Using observations from the NASA/ESA Hubble Space Telescope, astronomers have discovered over 250 ultra-faint dwarf galaxies in three extremely distant galaxy clusters Abell 2744, MACS0416 and MACS0717. Some of these galaxies formed just 600 million years after the Big Bang.
New images from the NASA/ESA Hubble Space Telescope show the faintest and earliest known galaxies in the Universe. This image shows the galaxy cluster MACSJ0717.5+3745 (MACS0717). It is one of the most massive galaxy clusters known, and it is also the largest known gravitational lens. Of all of the galaxy clusters known and measured, this cluster lenses the largest area of the sky. Image credit: NASA / ESA / HST Frontier Fields Team / STScI.
The team, headed by Dr Hakim Atek of the Ecole Polytechnique Fédérale de Lausanne in Switzerland, studied images of Abell 2744, MACS0416 and MACS0717 taken as part of the Hubble Frontier Fields program — a 3-year, 840-orbit program that explores the most distant regions of space through gravitational lensing effects around six different galaxy clusters.
“Clusters in the Frontier Fields act as powerful natural telescopes and unveil for us these faint dwarf galaxies that would otherwise be invisible,” said team member Dr Jean-Paul Kneib, also of the Ecole Polytechnique Fédérale de Lausanne.
“The faintest galaxies detected in these Hubble observations are fainter than any other yet uncovered in the deepest Hubble observations,” added team member Dr Johan Richard, of the Observatoire de Lyon in France.
This Hubble image shows the galaxy cluster MACS J0416.1–2403 (MACS0416). This is one of six being studied by the Hubble Frontier Fields program, which together have produced the deepest images of gravitational lensing ever made. Due to the huge mass of the cluster it is bending the light of background objects, acting as a magnifying lens. Image credit: NASA / ESA / HST Frontier Fields Team / STScI.
By looking at the light coming from the newly-discovered ultra-faint galaxies the astronomers found that the accumulated light emitted by these galaxies could have played a major role in the Epoch of Reionization.
Reionization started when the thick fog of hydrogen gas that cloaked the early Universe began to clear. UV light was now able to travel over larger distances without being blocked and the Universe became transparent to UV light.
Abell 2744, nicknamed Pandora’s Cluster, was the first of six targets within the Frontier Fields program, which together have produced the deepest images of gravitational lensing ever made. The cluster is thought to have a very violent history, having formed from a cosmic pile-up of multiple galaxy clusters. Image credit: NASA / ESA / HST Frontier Fields Team / STScI.
The scientists determined, for the first time with some confidence, that the smallest and most abundant of the galaxies in the study could be the major actors in keeping the Universe transparent.
By doing so, they have established that the Epoch of Reionization came to a close about 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 reionize the Universe,” Dr Atek said.
“We also needed to add in the contribution of a more abundant population of faint dwarf galaxies.”
This research was presented in a paper entitled “Are Ultra-faint Galaxies at z=6-8 Responsible for Cosmic Reionization? Combined Constraints from the Hubble Frontier Fields Clusters and Parallels,” by Hakim Atek et al., to appear in the Astrophysical Journal (arXiv.org preprint).
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Hakim Atek et al. 2015. Are Ultra-faint Galaxies at z=6-8 Responsible for Cosmic Reionization? Combined Constraints from the Hubble Frontier Fields Clusters and Parallels. ApJ, accepted for publication; arXiv: 1509.06764
