Dark Matter Was Missing From Early Galaxies, Study Reveals

Posted: Mar 18 2017, 4:17am CDT | by , Updated: Mar 18 2017, 4:39am CDT, in News | Latest Science News

 
Dark Matter was Missing from Early Galaxies, Study Reveals
Comparison of rotating disc galaxies in the distant Universe and the present day. Credit: ESO

Observations of distant galaxies suggest they were dominated by normal, visible matter

Among the most fascinating mysteries of the cosmos is the invisible, unidentified dark matter that makes up most of the universe. Scientists believe that universe consists of about 5 percent visible matter while nearly 30 percent of universe mass is made up of dark matter. But this was not always the case.

New observations indicate that early galaxies forming 10 billion years ago were dominated by normal or ordinary matter - opposite to present-day galaxies where the effects of mysterious dark matter seem to be much greater.

These surprising results are based on the observations of European Southern Observatory’s Very Large Telescope. Using ground-based telescope, researchers measured the rotation of six massive, star-forming galaxies in the distant Universe, at the peak of their formation 10 billion years ago and found that the outer regions of these distant galaxies seem to be rotating more slowly than regions closer to the core, suggesting that dark matter is present only in small quantities.

“Surprisingly, the rotation velocities are not constant, but drop with radius,” said lead researcher Professor Reinhard Genzel from the Max Planck Institute for Extraterrestrial Physics.

“The reason for this is twofold: First, 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. Second, these early disks were much more turbulent than the spiral galaxies we see in our cosmic neighborhood, so they did not need as much circular motion to be dynamically supported.”

The six galaxies mapped in this study were among a larger sample of hundred massive galaxies from early universe and the change in orbital speed gets more evident as researchers looked further and further back in time, into the early Universe. The results suggest that in the early Universe, about 3 to 4 billion years after the Big Bang, the gas in galaxies was denser at the center rather than the dark matter halos surrouding them. It is not until later times that we see its dominant effect on the orbital speed of modern disk galaxies.

“We have to be very careful when comparing these early massive and gas-rich rotating galaxies to the ones in our local universe,” said Natascha Förster Schreiber, co-author of the study. “Present-day spirals, such as our Milky Way, require additional dark matter in various amounts.”

While normal matter can be seen in the form of stars, planets or luminous gas and dust, the dark matter is not possible to detect using telescopes. It does not absorb, reflect or reemit light. It presence can only be felt from the gravitational effect it seems to have on visible matter.

The new findings also do not call into question the existence of dark matter. Instead, they suggest that dark matter was distributed differently in the past than it is now. They will add to our understanding of the role that dark matter played in early universe and how it dispersed over time.

Co-author Dr Stijn Wuyts from the University of Bath says. “We are learning that at all times, there is substantial room for other mass components than stars. At early cosmic times, molecular gas, the fuel for star formation, accounts almost entirely for this mass deficit. In nearby galaxies, such as our own Milky Way, dark matter becomes a more significant contributor to the mass budget.”

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