Galactic Rotation Curves Revisited: A Surprise For Dark Matter

Historically, galactic rotation curves have suggested that galaxies are surrounded by a vast amount of invisible matter, otherwise known as a dark matter halo. A few weeks ago, a team of astrophysicists published a result that completely contradicts these halo models and could even change the popular understanding of dark matter. The team found that galactic rotation curves can be calculated explicitly from a simple equation that only depends on the amount of visible matter in the galaxy. The exact implications of this finding are still unclear, but the authors do suggest a few possibilities.
For almost a century, galactic rotation curves have served as robust evidence for the existence of dark matter. A rotation curve is simply the radial velocity of the stars, dust, and gas that make up a galaxy plotted as a function of their distance from the galaxy’s center. Based on the gravitational pull of matter, one would expect that stars closest to the center of the galaxy would move faster than the stars near the galaxy's outer edge. However, in most galaxies, inner and outer stars move at roughly the same velocity. There is some additional gravitational pull on the outer stars that isn’t fully described by the amount of visible matter in a galaxy. For ages, most scientists have interpreted this result to mean that galaxies are surrounded by a halo of invisible dark matter. Image obtained under Creative Commons License. Credit: Gemini Observatory)
A galactic rotation curve is the radial velocity of the stars, dust, and gas that make up a galaxy plotted as a function of their distance from the galaxy’s center. Based on visible matter alone, one would expect that stars closest to the center of the galaxy would move faster than the stars near the galaxy’s outer edge (dashed line). However, in most galaxies inner and outer stars move at roughly the same velocity (solid line). There is some additional gravitational pull on the outer stars that isn’t fully described by the amount of visible matter in a galaxy. Most scientists have interpreted these rotation curves to mean that galaxies are surrounded by a halo of invisible dark matter. Image obtained under Creative Commons License. Credit: Gemini Observatory
At first glance, the group’s result suggests one could successfully develop a model of galactic rotation curves by modifying gravity, rather than adding in dark matter. However, astrophysicists have made several other observations of the universe that imply modifying gravity isn’t the best way to successfully describe nature. Alternatively, this result could imply a surprising coupling between regular and dark matter, making the two types of matter more correlated than expected. If this scenario were the case, the next step would be to try and probe this coupling in other dark matter experiments.
There is a great deal of excitement surrounding the announcement. It is rare in science to find such a simple equation, with no adjustable parameters that describe observed data. The finding also appears to apply to all spiral and irregular galaxies, regardless of shape or size. Such an elegant and universal relationship suggests a new discovery could be just around the corner.
Acknowledgements: Many thanks to Emma Tolley, a PhD graduate student in Physics. Emma is a member of the Harvard ATLAS group, and is currently searching for dark matter signatures at the LHC.
Managing Correspondent: Karri DiPetrillo
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