About 370,000 years after the Big Bang, the universe experienced a period that cosmologists refer to as “the cosmic dark ages.”
During this period, the universe was hidden by a hot, dense plasma that conceals all visible light, making it invisible to astronomers.
When the first stars and galaxies formed over the next hundred million years, the radiation ionized they emitted this plasma, making the universe transparent.
One of the biggest cosmological mysteries right now is when “cosmic reionization” began. To find out, astronomers have looked deeper into the cosmos (and further back in time) to discover the first visible galaxies.
Thanks to recent research from a team of astronomers from University College London (UCL), a brilliant galaxy has been observed that resembled the intergalactic medium 1
The research was presented last week (July 2) during the annual meeting of the European Astronomical Society (EAS) – due to the pandemic, this year’s meeting was virtual.
During his presentation, Romain Meyer (a doctoral student at UCL and leading author of the study) and his colleagues shared their findings, which is the first solid evidence that reonization was complete 13 billion years ago.
The team responsible for this discovery was led by Romain Meyer, a PhD student at UCL Astrophysics Group. He was joined by UCL researchers Dr Nicolas Laporte and Prof Richard S Ellis as well as Prof Anne Verhamme and Dr Thibault Garel at the University of Geneva. Their results are also the subject of a recently submitted document Monthly announcements from the Royal Astronomical Society.
Studying galaxies that existed during this early period in the universe is crucial to understanding the origin of the cosmos as well as its subsequent evolution.
According to our current cosmological models, the first galaxies were formed from merging star clusters, which in turn formed when the first stars in the universe gathered.
Over time, these galaxies blew out the radiation that removed the neutral gas in the intergalactic medium (IGM) for its electrons (AKA ionization process). Astronomers know this because we have clear evidence of it, in the form of the cosmic dark age and how the universe is transparent today.
But the most important questions about how and when this happened remain unknown. Like Dr. Meyer told Universe Today via email:
“By looking at distant galaxies we look into the early universe, because light has traveled for billions of years before it reaches us. This is great because we can look at what galaxies were like billions of years ago, but it has several disadvantages “.
To begin with, Meyer explained, distant objects are very weak and can only be observed using the most powerful ground-based and space-based telescopes.
At this distance, there is also the difficult question of redshift, where the expansion of the cosmos causes light from distant galaxies to extend its wavelength toward the red end of the spectrum.
In the case of galaxies that are billions of years old, the light has been shifted to only visible infrared (especially the UV light that Meyer and his colleagues were looking for).
To get a good look at A370p_z1, a brilliant galaxy 13 billion light years away, the team consulted using data from the Hubble Frontier Fields program – which astronomers are still analyzing.
The Hubble data indicated that this galaxy was highly displaced, indicating that it was particularly ancient.
They then made follow-up observations with the Very Large Telescope (VLT) to get a better sense of the galaxy’s spectra. In particular, they looked for the bright line emitted by ionized hydrogen, known as the Lyman-alpha line. Meyer said:
“The big surprise was to find that this line, detected at 9480 Ångström, was a double line. This is extremely rare to find in early galaxies, and this is only the fourth galaxy we know to have a dual Lyman alpha- line for the first billion years. The nice thing about double Lyman-alpha lines is that you can use them to conclude on a very important set of early galaxies: what fraction of energy photons they leak into the intergalactic medium. “
Another big surprise was the fact that the A370p_z1 seemed to allow 60 to 100 percent of its ionized photons into intergalactic space, and was probably responsible for ionizing the bubble IGM around it.
Galaxies that are closer to the Milky Way usually have evacuation fractions of about 5 percent (50 percent in some rare cases), but observations by IGM indicate that early galaxies must have had an average 10 to 20 percent evacuation fraction.
This discovery was extremely important as it could help resolve an ongoing debate in cosmological circles.
So far, the questions about when and how re-ionization has occurred have created two possible scenarios.
In one, there was a population of many weak galaxies that leaked about 10 percent of their energetic photons. In the second, there was an “oligarchy” of brilliant galaxies with a much larger percentage (50 percent or more) of outgoing photons.
In both cases, the evidence so far has suggested that the first galaxies were very different from today.
“Detecting a galaxy with almost 100 percent escape was really nice because it confirms what astrophysicists suspected: early galaxies were much different than present objects, and leaking energetic photons much more efficiently,” Meyer said.
Studying galaxies for the reionization era for Lyman alpha lines has always been difficult because of how they are surrounded by neutral gas that absorbs that signature hydrogen emission.
But we now have strong evidence that the re-ionization was complete 800 million years after the Big Bang, and that it was likely that a few brilliant galaxies were responsible.
If what Meyer and his colleagues observed is typical of galaxies with the reionization time, we can assume that reionization was caused by a small group of galaxies that created large bubbles of ionized gas around them that grew and overlapped.
As Meyer explained, this discovery could point the way toward the creation of a new cosmological model that accurately predicts how and when major changes in the early universe occurred:
This discovery confirms that early galaxies can be extremely effective at leaking ionizing photons, which is an important hypothesis of our understanding of “cosmic reionization” – the epoch when the intergalactic medium went from neutral to ionized 13 billion years ago (e.g. hydrogen atoms of these energetic photons).
According to Meyer, more objects like A370p_z1 need to be found so that astronomers can establish the average evacuation fractions for early galaxies.
Meanwhile, the next step will be to determine why these early galaxies were so effective at leaking energetic photons.
Several scenarios have been proposed, and by getting a better look at the early universe, astronomers can test them.
As Meyer certainly noted, much of this will depend on the next-generation telescope, which will take to space soon. The most notable of these is the James Webb Space Telescope (JWST), which (after several delays) is still scheduled sometime next year.
Here’s another significance for studies like these, which is how they will help the James Webb team decide which cosmological mysteries to investigate.
“With the James Webb Space Telescope, we will follow up this target deeper into the infrared to gain access to what was originally emitted in the optical light,” Meyer said.
“It will give us more insight into the physical mechanisms played in early galaxies. JWST’s mission is limited in time, which is why discovering these extreme objects is so important now: knowing which objects are special or extreme during the first billion years of our universe, we know what to look for when JWST is finally launched! “
Exciting times lie ahead for astronomers, astrophysicists, exoplanet hunters, SETI scientists and cosmologists. It’s hard to know who should be most excited, but something tells me it would be like asking a parent which of their children they love the most. Inevitably, the answer is always, “all of them!”
This article was originally published by Universe Today. Read the original article.