The Universe we live in is transparent, where the light of stars and galaxies shines against a background of light and dark, and among these, an ancient galaxy presents itself.
But that wasn’t always the case – in its early years, the Universe was filled with a haze of hydrogen atoms that obscured the light of the first stars and galaxies.
It is believed that intense ultraviolet light from the first generations of stars and galaxies burned off the hydrogen haze, transforming the Universe into what we see today.
The early Universe was filled with a haze of hydrogen atoms until the first stars and galaxies burned it up. (NASA/JPL-Caltech, CC BY)
While previous generations of telescopes lacked the ability to study these early cosmic objects, astronomers are now using the superior technology of the James Webb Space Telescope to study the stars and galaxies that formed shortly after the Big Bang.
I am a astronomer who studies the most distant galaxies of the Universe using the world’s leading ground and space telescopes.
Using new observations from the Webb Telescope and a phenomenon called gravitational lensing, my team confirmed the existence of the faintest galaxy currently known from the early Universe.
The galaxy, called JD1, is seen as it was when the Universe was just 480 million years old, or 4% of its current age.
A brief history of the beginning of the Universe
The first billion years of the Universe’s life were a crucial period in its evolution. In the first moments after the Big Bang, matter and light were bound together in a hot, dense “soup” of fundamental particles.
However, a fraction of a second after the Big Bang, the Universe expanded extremely quickly.
This expansion eventually allowed the Universe to cool enough for light and matter to separate from its “soup” and – some 380,000 years later – form hydrogen atoms.
Hydrogen atoms appeared as an intergalactic fog, and without light from stars and galaxies, the Universe was dark. This period is known as the cosmic dark ages.
The arrival of the first generations of stars and galaxies several hundred million years after the Big Bang bathed the Universe in extremely hot ultraviolet light, which burned – or ionized – the hydrogen fog. This process produced the transparent, complex, and beautiful Universe we see today.
Astronomers like myself call the first billion years of the Universe – when this haze of hydrogen was burning – the epoch of reionization.
To fully understand this time period, we studied when the first stars and galaxies formed, what their key properties were, and whether they were able to produce enough ultraviolet light to burn all the hydrogen.
The search for faint galaxies in the early Universe
The first step in understanding the epoch of reionization is finding and confirming the distances to galaxies that astronomers think might be responsible for this process.
Since light travels at a finite speed, it takes time to reach our telescopes, so astronomers see objects as they were in the past.
For example, light from the center of our galaxy, the Milky Way, takes about 27,000 years to reach us on Earth, so we see it as it was 27,000 years ago. This means that if we want to see the first moments after the Big Bang (the Universe is 13.8 billion years old), we need to look for objects at extreme distances.
Because the galaxies that reside in this time period are so far away, they appear extremely weak and small for our telescopes and emit most of their light in the infrared. That means astronomers need powerful infrared telescopes like the Webb to find them.
Before Webb, virtually all of the distant galaxies astronomers found were exceptionally bright and large, simply because our telescopes weren’t sensitive enough to see the smallest, faintest galaxies.
However, it is the latter population that is much more numerous, representative and likely the main drivers of the reionization process, not the bright ones.
Therefore, these faint galaxies are the ones that astronomers need to study in more detail. It’s like trying to understand human evolution by studying entire populations instead of a few very tall people. By allowing us to see faint galaxies, Webb is opening a new window into the study of the early Universe.
A typical early galaxy
JD1 is one of these “typical” faint galaxies. He was discovered in 2014 with the Hubble Space Telescope like a suspicious distant galaxy. But Hubble didn’t have the capability or sensitivity to confirm its distance – it could only make a guess.
Small and faint nearby galaxies can sometimes be confused with distant galaxies, so astronomers need to be sure of their distances before we can make claims about their properties. Distant galaxies therefore remain “candidates” until they are confirmed.
The Webb Telescope finally has the ability to confirm this, and JD1 was one of Webb’s first big confirmations of an extremely distant galaxy candidate found by Hubble. This confirmation classifies it as the faintest galaxy ever seen in the early Universe.
To confirm JD1, an international team of astronomers and I used Webb’s near-infrared spectrograph, NIRSpecto obtain an infrared spectrum of the galaxy.
The spectrum allowed us to pinpoint Earth’s distance and determine its age, the number of young stars it formed, and the amount of dust and heavy elements it produced.
A sky full of galaxies and a few stars. JD1, pictured in an enlarged box, is the faintest galaxy yet found from the early Universe. ( Guido Roberts-Borsani/UCLA; original images: NASA, ESA, CSA, Swinburne University of Technology, University of Pittsburgh, STScI)
Gravitational lensing, nature’s magnifying glass
Even for Webb, JD1 would be impossible to see without nature’s help. JD1 is located behind a large cluster of nearby galaxies called Abell 2744whose combined gravitational force bends and amplifies JD1’s light.
This effect, known as gravitational lensing, makes JD1 appear larger and 13 times brighter than it normally does.
Without gravitational lensing, astronomers would not have seen JD1, even with Webb.
The combination of JD1’s gravitational magnification and new images from another of Webb’s near-infrared instruments, the NIRCammade it possible for our team to study the structure of the galaxy with unprecedented detail and resolution.
This not only means that we as astronomers can study the inner regions of early galaxies, but also that we can begin to determine whether these early galaxies were small, compact, isolated sources or if they were merging and interacting with nearby galaxies.
By studying these galaxies, we are tracking down the building blocks that shaped the Universe and gave rise to our cosmic home.
Guido Roberts-Borsani, postdoctoral researcher in astrophysics, University of California, Los Angeles
not published TheConversation
Adapted from ScienceAlert
2023-08-09 22:56:29
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