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Paper of the month:  The Radial Acceleration Relation in Rotationally Supported Galaxies

A galaxy rotation curve is a plot of the orbital velocities of stars or gas in the observed galaxy versus the radial distance from the galaxy's center. What turns out is that galaxies in our universe seem to achieve too high velocities, such that the gravity generated by the observable matter (stars, gas) could not hold them together. This has led scientists to believe that there is some extra matter, not visible, that generates the extra gravity that galaxies need to stay intact. This extra matter is what we call dark matter, one of the most fascinating problems in physics nowadays. Unlike normal matter, dark matter does not interact with the electromagnetic force; therefore it does not absorb, emit or reflect light and because of this it is very hard to detect. It is possible that it interacts through gravity and weak force (WIMPs-weakly interacting massive particles), but it is also possible that it interacts only through gravity, which would make it even harder to spot.

November 28, 2016 by tiinatimonen

There are also other indications for the presence of this new type of non-baryonic matter, maybe the most famous is the Bullet Cluster, two colliding clusters of galaxies with an evidence of a split between visible matter and dark matter. However, it should be noted that there are alternative theories, which try to justify the observations, like the ones that modify the laws of gravity. The earliest was Milgrom's Modi ed Newtonian Dynamics (MOND) in 1983, which modi es Newton's law. Although these theories fail in describing observations such as the Bullet Cluster mentioned above, they seem to be still fascinating for many scientists.


In this paper the authors study the radial acceleration relation in Rotationally Supported Galaxies. To do this, the authors use the Spitzer Photometry and Accurate Rotation Curves (SPARC) database which includes 175 disk galaxies. The key point is that, given the observed distribution of stars and gas, baryonic mass models can be constructed and the acceleration due to the baryonic components can be determined.


The authors find that the observed centripetal acceleration correlates strongly with that predicted by the baryons for all galaxies of all types (see Fig. 1).


This relation allows to deduce the distribution of dark matter entirely in terms of baryons, hence the baryon and the dark matter mass are strongly coupled.


One possible explanation of this relation could come from new dynamical laws as proposed by MOND models but up to now there is no entirely satisfactory interpretation of the empricial function found by the authors.


Figure 1: The centripetal acceleration observed in rotation curves is plotted against that predicted for the observed distribution of baryons. Nearly 2700 individual data points for 153 SPARC galaxies are shown in grayscale.

Text by Andrea Caputo and Julia Gehrlein