Searching for Light in Darkness

 

To observe CDG-2, the researchers used data from three telescopes — Hubble, the European Space Agency’s Euclid space observatory and the Subaru Telescope in Hawaii — along with a novel approach that involved looking for objects called globular clusters. “These are very tight, spherical groupings of very olds stars, basically the relics of the first generation of star formation,” Li said.

Globular clusters are bright even if the surrounding galaxy is not, and previous observations have shown a relationship between them and the presence of dark matter in a galaxy, Li added. Because CDG-2 appears to have very few stars, there must be something else providing the mass that the clusters need to hold themselves together. Li and his colleagues assume that the source of the mass is dark matter.

The researchers found a set of four globular clusters in the Perseus Cluster, a group of thousands of galaxies immersed in a cloud of gas and one of the most massive objects in the universe. Further observations revealed a glow or halo around the globular clusters, suggesting the presence of a galaxy.

But how does a galaxy end up with few or no stars and mostly dark matter?

Astronomers believe, Li explained, that after the formation of the clusters early in the galaxy’s existence, larger surrounding galaxies stripped it of the hydrogen gas required to make more individual stars like our sun. “The material that this galaxy needed to continue to form stars was no longer there, so it was left with basically just a dark matter halo and the four globular clusters.” The process, he added, would leave behind a skeleton or ghost of “a galaxy that pretty much just failed.”

As a result of this formation mechanism, the galaxy only has 0.005% of the brightness of our own galaxy, Li said. “In terms of starlight, it is about 6 million times as bright as our sun. Our own galaxy’s brightness is about 20 billion times the brightness of the sun,” he noted.

Looking for globular clusters could be “an entirely new method to find these potentially dark galaxies,” Li argued, adding that they should exist in abundance. However, more observations are needed to detail the physical properties of CDG-2 and confirm how much dark matter it contains, which Li said could be achieved by using the James Webb Space Telescope.

‘Dark is slightly bright’

Studying potential dark galaxies is important because they provide nearly pristine views of the behavior of dark matter, according to Neal Dalal, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, who was not involved with the study.

“In big galaxies with lots of stars, like our Milky Way galaxy, the stars and gas can have a significant impact on the distribution of dark matter, making it difficult to disentangle the effects of ordinary matter from the effects of dark matter,” he wrote in an email. But in these extremely faint galaxies, there are so few stars and so little gas that the behavior of their dark matter must be nearly unaffected by the ordinary matter, he concluded. “Therefore we get a much cleaner probe of dark matter physics.”

One of the most interesting things about this galaxy is the way it was found, using globular clusters, said Robert Minchin, an astronomer at the National Radio Astronomy Observatory in Socorro, New Mexico, via email. “It seems odd, at first sight, to search for light from dark galaxies but, to misquote the Princess Bride, ‘There’s a big difference between mostly dark and all dark. Mostly dark is slightly bright,’” he noted. Minchin also did not participate in the work.

- Author: Jacopo Prisco, CNN