Maunakea, Hawaii – An international team of astronomers funded in part by NASA has found the farthest galaxy group identified to date. With the help of W. M. Keck Observatory on Maunakea, Hawaii, the team confirmed the trio of galaxies called EGS77 dates to a time when the universe was only 680 million years old, or less than 5% of its current age of 13.8 billion years.
More significantly, observations show the galaxies are participants in a sweeping cosmic makeover called reionization. The era began when light from the first stars changed the nature of hydrogen throughout the universe in a manner akin to a frozen lake melting in the spring. This transformed the dark, light-quenching early cosmos into the one we see around us today.
“The young universe was filled with hydrogen atoms, which so attenuate ultraviolet light that they block our view of early galaxies,” said James Rhoads at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who presented the findings on Jan. 5 at the 235th meeting of the American Astronomical Society in Honolulu. “EGS77 is the first galaxy group caught in the act of clearing out this cosmic fog.”
While more distant individual galaxies have been observed, EGS77 is the farthest galaxy group to date showing the specific wavelengths of far-ultraviolet light revealed by reionization. This emission, called Lyman alpha light, is prominent in all members of EGS77.
In its infancy, the universe was a hot, opaque plasma. After expanding and cooling for about 380,000 years, the universe formed the first atoms — more than 90% of them hydrogen. Hundreds of millions of years later, this gas formed the first stars and galaxies. But the very presence of this abundant gas poses challenges for spotting galaxies in the early universe.
In its earliest phase, the universe was a glowing plasma of particles, including electrons, protons, atomic nuclei, and light. Atoms could not yet exist. The universe was in an ionized state, similar to the gas inside a lighted neon sign or fluorescent tube.
This illustration of the EGS77 galaxy group shows the galaxies surrounded by overlapping bubbles of hydrogen ionized by ultraviolet light from their stars. By transforming light-quenching hydrogen atoms to ionized gas, UV starlight is thought to have formed such bubbles throughout the early universe, gradually transitioning it from opaque to completely transparent. CREDIT: NASA’S GODDARD SPACE FLIGHT CENTER
After the universe expanded and cooled for about 380,000 years, electrons and protons combined into the first atoms — more than 90% of them hydrogen. Hundreds of millions of years later, this gas formed the first stars and galaxies. But the very presence of this abundant gas poses challenges for spotting galaxies in the early universe.
Hydrogen atoms readily absorb and quickly re-emit far-ultraviolet light known as Lyman alpha emission, which has a wavelength of 121.6 nanometers. When the first stars formed, some of the light they produced matched this wavelength. Because Lyman alpha light easily interacted with hydrogen atoms, it couldn’t travel far before the gas scattered it in random directions.
“Intense light from galaxies can ionize the surrounding hydrogen gas, forming bubbles that allow starlight to travel freely,” said team member Vithal Tilvi, a researcher at Arizona State University in Tempe. “EGS77 has formed a large bubble that allows its light to travel to Earth without much attenuation. Eventually, bubbles like these grew around all galaxies and filled intergalactic space, reionizing the universe and clearing the way for light to travel across the cosmos.”
EGS77 was discovered as part of the Cosmic Deep And Wide Narrowband (Cosmic DAWN) survey, for which Rhoads serves as principal investigator. The team imaged a small area in the constellation Boötes using a custom-built filter on the National Optical Astronomy Observatory’s Extremely Wide-Field InfraRed Imager (NEWFIRM), which was attached to the 4-meter Mayall telescope at Kitt Peak National Observatory near Tucson, Arizona.
Because the universe is expanding, Lyman alpha light from EGS77 has been stretched out during its travels, so astronomers actually detect it at near-infrared wavelengths. We can’t see these galaxies in visible light now because that light started out at shorter wavelengths than Lyman alpha and was scattered by the fog of hydrogen atoms.
To help select distant candidates, the researchers compared their images with publicly available data of the same region taken by NASA’s Hubble and Spitzer space telescopes. Galaxies appearing brightly in near-infrared images were tagged as possibilities, while those appearing in visible light were rejected as being too close.
The team confirmed the distances to EGS77’s galaxies by using the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) on the Keck I telescope at the W. M. Keck Observatory on Maunakea, Hawaii. The three galaxies all show Lyman alpha emission lines at slightly different wavelengths, reflecting slightly different distances. The separation between adjacent galaxies is about 2.3 million light-years, or slightly closer than the distance between the Andromeda galaxy and our own Milky Way.
A paper describing the findings, led by Tilvi, has been submitted to The Astrophysical Journal.
“While this is the first galaxy group identified as being responsible for cosmic reionization, future NASA missions will tell us much more,” said co-author Sangeeta Malhotra at Goddard. “The upcoming James Webb Space Telescope is sensitive to Lyman alpha emission from even fainter galaxies at these distances, and may find more galaxies within EGS77.”
Astronomers expect that similar reionization bubbles from this era will be rare and hard to find. NASA’s planned Wide Field Infrared Survey Telescope (WFIRST) may be able to uncover additional examples, further illuminating this important transition in cosmic history.
MOSFIRE (Multi-Object Spectrograph for Infrared Exploration) is a highly-efficient instrument that can take images or up to 46 simultaneous spectra. Using a sensitive state-of-the-art detector and electronics system, MOSFIRE obtains observations fainter than any other near infrared spectrograph. MOSFIRE is an excellent tool for studying complex star or galaxy fields, including distant galaxies in the early Universe, as well as star clusters in our own Galaxy. MOSFIRE obtained first light in April 2012 and was made possible by funding provided by the National Science Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems.
Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.
The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.