An international research team led by Yale University postdoctoral researcher Hakim Atek recently discovered more than 250 distant galaxies, including some of the faintest, smallest galaxies in the universe. The team relied upon new images from the Hubble Space Telescope, focusing on a trio of cosmic magnifying glasses.

Scientists have long wondered how the universe pierced the heavy veil of hydrogen gas that enshrouded it for millions of years after the Big Bang. This opaque layer of hydrogen was thick enough to block ultraviolet light, and the process of clearing away the hydrogen is known as re-ionization.

Yet the universe's largest and brightest galaxies did not produce enough energy to account for re-ionization. That's where the newly discovered, faint galaxies proved crucial to understanding this cosmic phenomenon. Atek's team found that the accumulated light from these tiny galaxies - added to the other light - would be enough to cause re-ionization.

"The most exciting part of this work was the fact that we keep unveiling fainter and fainter galaxies, and they happen to be more and more abundant," said Atek, who conducted his research at Yale and the Ecole Polytechnique Federale de Lausanne, in Switzerland. "This raises the question: What are the faintest and smallest galaxies ever formed in the early universe?"

The research team looked at Hubble Frontier Fields images of three galaxy clusters. Powerful gravitational forces generated by these clusters magnify the faint light of galaxies located far behind them; this is called gravitational lensing.

"Hubble remains unrivaled in its ability to observe the most distant galaxies, and the sheer depth of the Hubble Frontier Fields data guarantees very precise understanding of the cluster magnification effect, allowing us to make discoveries like these," said co-author Mathilde Jauzac, of Durham University-UK and the University of KwaZulu-Natal-South Africa.

The research represents one of the largest samples of dwarf galaxies ever discovered from the early universe, just 600 to 900 million years after the Big Bang. With the new information, the researchers said, they estimate the universe became fully transparent about 700 million years after the Big Bang.

Publication of the research in the Astrophysical Journal is pending. Priyamvada Natarajan, a Yale professor of astronomy and physics, is a co-author of the paper. Other co-authors represent the Observatoire de Lyon, Aix Marseille Universite, and CNRS, in France; the Universite de Geneve, in Switzerland; the University of Hawaii; and the University of Arizona.