Back at the dawn of the Universe, astronomers have found a lot of cosmic proportions. At least 21 galaxies, which form stars at a huge rate, merge in the early stages of the formation of a cluster of galaxies. And it’s all happening 13 billion light-years away, just 770 million years after the Big Bang itself.
This is the first protocluster discovered so far, called LAGER-z7OD1, and today it has probably evolved into a group of galaxies 3.7 trillion times the mass of the Sun.
Such a large protocluster, so early in the Universe (barely a cosmic flicker since the curtain was lifted on life, the Universe and all), could contain some vital clues as to how the primordial smoke was wiped out and how the lights were turned on, sending light flowing freely through space.
Our Universe is a massively interconnected place. Galaxies may appear relatively autonomous, but more than half of all galaxies are gravitationally bound in groups or clusters, huge structures of hundreds to thousands of galaxies.
The beginnings of such clusters are not unknown in the initial universe. Protocols have been found up to LAGER-z7OD1, some even much larger, suggesting that clusters could begin to assemble much faster than previously thought.
But LAGER-z7OD1, according to a team of researchers led by astronomer Weida Hu of China University of Science and Technology, is special. It may reveal clues about one of the most mysterious stages in the history of the Universe: the era of reionization.
“The total volume of ionized bubbles generated by its member galaxies is comparable to the volume of the protocluster itself, indicating that we are witnessing the melting of individual bubbles and that the intergalactic medium within the protocluster is almost completely ionized, “they wrote in their diary.
“LAGER-z7OD1 therefore provides a unique natural laboratory to investigate the reionization process.”
You see, space was not always the charming, transparent place it is today. For the first 370 million years or so, it was filled with a warm, lukewarm mist of ionized gas. Light could not travel freely through this mist; free electrons were scattered and that was all.
Once the universe cooled enough, protons and electrons began to recombine into neutral hydrogen atoms. This meant that light — not that there was much, yet — could finally travel through space.
When the first stars and galaxies began to form, their ultraviolet light reionized the ubiquitous neutral hydrogen throughout the Universe: first in bubbles located around the ultraviolet sources and then in larger areas. as the ionized bubbles connected and overlapped, allowing the entire spectrum of electromagnetic radiation to flow freely.
About 1 billion years after the Big Bang, the Universe was completely reionized. This means that it is more difficult to probe beyond this point (about 12.8 light-years away), but it also means that the reionization process itself is difficult to understand.
Ideally, you’ll need really bright objects whose ionizing radiation can cut off neutral hydrogen, and that’s what Hu and his team were looking for with Lyman’s alpha galaxies in the Epoch of Reionization survey. These are small galaxies in the early universe that form stars at an insane rate, which means that they can be detected at fairly large distances, well within the time of reionization. This makes them useful probes of the period.
In their research, the researchers found LAGER-z7OD1, an excessive region of galaxies in a three-dimensional volume of space measuring 215 million by 98 million by 85 million light-years. This volume contained two different subprotocols that merged into a larger one, with at least 21 galaxies, 16 of which have been confirmed.
The total volume of ionized space around the galaxies was slightly higher than the volume of LAGER-z7OD1.
“This demonstrates substantial overlaps between individual bubbles, indicating that individual bubbles fuse into one or two giant bubbles,” the researchers wrote.
Thus, the protocol not only represents an excellent example of this type, as it provides a new data point to study how these structures form and emerge, as well as star formation in the initial universe, but which offers a unique window to the formation and combination of ionized bubbles in the middle of the reionization era.
However, what ideas will emerge have yet to be discovered. As the researchers point out, this will be the work of future more powerful telescopes that will be able to better observe the finer details of the reionization process.
The team’s research has been published in Nature Astronomy.