An international team of scientists has generated the most accurate statistical description yet of early galaxies as they existed in the Universe about a half billion years after the Big Bang. In a paper published online in the journal Nature Communications, they describe the use of a novel statistical method to analyze data captured by the NASA/ESA Hubble Space Telescope during deep-sky surveys.
These panels show different components of near-infrared background light detected by the NASA/ ESA Hubble Space Telescope in deep-sky surveys. The one on the upper left is a mosaic of images taken over a ten-year period. When all the stars and galaxies are masked, the background signals can be isolated, as seen in the second and third panels. The one on the upper right reveals ‘intrahalo light’ from rogue stars torn from their host galaxies, and the lower panel captures the signature of the first galaxies formed in the Universe. Image credit: Ketron Mitchell-Wynne / University of California, Irvine.
The method enabled the astronomers to parse out signals from the noise in Hubble’s images, providing the first estimate of the number of primordial galaxies in the early Universe.
The researchers concluded that there are close to ten times more of these galaxies than were previously detected in deep Hubble surveys.
“The time period under investigation is known as the epoch of reionization,” said lead author Ketron Mitchell-Wynne of the University of California, Irvine.
Coming after the Big Bang and a few hundred million years in which the Universe was dominated by photon-absorbing neutral hydrogen, the epoch of reionization was characterized by a phase transition of hydrogen gas due to the accelerated process of star and galaxy formation.
“It’s the furthest back you can study with Hubble,” Mitchell-Wynne said. “Hubble’s cameras utilize charge-coupled devices, high-quality electronic image sensors first used in astronomy that later were employed in professional video cameras.”
Mitchell-Wynne and co-authors looked at data spanning optical and infrared wavelengths. Photons in the infrared spectrum come directly from stars and galaxies.
Co-author Prof Asantha Cooray, also from the University of California, Irvine, pointed to recent probes into extragalactic infrared background light by the California Institute of Technology’s CIBER instrument.
“CIBER measured the infrared background at two wavelengths, 1.1 and 1.6 microns. These measurements led the CIBER group to confirm the existence of ‘intrahalo light’ from stars distributed outside galaxies,” Prof Cooray said.
“We believe it’s true that there is intrahalo light, but we made a new discovery by looking at five infrared bands with Hubble,” he said.
“We sort of overlap with CIBER and then go into short optical wavelengths, and we see in addition to intrahalo light a new component – stars and galaxies that formed first in the Universe.”
“From the CIBER analysis, we knew there would be a detection of intrahalo light in the infrared bands. We didn’t really know what to expect in the optical ones,” Mitchell-Wynne said.
“With Hubble data, we saw a large drop in the amplitude of the signal between the two. With that spectra, we started to get a little more confident that we were seeing the earliest galaxies.”
Prof Cooray added: “for this research, we had to look closely at what we call ‘empty pixels,’ the pixels between galaxies and stars.”
“We can separate noise from the faint signal associated with first galaxies by looking at the variations in the intensity from one pixel to another. We pick out a statistical signal that says there is a population of faint objects. We do not see that signal in the optical wavelengths, only in infrared. This is confirmation that the signal is from early times in the Universe.”
“These primordial galaxies were very different from the well-defined spiral and disc-shaped galaxies currently visible in the Universe. They were more diffuse and populated by giant stars.”
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Ketron Mitchell-Wynne et al. 2015. Ultraviolet luminosity density of the Universe during the epoch of reionization. Nature Communications 6, article number: 7945; doi: 10.1038/ncomms8945
