centre of two UCDs," Afanasiev said. "However we assume that the majority of UCDs still should have a central supermassive black hole, as it is dictated by what we believe is their main formation mechanism."

This article has been update to include additional comments from Anton Afanasiev.