Dark matter left early galaxies alone

Early spiral galaxies spun much slower than those in the current Universe, implying that dark matter did not dominate their mass at these times.


Galaxies in the nearby Universe (left) have more dark matter near their core than earlier galaxies (right), meaning they spin faster.
Credit: ESO/L. Calçada

Dark matter’s pull holds sway over galaxies in the Universe today, but new observations show that this wasn’t always the case. Evidence found by ESO’s Very Large Telescope (VLT) has found that 10 billion years ago the mysterious substance had little effect on galaxies, and they were instead dominated by normal, or baryonic, matter.

While normal matter emits radiation, allowing us to see it, dark matter does not. Its presence can only be inferred from the motion of luminous matter, such as how its mass affects the rotation of spiral galaxies. In the present day Universe, most spiral galaxies spin faster than would be expected if only normal matter were present, implying some invisible mass is causing it to spin faster. Dark matter is the name given to this mass.

Read more stories about dark matter in galaxies from BBC Sky at Night Magazine:

The team using the VLT observed six massive galaxies that are so distant we are observing them as they were 10 billion years ago, during the peak of the Universe’s galaxy formation. Unlike local galaxies, these spirals appear to be rotating more slowly in the outer regions than at the core.

“Surprisingly, the rotation velocities are not constant, but decrease further out in the galaxies,” says Reinhard Genzel from the Max Planck Institute for Extraterrestrial Physics, and who led the study. “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 neighbourhood.”

The suggestion is that three to four billion years after the Big Bang, the gas in galaxies had flattened out into rotating discs, allowing them to form their spiral structure, but the dark matter halos around them were still large and spread out. It took billions of years longer for dark matter to condense to the point it was dense enough to effect the rotation of the galactic spiral arms.


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