MAUNAKEA, Hawaii – A team of astrophysicists
using the W. M. Keck Observatory in Hawaii has successfully measured the
farthest galaxy ever recorded and more interestingly, captured its
hydrogen emission as seen when the Universe was less than 600 million
years old. Additionally,
the method in which the galaxy called EGSY8p7 was detected gives
important
insight into how the very first stars in the Universe lit-up after the
Big
Bang. The paper will be published shortly in the Astrophysical Journal
Letters.
Using Keck Observatory’s powerful infrared
spectrograph called MOSFIRE, the team dated the galaxy by detecting its Lyman-alpha
emission line – a signature of hot hydrogen gas heated by strong ultraviolet
emission from newly born stars. Although this is a frequently detected
signature in galaxies close to Earth, the detection of Lyman-alpha emission at
such a great distance is unexpected as it is easily absorbed by the numerous
hydrogen atoms thought to pervade the space between galaxies at the dawn of the
Universe. The result gives new insight into `cosmic reionization’, the process
by which dark clouds of hydrogen were split into their constituent protons and
electrons by the first generation of galaxies.
“We frequently see the Lyman-alpha emission
line of hydrogen in nearby objects as it is one of most reliable tracers of
star-formation,” said California Institute of Technology (Caltech) astronomer, Adi
Zitrin, lead author of the discovery paper. “However, as we penetrate deeper
into the Universe, and hence back to earlier times, the space between galaxies
contains an increasing number of dark clouds of hydrogen which absorb this
signal.”
Recent work has found the fraction of galaxies
showing this prominent line declines markedly after when the Universe was about
a billion years old, which is equivalent to a redshift of about 6. Redshift is a
measure of how much the Universe has expanded since the light left a distant
source and can only be determined for faint objects with a spectrograph on a
powerful large telescope such as the Keck Observatory’s twin 10-meter telescopes,
the largest on Earth.
"The surprising aspect about the present
discovery is that we have detected this Lyman-alpha line in an
apparently faint galaxy at a redshift of 8.68, corresponding to a time
when the Universe should be full of
absorbing hydrogen clouds,” said co-author and Caltech astronomer
Richard Ellis.
“Quite apart from breaking the earlier record redshift of 7.73, also
obtained
at the Keck Observatory, this detection is telling us something new
about how
the Universe evolved in its first few hundred million years.”
Computer simulations of cosmic reionization
suggest the Universe was fully opaque to Lyman-alpha radiation in the first 400
million years of cosmic history and then gradually, as the first galaxies were
born, the intense ultraviolet radiation from their young stars, burned off this
obscuring hydrogen in bubbles of increasing radius which, eventually, overlapped
so the entire space between galaxies became `ionized’, that is composed of free
electrons and protons. At this point the Lyman-alpha radiation was free to
travel through space unimpeded.
It may be that the galaxy we have
observed, EGSY8p7, which is unusually (intrinsically) luminous, has
special properties that enabled it to create a large bubble of ionized
hydrogen much earlier than is possible for more typical galaxies at
these times,” said Sirio Belli, a Caltech graduate student who helped
undertake the key observations. “EGSY8p7 was found to be both luminous
and at high redshift, and its colors measured by the Hubble and Spitzer
Space Telescopes indicate it may be powered by a population of unusually
hot stars.”
Because the discovery of such an early source
with powerful Lyman-alpha is somewhat unexpected, it provides new insight into
the manner by which galaxies contributed to the process of reionization.
Conceivably the process is patchy with some
regions of space evolving faster than others, for example due to variations in
the density of matter from place to place. Alternatively, EGSY8p7 may be the
first example of an early generation which unusually strong ionizing radiation.
“In some respects, the period of cosmic reionization
is the final missing piece in our overall understanding of the evolution of the
Universe,” says Zitrin. “In addition to pushing back the frontier to a time
when the Universe was only 600 million years old, what is exciting about the
present discovery is that the study of sources such as EGSY8p7 will offer new
insight into how this process occurred.”
The Caltech team
reporting on this discovery consists of Zitrin, Ellis, and Belli who
lead an international collaboration involving astronomers at Yale and
the University of Arizona, and fellow European researchers from Leiden
University in the Netherlands and the University of Durham and the
Univeristy College London in England.
The
research was funded in part by NASA.
The
W. M. Keck Observatory operates the largest, most scientifically productive
telescopes on Earth. The two, 10-meter optical/infrared telescopes near the
summit of Mauna Kea on the Island of Hawaii feature a suite of advanced
instruments including imagers, multi-object spectrographs, high-resolution
spectrographs, integral-field spectrographs and world-leading laser guide star
adaptive optics systems.
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 was made possible by funding provided by the National
Science Foundation and astronomy benefactors Gordon and Betty Moore.
Keck
Observatory is a private 501(c) 3 non-profit organization and a scientific
partnership of the California Institute of Technology, the University of
California and NASA
Science contact
Adi Zitrin
California Institute of Technology
626-278-5854
California Institute of Technology
626-278-5854
Media contact
Richard Ellis
California Institute of Technology
626-676-5530
rse@astro.caltech.edu
Source: W.M. Keck Observatory