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 larger, brighter and more massive than the biggest globular clusters—large groups of ancient stars that are closely packed together in a somewhat spherical form—in the Milky Way. But at the same time, they are significantly more compact than typical dwarf galaxies of comparable luminosity, according to Steffen Mieske, an astronomer at the European Southern Observatory’s Very Large Telescope (VLT) facility in the Chilean Atacama Desert, who was not involved in the research.
The radius of UCDs tends not to exceed three hundred light years, while their mass only reaches up to several tens of millions of solar masses. By comparison, the radius of the Milky Way is about 50,000 light years and is thought to be hundreds of billions of times that of our Sun.
But despite Fornax’s status as a dwarf, the scientists from Lomonosov Moscow State University (MSU), Russia, 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 one of the VLT’s 8-meter telescopes. 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.
"To be able to say with complete assurance that this hypothesis is correct, we need to discover more supermassive black holes in UCDs,” Anton Afanasiev, first author of the study from the Faculty of Physics at MSU, said in a statement.
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.
