Light from Universe’s first galaxies observed

image_3222e-Early-GalaxiesThe three panels show the different components of extragalactic background light detected with the Hubble Space Telescope. Image Credit: Ketron Mitchell-Wynne, UC Irvine. Text: Scientific American.

In between the thousands of bright galaxies that populate many Hubble Space Telescope photos of the distant cosmos are empty dark spots—tantalizing patches that could be chock-full of more galaxies if only we could see them. Now, astronomers have taken another look at those empty patches and spotted faint light streaming from stars formed only 500 million years after the Big Bang. The new results (pdf) suggest this light came from some of the first galaxies ever formed, which could be 10 times more numerous than previously thought.

This so-called “extragalactic background light” likely dates from roughly 250 million years after the Big Bang. Shortly after the birth of the universe, space was filled with a hot, dense fog of ionized gas. But over hundreds of thousands of years, the gas expanded and cooled, allowing giant clouds of hydrogen and helium to collapse and form the first stars. Ever since these stars first ignited, their light—and all the light from successive generations of stars—has been filling the universe, creating a pervasive glow throughout the darkest depths of space.

Although the extragalactic background radiation has proved arduous to conclusively detect, the light seen in the Hubble photos looks to be the most distant background light yet. Using data from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) and the Great Observatories Origins Deep Survey (GOODS), the team was able to separate out the light from later stars and galaxies, isolating the contribution from the first stars.

Graduate Student Ketron Mitchell-Wynne from the University of California, Irvine, and his colleagues looked for fluctuations in the intensity of the seemingly dark and empty pixels in Hubble photos taken from 2002 to 2012 to measure the elusive first light. The fluctuations helped them statistically determine that they were seeing a faint signal associated with the first stars and not simply noise. They then subtracted any light added by the stars within our galaxy, light added by the nearby galaxies, and even light added by the rogue stars that have been torn from their host galaxies and now occupy intergalactic space, until they were left with light solely from the early universe. [continues]

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