Hubble spies Big Bang frontiers

Observations by the NASA/ESA Hubble Space Telescope have taken advantage of gravitational lensing to reveal the largest sample of the faintest and earliest known galaxies in the Universe. Some of these galaxies formed just 600 million years after the Big Bang and are fainter than any other galaxy yet uncovered by Hubble. The team has determined, for the first time with some confidence, that these small galaxies were vital to creating the Universe that we see today.

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 [Credit: NASA/ESA/
HST Frontier Fields Team/STScI]

An international team of astronomers, led by Hakim Atek of the Ecole Polytechnique Federale de Lausanne, Switzerland, has discovered over 250 tiny galaxies that existed only 600-900 million years after the Big Bang [1] -- one of the largest samples of dwarf galaxies yet to be discovered at these epochs. The light from these galaxies took over 12 billion years to reach the telescope, allowing the astronomers to look back in time when the universe was still very young.

Although impressive, the number of galaxies found at this early epoch is not the team's only remarkable breakthrough, as Johan Richard from the Observatoire de Lyon, France, points out, "The faintest galaxies detected in these Hubble observations are fainter than any other yet uncovered in the deepest Hubble observations."

By looking at the light coming from the galaxies the team discovered that the accumulated light emitted by these galaxies could have played a major role in one of the most mysterious periods of the Universe's early history -- the epoch of reionisation. Reionisation started when the thick fog of hydrogen gas that cloaked the early Universe began to clear. Ultraviolet light was now able to travel over larger distances without being blocked and the Universe became transparent to ultraviolet light [2].

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 [Credit: NASA/ESA/HST Frontier Fields Team/STScI]

By observing the ultraviolet light from the galaxies found in this study the astronomers were able to calculate whether these were in fact some of the galaxies involved in the process. The team 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 reionisation -- which ends at the point when the Universe is fully transparent -- came to a close about 700 million years after the Big Bang [3].

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

To make these discoveries, the team utilised the deepest images of gravitational lensing made so far in three galaxy clusters, which were taken as part of the Hubble Frontier Fields programme [4]. These clusters generate immense gravitational fields capable of magnifying the light from the faint galaxies that lie far behind the clusters themselves. This makes it possible to search for, and study, the first generation of galaxies in the Universe.

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 
[Credit: NASA/ESA/HST Frontier Fields Team/STScI]

Jean-Paul Kneib, co-author of the study from the Ecole Polytechnique Federale de Lausanne, Switzerland, explains, "Clusters in the Frontier Fields act as powerful natural telescopes and unveil these faint dwarf galaxies that would otherwise be invisible."

Co-author of the study Mathilde Jauzac, from Durham University, UK, and the University of KwaZulu-Natal, South Africa, remarks on the significance of the discovery and Hubble's role in it,"Hubble remains unrivalled in its ability to observe the most distant galaxies. The sheer depth of the Hubble Frontier Field data guarantees a very precise understanding of the cluster magnification effect, allowing us to make discoveries like these."

These results highlight the impressive possibilities of the Frontier Fields programme with more galaxies, at even earlier time, likely to be revealed when Hubble peers at three more of these galaxy clusters in the near future.

The findings appear in the Astrophysical Journal (arXiv.org preprint).

Notes

[1] The calculated redshift for these objects is between z = 6 and z = 8.

[2] Neutral hydrogen gas absorbs all the high-energy ultraviolet light emitted by hot young stars very efficiently. At the same time, the absorbed ultraviolet light ionises the hydrogen. The very low density ionised hydrogen gas filling the universe became fully transparent. The hot stars carve out transparent bubbles in the gas and once all these bubbles merge to fill all of space, reionisation is said to be complete and the Universe becomes transparent to ultraviolet light.

[3] This corresponds to a redshift of about z = 7.5.

[4] The Hubble Frontier Fields is a three-year, 840-orbit programme which will yield the deepest views of the Universe to date, combining the power of Hubble with the gravitational amplification of light around six different galaxy clusters to explore more distant regions of space than could otherwise be seen.

Source: ESA/Hubble Information Centre [October 23, 2015]