Ancient galaxies cast long shadows across the universe, scientists say

Astronomers have just found a new way to locate galactic protoclusters, ancient and elusive clumps of gas and galaxies in the early universe: they can search for protocluster shadows.

The new technique and the results are described in an article by researchers from the Carnegie Institute for Science published in Nature.

Protoclusters in the early universe are the precursors to some of the most massive structures known, galaxy clusters, which may consist of thousands of galaxies bound together by their own gravity. Thus, studying protoclusters in the early universe can give astronomers a better understanding of how the clusters of galaxies they see closer to Earth evolved and formed.

Until recently, however, the only way astronomers could identify protoclusters was to search for areas of the early universe with a high density of galaxies. The sensitivity of these readings depends on the dust content and star formation activity in the galaxies.

The new method detailed in the article takes a different approach based on observations in ultraviolet light taken by Magellan telescopes in Chile. The researchers found that the hydrogen clouds in the protocluster absorbed the light passing through them and this created a kind of visible shadow on the galaxies behind the protoclusters.

The discovery is a good example of the value of deploying multiple techniques in astronomy, according to Carnegie astrophysicist and study co-author Gwen Rudie.

“One of the key lessons from this work is that when we map the distant universe, it’s important to bring together multiple perspectives,” she said in a statement. “Using just one technique can give a misleading picture.”

A surprising finding, the researchers say, is that the identified protoclusters show fewer galaxies than they would have expected. At least, galaxies that astronomers can see.

“We infer that half of the expected members of their galaxy are missing from our survey because they are abnormally dark,” the researchers write in the paper. “We attribute this to a surprisingly strong and early influence of the protocluster environment on the evolution of these galaxies that reduced their star formation or increased their dust content.”

Since these galaxies can appear dim in ultraviolet wavelengths, the researchers note that future observations should focus on multiple wavelengths in protoclusters. Such an investigation should find higher densities of galaxies in protoclusters. If not, it could force scientists to reconsider what they think they know about the evolution of the early cosmos.