Team of astronomers from the European Southern Observatory, the University of Heidelberg (Germany) and the University of Queensland (Australia) presented a study which analyzes ultra-compact dwarf galaxies (UCDs) and the amount of light they produce. The main parameter characterizing the distribution of mass of stars and their relative luminosity is so-called mass-to-light ratio, which, surprisingly, in UCDs is approximately 50% higher than it could be predicted on the basis of current astrophysical models. Authors of the study say that this phenomenon may be caused by black holes residing in the centers of such compact galaxies.

Ultracompact dwarf galaxies were first discovered approximately 15 years ago, in 1998, during observation of Fornax galaxy cluster. According to their size, they are situated somewhere between star clusters and ‘regular’ dwarf galaxies, which are to some extent larger compared to UCDs. Masses of these objects vary from 2*106 solar masses for smallest, to 108 solar masses for largest UCDs, while sizes fall within 100 parsecs (pc).

There are different theories regarding formation of ultracompact dwarf galaxies. Some scientists say, these objects are the remains of galactic nuclei, while others tend to consider them as the supermassive star clusters which belong to other galaxies. The lower-end value of the mass range of 2*106 solar masses was chosen as a limit where star cluster can be considered to be a small UCD.

German and Australian team investigated massive ultracompact dwarf galaxies. The reason for such choice was the elevated mass-to-light ratio the largest magnitude of which is typically observed in the UCDs with the largest stellar mass density, and as it was already mentioned, their M/L ratio exceeds the values predicted by cosmological models by approximately 50%.

Authors of the study analyzed the possibility that such behavior can be caused by massive black holes. These objects swirl the surrounding stars, thus increasing the heat and light they generate. Scientists mention the idea that the dark matter could be the cause of this phenomenon, but it is not very likely since the available mass of dark matter predicted by theoretical models is at least several times too small to have a significant impact on stellar movement inside compact dwarf galaxies.

If not dark matter, then what else? Astronomers say, that central black hole could be another source of concentrated dark matter. And, since such black hole theoretically is located in the center of UCD, it can influence the velocities of stars around it significantly enough to cause an increase in generated amount of light.

The team performed calculations to verify this hypothesis. They determined, that black hole should contain approximately 10-15% of the overall mass of the UCD in order to explain higher mass-to-light ratio. Scientists also support a theory, that UCDs are formed from larger galaxies, approximately 109 more massive than our Sun, which lost significant part of their outer mass via process of tidal stripping. This process occurs when larger galaxies literally rob their smaller counterparts of their peripheral stars, leaving galaxy nucleus with a supermassive black hole in its center, which, in turn, determines the increased mass-to-light ratio.

Story by Alius Noreika, source: technology.org