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.

This artist’s impression shows the Milky Way galaxy. The blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter, which was first introduced by astronomers to explain the rotation properties of the galaxy and is now also an essential ingredient in current theories of the formation and evolution of galaxies. New measurements show that the amount of dark matter in a large region around the Sun is far smaller than predicted and have indicated that there is no significant dark matter at all in our neighbourhood. Credit: ESA / NASA

This artist’s impression shows the Milky Way galaxy. The blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter, which was first introduced by astronomers to explain the rotation properties of the galaxy and is now also an essential ingredient in current theories of the formation and evolution of galaxies. New measurements show that the amount of dark matter in a large region around the Sun is far smaller than predicted and have indicated that there is no significant dark matter at all in our neighbourhood. Credit: ESA / NASA

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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.

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

Dark matter is the name given to this mass.

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.

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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.