Milky Way has a Dark Matter deficit?

A new set of galactic formation simulations has suggested that our Milky Way could actually contain only 1/4 to a 1/3 of the previously accepted quantity of Dark Matter.

When scientists observe other spiral galaxies in the Universe one property which is commonly investigated is how the rotational velocity of the stars is related to the radius from the galactic core. The original theoretical prediction was that the rotational velocities would increase rapidly in the centre (v proportional to r), by treating the core as a solid body and then on the outer regions could be treated as having Keplerian orbits (similar to those of the planets around the sun) (v proportional to r^-2).

However when astronomers pointed there telescopes toward galaxies and analysed the rotation curve they found one reoccurring difference between theory and observation. This difference is that at an increasing distance from the centre the rotational velocity does not fall off and instead generally remain flat. This describes a system in which (at a certain distance from the core) the orbital velocities of the stars remains constant independent of radius.

Galaxy Rotation Curve

In order for this new information to be incorporated into the galactic model it was decided that there must exist a halo of some unseen matter which is driving the extra acceleration required further from the centre. Due to this and measurements of weak gravitational lensing in certain galaxy clusters it was decided that this unseen ‘Dark’ matter must make up around 80% of the matter in the universe.

A recent survey conducted at the University of California has provided two simulations of galaxy formation first with a dark halo mass of 2 trillion suns and the other 800 billion suns. It was found that the lower quantity of dark matter actually provided a better fit for the current morphology and mass of our galaxy. Does this point towards our galaxy being irregular in comparison to the average spiral galaxy? or could it perhaps point towards the current theory of dark matter being incomplete?

 source: New Scientist (


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