“The Cosmic Seagull” –Distant, Dark World Found Powered By a Hidden Force


The outermost stars in the Cosmic Seagull, a galaxy 11.3 billion light-years away, speed too fast to be propelled by the gravity of the galaxy’s gas and stars alone, indicating the hidden presence of dark matter, astrophysicist  “In our nearby universe, you see these halos of dark matter around galaxies like ours,” says Verónica Motta of the University of Valparaíso in Chile and her colleagues in the August 8 arXiv.org. “So we should expect that in the distant past, that halo was there, too.”

Motta and her team used radio telescopes at the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the acceleration of the speed of gas across the Cosmic Seagull’s disk, from the center out to about 9,800 light-years, revealing that the galaxy’s far reaches are dominated by the invisible force we call dark matter.

Her team’s finding contrasts with a recent claim that such distant galaxies are oddly lacking in dark matter according to a 2017 study by astronomer Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who found that due to the scarcity of dark matter in 100 distant galaxies the slower stars are found at the edges and faster stars in the central regions.

The Cosmic Seagull galaxy lies behind the Bullet Cluster of galaxies , whose regular matter (red, depicting X-rays emitted by hot gas) seems separate from its dark matter (blue, inferred by distortion of galaxies in the background). The Bullet Cluster’s mass magnified the Seagull’s image and let astronomers observe the smaller galaxy in detail.


“In the astrophysical community, the [Genzel] result has been viewed with both excitement and skepticism,” says cosmologist Richard Ellis of University College London, who was not involved in either work. “It makes a lot of sense for others to examine galaxies at these [distances] in different ways.”

Motta and her colleagues were able to probe dark matter in the most distant galaxy yet, thanks to gravitational lensing and the massive cosmic train wreck called the Bullet Cluster. The distortion from gravitational lensing reminded Motta’s team of the seagull logo of a popular music festival in Viña del Mar, Chile. But it also made the galaxy appear magnified by a factor of 50 — setting a new record.

“Motta et al have exquisite data,” but their observations are limited, Ellis wrote in an e-mail to Science News. The team looked at only one galaxy, and that galaxy is much smaller and less massive than those that seem short on dark matter. Furthermore, the observations don’t cover the entire galactic disk, so the stars may be slower farther out than the team can see.

Motta agrees that a distant slowdown is possible, although her observations cover the same portion of the galaxy’s disk as the study of galaxies that seem light on dark matter.

“We are roughly at the place in which we should see the turning point” from fast to slow stars, if it exists, she says. “But we need to extend the study to get that.” Her team has been granted more time with ALMA next year to keep looking.

The Daily Galaxy via arXiv.org  and Science News 


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