Distant galaxies reveal clearing of the cosmic fog

By carefully studying some of the most distant galaxies, scientists have been able to establish a timeline for reionization for the first time.
By | Published: October 12, 2011 | Last updated on May 18, 2023
This artist’s impression shows galaxies at a time less than a billion years after the Big Bang, when the universe was still partially filled with hydrogen fog that absorbed ultraviolet light. New observations with ESO’s Very Large Telescope are probing this important phase of the early universe by studying the light from some of the most distant galaxies ever detected. Credit: ESO/M. Kornmesser
Scientists have used the European Southern Observatory’s (ESO) Very Large Telescope (VLT) to probe the early universe at several different times as it was becoming transparent to ultraviolet light. This brief but dramatic phase in cosmic history — known as reionization — occurred around 13 billion years ago. By carefully studying some of the most distant galaxies ever detected, the team has been able to establish a timeline for reionization for the first time. They have also demonstrated that this phase must have happened more quickly than astronomers previously thought.

An international team of astronomers used the VLT as a time machine to look back into the early universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang.

The new observations have allowed astronomers to establish a timeline for what is known as the age of reionization for the first time. During this phase, the fog of hydrogen gas in the early universe was clearing, allowing ultraviolet light to pass unhindered for the first time.

The new results build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the past three years.

“Archaeologists can reconstruct a timeline of the past from the artifacts they find in different layers of soil,” said Adriano Fontana from the INAF Rome Astronomical Observatory. “Astronomers can go one better: We can look directly into the remote past and observe the faint light from different galaxies at different stages in cosmic evolution. The differences between the galaxies tell us about the changing conditions in the universe over this important period, and how quickly these changes were occurring.”

Different chemical elements glow brightly at characteristic colors. These spikes in brightness are known as emission lines. One of the strongest ultraviolet emission lines is the Lyman-alpha line, which comes from hydrogen gas. It is bright and recognizable enough to be seen even in observations of faint and faraway galaxies.

Spotting the Lyman-alpha line for five very distant galaxies allowed the team to do two key things: First, by observing how far the line had been shifted toward the red end of the spectrum, they were able to determine the galaxies’ distances, and, therefore, how soon after the Big Bang they could see them. This let them place them in order, creating a timeline that shows how the galaxies’ light evolved over time. Secondly, they were able to see the extent to which the Lyman-alpha emission, which comes from glowing hydrogen within the galaxies, was reabsorbed by the neutral hydrogen fog in intergalactic space at different points in time.

“We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample,” said Laura Pentericci of INAF Rome Astronomical Observatory. “When the universe was only 780 million years old, this neutral hydrogen was quite abundant, filling from 10 percent to 50 percent of the universe’s volume. But only 200 million years later, the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionization must have happened quicker than astronomers previously thought.”

As well as probing the rate at which the primordial fog cleared, the team’s observations also hint at the likely source of the ultraviolet light that provided the energy necessary for reionization to occur. There are several competing theories for where this light came from — two leading candidates are the universe’s first generation of stars and the intense radiation emitted by matter as it falls toward black holes.

“The detailed analysis of the faint light from two of the most distant galaxies we found suggests that the very first generation of stars may have contributed to the energy output observed,” said Eros Vanzella of the INAF Trieste Observatory in Italy. “These would have been very young and massive stars, about 5,000 times younger and 100 times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent.”

The highly accurate measurements required to confirm or disprove this hypothesis, and show that the stars can produce the required energy, necessitate observations from space or from ESO’s planned European Extremely Large Telescope, which will be the world’s largest eye on the sky once completed early next decade.

Studying this early period in cosmic history is technically challenging because accurate observations of extremely distant and faint galaxies are needed, a task that can only be attempted with the most powerful telescopes. For this study, the team used the great light-gathering power of the 8.2-meter VLT to carry out spectroscopic observations, targeting galaxies first identified by the NASA/ESA Hubble Space Telescope and in deep images from the VLT.