Scientists have created the most accurate statistical description of the earliest-known galaxies known to science. These observations provide the first estimate of the number of primordial galaxies in the early universe.
Astronomers from the University of California, Irvine and Baltimore's Space Telescope Science Institute described faint, 500 billion-year-old galaxies that formed right after the Big Bang in stunning detail. The team used a new statistical analysis method to analyze Hubble Space Telescope data, allowing them to "parse out" noise from the Hubble's deep-sky images. The findings suggest there are 10 times more small, primordial galaxies than previously believed. The "epoch of reionization" occurred right after the Big Bang, and was characterized by darkness as photons were absorbed with neutral hydrogen. During this time, a phase transition of hydrogen gas was caused by the accelerated process of star and galaxy formation.
To make their findings, the researchers looked at data spanning optical and infrared wavelengths. Light in the infrared spectrum comes directly from stars and galaxies, and to observe this light the team turned to probes into extragalactic infrared background light by the California Institute of Technology's CIBER instrument. The measurements allowed the team to confirm the existence of "intrahalo light" from stars on the outside of galaxies.
"We believe it's true that there is intrahalo light, but we made a new discovery by looking at five infrared bands with Hubble. 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," said UCI cosmologist Asantha Cooray.
The CIBER analysis suggested there would be intrahalo light in the infrared bands, but the researchers were still not entirely sure what to expect in the optical ones. The Hubble data showed a large drop in the amplitude of the signal between the two, confirming they were looking at some of the earliest galaxies in the universe.
"For this research, we had to look closely at what we call 'empty pixels,' the pixels between galaxies and stars," Cooray said. "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."
The researchers believe primordial galaxies were much different from the spiral and disc-shaped galaxies we see today. The primordial galaxies were believed to be extremely diffuse and had higher populations of massive stars.
"This is a very exciting finding," said Henry C. Ferguson, an astronomer at Baltimore's Space Telescope Science Institute and co-principal investigator for CANDELS. "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."
The findings were published in a recent edition of the journal Nature Communications.
