Galaxies In the Early Universe Appear To Have Less Dark Matter Than Expected

Comparison of rotating disc galaxies in the local (left) and in the distant universe (right). Dark matter allows the outer edge disk to rotate faster in the former. ESO/L. Calçada

An international group of astronomers have discovered a distinct lack of dark matter in the disks of spiral galaxies 10 billion years ago.

The observation, reported in Nature, suggests that dark matter did not condense as quickly as the gas, leaving the core of galaxies spinning much faster than the edge of the disk. Early galaxies are richer in gas and more compact than spiral galaxies today.

The result is quite striking. Dark matter was first proposed to explain how both the core and edges of spirals rotate with the same velocity, but here we have galaxies that exhibit the expected behavior from gas-dominated objects – namely that the velocity decreases the further from the core you get.

“Surprisingly, the rotation velocities are not constant, but decrease further out in the galaxies,” lead author Reinhard Genzel, from the Max Planck Institute for Extraterrestrial Physics, said in a statement. “There are probably two causes for this. Firstly, most of these early massive galaxies are strongly dominated by normal matter, with dark matter playing a much smaller role than in the local universe. Secondly, these early discs were much more turbulent than the spiral galaxies we see in our cosmic neighborhood.”

Rotation rates of modern spiral galaxies (left) and those in the distant universe (right)

It appears that these two effects are particularly dominant during the peak of star formation and mass assembly of galaxies, which occurred between 3 and 4 billion years after the Big Bang 13.8 billion years ago. According to the detailed models, there’s an equivalent amount of dark and normal matter in these early galaxies, although the motion is dominated by the latter.

While the paper looked at the data for a handful of galaxies, the observations were part of a series of larger studies looking at 240 star-forming galaxies that confirm these findings. The researchers used the European Southern Observatory's (ESO) Very Large Telescope to investigate these objects.

While these distant galaxies are clearly spirals, they are not direct ancestors of the Milky Way. Spiral galaxies in the local universe require a lot more dark matter, so passive spheroidal galaxies are more likely to have been their descendants.

And while dark matter outweighs normal matter five to one, they have the same importance in galaxy evolution.

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