Using data from the Hubble Space Telescope, a team of researchers has discovered galaxies previously unknown to astronomers. And there isn't just a handful of these hidden galaxies: They are rolling out of every crevice in the universe. But how did the sky get so packed without anyone ever realizing it?
The team used a new statistical method to analyze Hubble's observations. They examined what looked like "empty pixels" in lots of different wavelengths of light. This new way of examining the data allowed researchers to see galaxies that were previously hidden. The results have been published in Nature Communications.
"For this research, we had to look closely at what we call 'empty pixels,' the pixels between galaxies and stars," explained research leader Asantha Cooray, from the University of California, Irvine (UCI), in a statement. "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."
Three comparisons of the sky as Hubble sees it, the sky stripped of stars and galaxies and the light from the early universe. Ketron Mitchell-Wynne / UCI.
So not only are these galaxies some of the farthest objects that Hubble can detect, but the researchers also predict that there are 10 times more galaxies like this than were previously detected in deep Hubble surveys.
Cooray thinks that the galaxies are so faint because they are so young. Galaxies then were different to how we think of them now: They were more diffuse and populated with giant stars – not the well-structured galaxies, such as spiral galaxies, that we have today.
The period these youthful galaxies are from is called "epoch of reionization." After the Big Bang, the universe was filled with neutral hydrogen. This hydrogen was so dense that it absorbed all the light particles (photons), which meant there was total darkness. The epoch of reionization is the period when all this neutral hydrogen became almost completely ionized. This happened when the universe was only a few hundred million years old. These early ionized particles laid the foundation for star and galaxy evolution. Learning more about this time is critical for understanding the basic makeup of our universe.
"It's the furthest back you can study with the Hubble Space Telescope," said lead author Ketron Mitchell-Wynne from UCI.
But the details of these galaxies scant. Astronomers would need an even more powerful telescope to get a good look at one of them individually. Fortunately, there is a telescope specialized in looking at light from the early universe that has a planned launch date of 2018: the James Webb Telescope.
"This is a very exciting finding," said Henry Ferguson, an astronomer at Baltimore's Space Telescope Science Institute. "It's the first time that we've been able to convincingly measure this subtle signature of early galaxies with Hubble, giving us a firmer handle on what to look for when the James Webb Space Telescope launches a few years from now."
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