Faintest Early-Earth Galaxy Could Solve Mystery of the Cosmic Dark Ages

May 25, 2016 | Joanne Kennell

Composite image of the MACS2129.4-0741 galaxy cluster
Photo credit: BRADAC/HST/W. M. Keck Observatory. Composite image of the galaxy cluster from three different filters on the Hubble Space Telescope. The wave charts show the spectra of the imaged systems.

How did they end?

An international team of scientists has detected and confirmed the faintest early-universe galaxy ever. Using the W. M. Keck Observatory on the summit of Mauna Kea in Hawaii, the researchers detected the galaxy just as it was 13 billion years ago.

Tommaso Treu, a professor of physics and astronomy at the University of California, Los Angeles, (UCLA) College and a co-author of the research, said the discovery could be a step toward solving one of the biggest mysteries in astronomy: how a period known as the “cosmic dark ages” ended.

The research team was led by Marusa Bradac, a professor at UC Davis. Co-authors included Matthew Malkan, a UCLA professor of physics and astronomy, and UCLA graduate students Charlotte Mason, Takahiro Morishita, and Xin Wang.

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The scientists made the discovery using an effect known as gravitational lensing — similar to an image behind a glass lens appearing distorted because of how the glass warps and magnifies the light — to see the faint object, which formed just after the Big Bang.

The galaxy was hidden behind a galaxy cluster known as MACS2129.4-0741, which is so large that it created three different images of the galaxy (as seen in the picture above). The fact that they share peaks at the same wavelength (blue/green) shows that they belong to the same source — the ultra-faint galaxy.

According to the Big Bang, the universe cooled as it expanded. As it cooled, protons captured electrons to form hydrogen atoms, which in turn made the universe opaque to radiation — the start of the cosmic dark ages, Treu explained in a UCLA news release.

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"At some point, a few hundred million years later, the first stars formed and they started to produce ultraviolet light capable of ionizing hydrogen," Treu said. "Eventually, when there were enough stars, they might have been able to ionize all of the intergalactic hydrogen and create the universe as we see it now."

This ultraviolet light-producing process, called cosmic reionization, happened about 13 billion years ago, but scientists are still not sure if there were enough stars to do it or if more exotic sources, like gas falling onto supermassive black holes, were responsible.

"Currently, the most likely suspect is stars within faint galaxies that are too faint to see with our telescopes without gravitational lensing magnification," Treu said. "This study exploits gravitational lensing to demonstrate that such galaxies exist, and is thus an important step toward solving this mystery."

The results were published in the Astrophysical Journal Letters.

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