Everyone loves a rebel. We find individuals or items that break expectations alluring, mysterious, and we find ourselves craving to know everything we can about them. Ready for your latest rule breaker? Meet the 19 dwarf galaxies that do not fit astronomers mold for typical evolution. These galaxies make up the basis of a recently published study in Nature Astronomy, that massively expands the number of galaxies that exhibit the curious quality where baryonic matter, or ordinary atomic matter, outweighs dark matter, or non atomic matter that can not be detected in any part of the electromagnetic spectrum. Thus dark matter is essentially “invisible” to astronomical instruments.
Dark matter comes from a fairly standard cosmological model that depicts the Universe as 26.6% dark matter and only 4.9% as the baryonic matter. Because dark matter makes up so much of the Universe, astronomers have theorized that dark matter is the major force behind creations of the spaces in which galaxies form and how galaxies develop their characteristic structures. Dwarf galaxies, due to their smaller size and weaker gravitational interactions, are very susceptible to dark and baryonic matter. Because these galaxies tend to have trouble maintaining the baryonic matter, such as gas clouds, dark matter typically dominates the mass observed by astronomers. Or at least they did, until these 19 galaxies were observed defying what has been seen as almost astronomical law.
However, these are not the first galaxies to display this baffling aspect. In 2016, Kyle Oman, from the University of Victoria, British Columbia, Canada, and his research team published a paper in Monthly Notices of the Royal Astronomical Society, on their discovery of two dwarf galaxies that seemed to also
have an unusually small amount of dark matter. Driven to uncover more of these baryon-dominated dwarf galaxies (BDDGs), Qi Guo, an astrophysicist at the University of Chinese Academy of Science, and her collaboration decided to use existing catalogue data from the Arecibo Observatory radio telescope. The data was used in combination with hydrogen gas galaxies from the Seventh Data Release of the Sloan Digital Sky Survey to create a sample of 324 potential BDDGS. To experimentally measure the weight of the galaxies, Guo and the researchers used the estimated circular velocity of the hydrogen cycling around the galaxies and the radius at which the hydrogen gas was detected, to compute the mass of the dwarf galaxy. In general, the faster the galaxy was moving, the more total mass there was in the galaxy. To determine how much of the total mass of the dwarf galaxies were baryonic, or normal matter, the researchers used observed light from the stars and the mass of hydrogen within the galaxy. From the total mass and the mass of all the baryonic material, the mass of the dark matter was tabulated. When the ratio of the total mass and the baryonic mass were analyzed for all 324 galaxies, only 19 showed a low value, which indicated that they had more baryonic mass at a radius where dark matter should theoretically be dominating similar to the other 305 galaxies in the study’s sample. Thus the question arises: where could this dark matter be and how does this affect the standard cosmological model?
Guo and her team developed different models to try and come up with a conclusive theory on the phenomenon that they were observing. One of the simulations predicted that interactions between BDDGs and other galaxies with high densities could be responsible for the formation of the bar
yon-dominated dwarf galaxies. If two galaxies had interacted in some way, one of the galaxies could have “stripped” the dark matter from the other galaxy leaving it with the lower circular velocity that was observed. This idea is not completely foolproof, however, since only 5 of the 19 BDDGs are in a galaxy cluster; the other 14 reside at too far of a distance from another galaxy or cluster to have that type of interaction. Other models were tested, but none were able to include a significant percentage of the baryon-dominated galaxies.
Thus, researchers arrived right back at the beginning, with 19 curious and rebellious dwarf galaxies that have refused to conform. Sadly, these galaxies do not have the numbers to develop an entirely new cosmological theory, but they are showing the possibility of an “evolution revolution” that could change the way we think about the ways of the Universe.
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