National Optical Astronomy Observatory | 2017 Jul 11
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.
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)
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. ...
First Results from the Lyman Alpha Galaxies in the Epoch of Reionization
(LAGER) Survey: Cosmological Reionization at z ~ 7 - Zhenya Zheng et al