An international team of scientists from the United States and China found that when the Universe was 800 million years old, small galaxies with active star formation were formed in it. The beginning of this process coincided with the end of the Dark Ages - a period when there were no bright sources of light in space, and all space was filled with neutral hydrogen and helium. Press release posted on Phys.org,
According to modern scientific concepts, 300-1000 million years after the Big Bang, the first galaxies began to form in the Universe. Their radiation contributed to the ionization of intergalactic gas, as a result of which the Dark Ages gave way to the era of reionization.
To detect these relict star systems, scientists have determined the position of the so-called Lyman limit in the spectrum of radiation emitted by distant galaxies. The Lyman limit is the minimum possible wavelength of a photon emitted by excited hydrogen. Because ancient stars are predominantly hydrogen, they only emit ultraviolet light at wavelengths above this limit.
The further the galaxy is from the Earth, the faster it moves away from us and the stronger the Lyman limit is shifted to the red (longwave) side due to the Doppler effect. The redshift can be used to determine the distance to the star system. Although ultraviolet light is absorbed by the Earth's atmosphere, sufficiently "reddened" radiation reaches the surface and can be detected by ground-based observatories. Astronomers using the Lyman limit were able to detect hundreds of galaxies for which the redshift (z) exceeds 6. These objects existed when the universe was less than a billion years old.
To identify the most ancient of these galaxies, the researchers turned their attention to the brightness of the objects. The ultraviolet light emitted by them is easily scattered by the surrounding neutral gas. As a result, the light source appears to be "hazy". Since there was a lot of neutral hydrogen at the very beginning of the era of reionization, galaxies of this time look more blurry than those that formed later.
Astronomers using this approach have found 23 star systems that are the most likely candidates for the very first galaxies that emerged after the end of the dark ages. Their redshift reached seven, which corresponds to the age of the universe 800 million years (about 13 billion years ago).