"Lyman-Alpha Galaxies in the Epoch of Reionization” (LAGER) understood the reionization on the Cosmic Dark Ages and learned that the first galaxies from before 800 million years ago.
LAGER, initiated by WANG Junxian(University of Science and Technology of China) with an international team of astronomers from China, the US, uses a specially built narrowband filter NB964 for the superb large-area Dark-Energy Camera (DECam) on the NOAO/CTIO 4-m Blanco telescope. This project has collected an early (about 800 million years after the Big Bang, about 6% of present Universe age) samples of galaxies where intergalactic gas was 50% ionized at that epoch after systematically searching Lyman alpha emitting galaxies (at a redshift of z~7).
Long ago, about 300,000 years after the beginning of the Universe (the Big Bang), the Universe was dark. In the absence of stars and galaxies, Universe was filled with neutral hydrogen gas. On the end of Cosmic Dark Ages, the first galaxies appeared due to gravity interaction, and their energetic radiation ionized their surroundings, the intergalactic gas, illuminating and transforming the Universe.
This process during which the Universe underwent phrase transition is called reionization. Although the astronomers have known that this happened between approximately 300 million years and 1 billion years after The Big Bang and the first galaxies played a pivotal role in it, to give detailed procedure and to determine when the first galaxies formed are still challenging in Astronomy Physics. The observation on Lyman alpha emitting galaxies (LAEs) is a crucial method to study cosmological reionization. Specifically, the photons emitted from early objects were scattered by the diffuse neutral hydrogen gas, like the light beam in the mist, leaving the galaxies blocked. The greater the surrounding ionized, the more mist reduced. If the surrounding were totally ionized, the mist would disappear.
False color image of a 2 square degree region of the LAGER survey field. 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; (Image by ZHENG Zhenya (SHAO) & WANG Junxian (USTC)).
The first research paper of LAGER is going to open a new world. Over the past decade, the international progress of searching the galaxies at a redshift of z~7 and further Lyman alpha has moved very slowly because of the difficulties in observations and research of 800 million years Universe is the most frontier of the reionization. LAGER has discovered 23 candidate LAEs in the first survey field (4 in total) that was present 800 million years after the Big Bang, the largest sample detected to date at that epoch.
Detecting the sample, it found that the amount of Lyman alpha at 1billion year ago was about four times of the amount at earlier 800 million years. 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 resulted from the suppression of their Lyman alpha emission by neutral intergalactic gas, which means the mist began to dissipate at less than 6% Universe age and the first galaxies which accounted for a large proportion of early Universe need to form before 800 million years ago.
Source: http://en.ustc.edu.cn/news/201707/t20170728_280492.html
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