Based on data collected through the NASA/ESA Hubble Space Telescope, a group of astronomers discovered 252 faint dwarf galaxies within three distant galaxy clusters. The discovery is a spectacular glimpse into the formation of the early universe 600 to 900 million years after the Big Bang.

Scientists said that the dwarf galaxies found at galaxy clusters Abell 2744, MACS0416 and MACS0717 are fainter than any other galaxy in the observable universe. They said that it took 12 billion years for the light from these galaxies to reach the Hubble Space Telescope, and this enabled scientists to trace back the galaxies at their earliest forms.

In a paper presented in the Astrophysical Journal, Hakim Atek and his colleagues explained that the light emitted by the dwarf galaxies may have contributed to the epoch of reionization, a period in the early universe when thick hydrogen gas that covered everything began to clear. Ultraviolet light was then able to travel over great distances without being blocked by the hydrogen fog.

Researchers concluded that these dwarf galaxies contributed greatly in keeping the universe transparent, while they explained that the epoch of reionization ended 700 million years after the Big Bang.

"If we took into account only the contributions from bright and massive galaxies, we found that these were insufficient to reionise the Universe," said Atek. "We also needed to add in the contribution of a more abundant population of faint dwarf galaxies."

As part of the Hubble Frontier Fields program, a three-year space project which explores the farthest parts of the universe, researchers examined images of galaxy clusters through gravitational lensing effects. The galaxy clusters had accumulated and produced gravitational fields that magnified light from nearby faint galaxies found behind the clusters.

Jean-Paul Kneib, co-author of the study, said that these clusters are powerful natural telescopes that unveil faint dwarf galaxies that would otherwise be invisible.

Mathilde Jauzac, a fellow researcher, was impressed at the abilities of the Hubble to observe even the most distant galaxies. She added that the data shows a precise understanding of the cluster magnification effect.

Researchers hope that data regarding three more of the six distant galaxy clusters which the Hubble is currently observing can be revealed in the near future.

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