EGS77 is the farthest galaxy group identified to date. It dates
back to a time when the universe was only 680 million years old, or
less than 5 percent of its current age of 13.8 billion years. Credit: Nasa's Goddard Space Flight Center
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.
This animation shows EGS77’s place in cosmic history, flies to the
galaxies, and illustrates how ultraviolet light from their stars create
bubbles of ionized hydrogen around them. Credit: Nasa's Goddard Space Flight Center
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.
Source: W.M. Keck Observatory/News
About MOSFIRE
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 and astronomy benefactors Gordon and Betty Moore. It is currently the most in-demand instrument at the W. M. Keck Observatory.
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.
