Fornax UCD3 is a part of the Fornax galaxy cluster (above) and belongs to a very rare and unusual class of galaxies – ultra-compact dwarfs (UCDs). The mass of such UCD galaxies reaches several dozen million solar masses while the radius typically does not exceed three hundred light years. Note that our Milky Way Galaxy is 120,000 light years across. This ratio between mass and size makes UCDs the densest stellar systems in the Universe.
“We have discovered a supermassive black hole in the center of Fornax UCD3. The black hole mass is 3.5 million that of the Sun, similar to the central black hole in our own Milky Way” explained physicist Anton Afanasiev, the first author of the article, at Moscow State University.
In the course of the study, the scientists used the data collected with SINFONI, an infrared integral field spectrograph installed at one of the 8-m VLT telescopes in Chile operated by the European Southern Observatory. Having analyzed the observed spectra, the authors derived the dependence between stellar velocity dispersion and radius in Fornax UCD3. Velocity dispersion quantifies the average variation between the individual stellar line-of-sight velocity and the mean velocity of the entire stellar population.
In the presence of a massive body, such as a black hole, the stars are influenced by its gravity and accelerate in various directions. Their average speed does not grow but the dispersion increases considerably. This is the very effect that was observed in this galaxy: the velocity dispersion in its center is so high that it can only be explained by the presence of a massive central black hole.
The scientists also compared the dependence of velocity and dispersion with dynamic models based on different assumptions of the black hole mass. They found that the model with a black hole mass of 3.5 million solar masses agreed with the observations best. They also considered the possibility that no black hole was present at all, but that hypothesis was excluded with at least three sigma statistical significance (99.7%).
The black hole discovered by the authors is the fourth ever to be found in UCDs and corresponds to 4% of the total galaxy mass. In “normal” galaxies this ratio is considerably lower, about 0.3%. The existence of massive black holes in UCDs is a strong argument for the tidal origin of such galaxies. According to this hypothesis, an average-sized galaxy passed a bigger and more massive one on a certain stage of its evolution. As a result, the influence of tidal forces stripped the majority of stars from the gravitational potential of the original average-sized galaxy. The remaining compact nucleus has become what we know 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. This is one of the prospects of this work.
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Lomonosov Moscow State University