Astronomers have used the Hubble Space Telescope to produce one of the largest samples of early galaxies known. The objects are also the smallest galaxies ever seen by Hubble from that time.
The international team, led by Hakim Atek of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, used a phenomenon called gravitational lensing to look further into the universe than ever before. Gravitational lenses form when galaxies, or clusters of galaxies, are so massive that they deform space-time around them, magnifying and deforming the light coming from background objects.
The scientists studied images from three galaxy clusters observed by the Hubble Frontier Fields program. The full program will study an additional three galaxy clusters and explore some of the most distant regions of space. The research is published in the Astrophysical Journal.
The team found 250 galaxies dating between 600 and 950 million years after the Big Bang. The galaxies lived through the epoch of reionization, a mysterious phase of the universe that happened between 150 million and 1 billion years after the Big Bang. During this time, the universe was opaque because hydrogen clouds blocked ultraviolet (UV) light. UV light is powerful enough to free the electron in hydrogen atoms, turning hydrogen gas into plasma. As more and more stars formed, more UV light was emitted and so more hydrogen was turned into plasma and, over time, this made the universe transparent.
The team believes that these primordial galaxies played a major role during the reionization epoch. The scientists are confident that the smallest galaxies were also the most common, and the total light emitted by these galaxies is comparable with emissions from the largest galaxies.
“The bright and massive galaxies alone are not enough to account for reionization,” said Atek in a statement. “We need to take into account the contribution of a more abundant population of faint dwarf galaxies.”
The discovery shows the potential of the Frontier Fields program: Astronomers are able to probe the very dawn of galaxies by exploiting the cluster of galaxies’ gravitational lensing, detailed descriptions of the magnification patterns and Hubble’s own power.
The launch of the more powerful James Webb Space Telescope in a few years could push that frontier even further, to observe the light of the first stars in the universe.
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