False color image of a 2 square degree region of the LAGER survey field,
created from images taken in the optical at 500 nm (blue), in the
near-infrared at 920 nm (red), and in a narrow-band filter centered at
964 nm (green). The last is sensitive to hydrogen Lyman alpha emission
at z ~ 7. The small white boxes indicate the positions of the 23 LAEs
discovered in the survey. The detailed insets (yellow) show two of the
brightest LAEs; they are 0.5 arcminutes on a side, and the white circles
are 5 arcseconds in diameter. Image Credit: Zhen-Ya Zheng (SHAO) &
Junxian Wang (USTC).
Milestones in the history of the Universe
(not to scale). The intergalactic gas was in a neutral state from about
300,000 years after the Big Bang until light from the first generation
of stars and galaxies began to ionize it. The gas was completely ionized
after 1 billion years. The LAGER study takes a close look at the state
of the Universe at 800 million years (yellow box) to investigate when
and how this transformation occurred. Image Credit: NAOJ.
Astronomers studying the distant Universe have found that small
star-forming galaxies were abundant when the Universe was only 800
million years old, a few percent of its present age. The results suggest
that the earliest galaxies, which illuminated and ionized the Universe,
formed at even earlier times.
Long ago, about 300,000 years after the beginning of the Universe
(the Big Bang), the Universe was dark. There were as yet no stars and
galaxies, and the Universe was filled with neutral hydrogen gas. At some
point the first galaxies appeared, and their energetic radiation
ionized their surroundings, the intergalactic gas, illuminating and
transforming the Universe.
While this dramatic transformation is known to have occurred sometime
in the interval between 300 million years and 1 billion years after the
Big Bang, determining when the first galaxies formed is a challenge.
The intergalactic gas, which is initially neutral, strongly absorbs and
scatters the ultraviolet light emitted by the galaxies, making them
difficult to detect.
To home in on when the transformation occurred, astronomers take an
indirect approach. Using the demographics of small star-forming galaxies
to determine when the intergalactic gas became ionized, they can infer
when the ionizing sources, the first galaxies, formed. If star forming
galaxies, which glow in the light of the hydrogen Lyman alpha line, are
surrounded by neutral hydrogen gas, the Lyman alpha photons are readily
scattered, much like headlights in fog, obscuring the galaxies. When the
gas is ionized, the fog lifts, and the galaxies are easier to detect.
A new study taking this approach has discovered 23 candidate Lyman
alpha emitting galaxies (LAEs) that were present 800 million years after
the Big Bang (at a redshift of z~7), the largest sample detected to
date at that epoch. The study, “Lyman-Alpha Galaxies in the Epoch of
Reionization” (LAGER), was carried out by an international team of
astronomers from China, the US, and Chile using the Dark Energy Camera
(DECam) on the CTIO 4-m Blanco telescope.
While the study detected many LAEs, it also found that LAEs were 4
times less common at 800 million years than they were a short time
later, at 1 billion years (at a redshift of z~5.7). The results imply
that the process of ionizing the Universe began early and was still
incomplete at 800 million years, with the intergalactic gas about half
neutral and half ionized at that epoch.
The low incidence rate of LAEs at 800 million years results from the
suppression of their Lyman alpha emission by neutral intergalactic gas.
The study shows that “the fog was already lifting when the universe
was 5% of its current age”, explained Sangeeta Malhotra (Goddard Space
Flight Center and Arizona State University), one of the co-leads of the
survey.
Junxian Wang (USTC), the organizer of the study, further explained,
“Our finding that the intergalactic gas is 50% ionized at z ~ 7 implies
that a large fraction of the first galaxies that ionized and
illuminated the universe formed early, less than 800 million years after
the Big Bang.”
For Zhenya Zheng (Shanghai Astronomical Observatory, CAS), the lead author of the paper describing these results, “800 million years is the current frontier in reionization studies.” While hundreds of LAEs have been found at later epochs, only about two dozen candidate LAEs were known at 800 million years prior to the current study. The new results dramatically increase the number of LAEs known at this epoch.
“None of this science would have been possible
without the widefield capabilities of DECam and its community pipeline
for data reduction,” remarked coauthor James Rhoads. “These capabilities
enable efficient surveys and thereby the discovery of faint galaxies as
well as rare, bright ones.”
To build on these results, the team is “continuing the search for
distant star forming galaxies over a larger volume of the Universe”,
said Leopoldo Infante (Pontificia Catolica University of Chile and the
Carnegie Institution for Science), “to study the clustering of LAEs.”
Clustering provides unique insights into how the fog lifts. The team is
also investigating the nature of these distant galaxies.
Reference:
“First Results from the Lyman Alpha Galaxies in the Epoch of Reionization (LAGER) Survey: Cosmological Reionization at z ~ 7,” Zhenya Zheng et al. 2017, Astrophysical Journal Letters, 842, 22.
Preprint: https://arxiv.org/abs/1703.02985
Cerro Tololo Inter-American Observatory is managed by the National
Optical Astronomy Observatory, which is operated by the Association of
Universities for Research in Astronomy Inc. (AURA) under a cooperative
agreement with the National Science Foundation.
Science Contacts
Dr. Junxian Wang
Department of Astronomy
University of Science and Technology of China
96 Jinzhai Road Hefei, Anhui 230026 China
Email: jxw@ustc.edu.cn
Dr. Sangeeta Malhotra
ASU School of Earth and Space Exploration
and
Astrophysics Science Division,
Goddard Space Flight Center
8800 Greenbelt Road
Greenbelt, Maryland 20771
Email: sangeeta.malhotra@asu.edu
