An international team of astronomers led by UC Santa Cruz and Yale University has pushed back the cosmic frontier of galaxy exploration to a time when the universe was only 5 percent of its present age. The team discovered an exceptionally luminous galaxy more than 13 billion light years from Earth and determined its exact distance using the powerful MOSFIRE instrument on the Keck I 10-meter telescope at the W. M. Keck Observatory in Hawaii.
These observations, published May 5 in Astrophysical Journal Letters, confirmed it to be the most distant galaxy currently measured, setting a new record. Known as EGS-zs8-1, the galaxy was originally identified based on its particular colors in images from NASA's Hubble and Spitzer Space Telescopes. It is one of the brightest and most massive objects in the early universe.
"One of the most dramatic discoveries from Hubble and Spitzer in recent years is the unexpected number of these very bright galaxies at early times close to when the first galaxies formed. We still don't fully understand what they are and how they relate to the very numerous fainter galaxies," said coauthor Garth Illingworth, professor of astronomy and astrophysics at UC Santa Cruz.
Forming stars
The new distance measurement also enabled the astronomers to determine that EGS-zs8-1 was still forming stars very rapidly, about 80 times faster than our galaxy does today.
"It has already built more than 15 percent of the mass of our own Milky Way today. But it had only 670 million years to do so. The universe was still very young then," said lead author Pascal Oesch of Yale University, who began studying these bright young galaxies as a Hubble Postdoctoral Fellow at UC Santa Cruz.
Only a handful of galaxies currently have accurate distances measured in this very early universe. "Every confirmation adds another piece to the puzzle of how the first generations of galaxies formed in the early universe," said coauthor Pieter van Dokkum of Yale. "Only the largest telescopes are powerful enough to reach to these large distances."
The discovery was only possible thanks to the relatively new MOSFIRE instrument on the Keck I telescope, which allows astronomers to efficiently study several galaxies at the same time. Harland Epps, professor emeritus of astronomy and astrophysics at UC Santa Cruz, designed the optics for MOSFIRE.
Measuring galaxies at these extreme distances and characterizing their properties is a main goal of astronomy over the next decade. The observations see EGS-zs8-1 at a time when the universe was undergoing very important changes: The hydrogen between galaxies was transitioning from a neutral to an ionized state. "It appears that the young stars in the early galaxies like EGS-zs8-1 were the main drivers for this transition called reionization," said coauthor Rychard Bouwens of the Leiden Observatory, who began studying the earliest galaxies with Illingworth as a postdoctoral fellow at UC Santa Cruz.
New questions
These new Keck Observatory, Hubble, and Spitzer observations together also pose new questions. They confirm that massive galaxies already existed early in the history of the universe, but that their physical properties were very different from galaxies seen around us today. Astronomers now have very strong evidence that the peculiar colors of early galaxies seen in the Spitzer Space Telescope images originate from a very rapid formation of massive, young stars, which interacted with the primordial gas in these galaxies.
The new observations underline the very exciting discoveries that NASA's James Webb Space Telescope (JWST) will enable when it is launched in 2018. In addition to pushing the cosmic frontier to even earlier cosmic times, JWST will be able to dissect the galaxy light of EGS-zs8-1 seen with the Spitzer Space Telescope and will provide astronomers with much more detailed insights into its gas properties.
"Our current observations indicate that it will be very easy to measure accurate distances to these distant galaxies in the future with the James Webb Space Telescope," Illingworth said. "The result of JWST's upcoming measurements will provide a much more complete picture of the formation of galaxies at the cosmic dawn."
