A few years ago, researchers discovered the existence of galaxies that had a bit too much regular matter in them. A new study now adds 19 more objects to this collection, a find that continues to create headaches for researchers.
Our models envision galaxies forming in the gravitational wells of wide concentrations of dark matter. This is a hypothetical form of matter that doesn’t interact with light, just gravity. Most galaxies in the universe conform to a precise ratio of regular matter to dark matter, the latter outweighing it by about one to five.
In dwarf galaxies, these ratios can be even more extreme, from 10 to even a thousand times more dark than regular matter. But in these particular galaxies, regular matter actually dominates over dark matter. And it is not just bizarre, it is an issue that challenges the standard model of cosmology. As reported in Nature Astronomy a few weeks ago, researchers described the discovery of these 19 dark matter-poor dwarf galaxies. In particular, 14 are isolated away from the influence of heavier galaxies.
"This result is very hard to explain using the standard galaxy formation model in the context of concordance cosmology, and thus encourages people to revisit the nature of dark matter," lead author Professor Qi Guo, from the Chinese Academy of Science, said in a statement.
The discovery was possible by combing data from the Sloan Digital Sky Survey and the release of 40 percent of the data from the Arecibo Legacy Fast (ALFA) catalog. Most dwarf galaxies studied before were “local” around the Milky Way or other relatively nearby galaxies. Dwarf galaxies further afield were too faint to be clearly visible and couldn’t be studied in detail.
The team is now calling for deeper observations of these galaxies. They know that regular matter dominates the galaxies' central few thousands of light-years, but more data is needed to precisely measure the ratio between dark and non-matter in these objects.
If the findings are confirmed, these galaxies will provide more weight to the growing body of evidence that galaxies can form without a lot of dark matter in them or that they can lose it, although we do not know yet how that may happen.