Some of the most massive galaxy clusters’ ancestors have been hiding in plain sight. New research led by Carnegie’s Andrew Newman shows a new technique for identifying the precursors of the most extreme galactic environments. The findings of the team have been published in Nature.
Galaxies, like all of us, are shaped and molded by their surroundings. To obtain a complete picture of the various physical influences on the lifecycle of a galaxy, it is critical to trace the emergence of properties caused by environmental factors as they emerge.
“We’ve known for a long time that the colors, masses, and shapes of galaxies are affected by their cosmic environment,” Newman explained.
Galaxy clusters are the most massive structures in the universe, held together by gravity and containing thousands of galaxies. Protoclusters, the forerunners of these massive galactic environments, are among the earliest known structures in the universe and are ideal for studying the early stages of a galaxy’s lifecycle.
The Big Bang created the universe as a hot, murky soup of expanding particles. Some parts of the universe were denser than others, and their gravity eventually overcame the universe’s expansion, causing material to collapse inward, forming clumps of structure that would eventually give birth to groups or clusters of galaxies.
However, the team discovered that these protoclusters can be surprisingly difficult to detect.
Most protoclusters have been discovered so far by searching the sky for regions with an unusually large number of distant galaxies. The team, which included Carnegie’s Gwen Rudie, Guillermo Blanc, Mahdi Qezlou, Daniel Kelson, Alan Dressler, and Observatories Director John Mulchaey, devised a new method for locating protoclusters.
The key to their method is that the protocluster’s intergalactic hydrogen gas absorbs light passing through it, casting a shadow on the galaxies behind it, beyond the protocluster’s boundaries. The team was able to identify the likely locations of protoclusters by observing such shadows with the Magellan telescopes at Carnegie’s Las Campanas Observatory in Chile.
“We were very surprised to find so few galaxies inside the protoclusters that cast the strongest shadows,” Newman said. “This is why these protoclusters were missed by previous searches.”
Their findings imply that the unseen galaxies in the overlooked protoclusters must have evolved differently, making them dimmer and thus harder to detect. The team intends to conduct additional searches to find these missing galaxies and learn more about their unusual properties.
“It’s interesting that these galaxies’ evolutionary paths diverged so early in the history of the universe, long before the clusters they now live in were assembled,” Blanc said.
“One of the key lessons from this work is that as we map the distant universe, it’s important to bring together multiple perspectives; using only one technique can give a misleading picture,” Rudie added.
Future protocluster searches, according to the researchers, will be able to test their findings against a larger population.