Universe’s Early Galaxies Were Less Influenced by Dark Matter

New observations from ESO’s Very Large Telescope (VLT) indicate that massive, star-forming galaxies during the peak epoch of galaxy formation, 10 billion years ago, were dominated by normal matter. This is in contrast to present-day galaxies, where the effects of dark matter seem to be much greater.

Schematic representation of rotating disc galaxies in the early Universe (right) and the present day (left). New VLT observations suggest that such massive star-forming disc galaxies in the early Universe were less influenced by dark matter (shown in red), as it was less concentrated. As a result the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe. Image credit: ESO / L. Calçada.
We see baryonic (normal) matter as brightly shining stars, glowing gas and clouds of dust. But dark matter does not emit, absorb or reflect light and can only be observed via its gravitational effects.
The presence of dark matter can explain why the outer parts of nearby spiral galaxies rotate more quickly than would be expected if only the normal matter that we can see directly were present.
Now, Dr. Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics and co-authors have used VLT’s K-band Multi Object Spectrograph(KMOS) and Spectrograph for INtegral Field Observations in the Near Infrared(SINFONI) instruments to measure the rotation of six massive, star-forming galaxies in the distant Universe, at the peak of galaxy formation 10 billion years ago.
What the team found was intriguing: unlike spiral galaxies in the modern Universe, the outer regions of these distant galaxies seem to be rotating more slowly than regions closer to the core — suggesting there is less dark matter present than expected.
"Surprisingly, the rotation velocities are not constant, but decrease further out in the galaxies,” Dr. Genzel said.
"There are probably two causes for this,” he added.
"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.”
Both effects seem to become more marked as astronomers look further and further back in time, into the early Universe.
This suggests that 3 to 4 billion years after the Big Bang, the gas in galaxies had already efficiently condensed into flat, rotating discs, while the dark matter halos surrounding them were much larger and more spread out.
Apparently it took billions of years longer for dark matter to condense as well, so its dominating effect is only seen on the rotation velocities of galaxy discs today.
This explanation is consistent with observations showing that early galaxies were much more gas-rich and compact than today’s galaxies.
(From: http://www.sci-news.com/astronomy/early-galaxies-less-influenced-dark-matter-04703.html)

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