JWST recently captured three of the oldest galaxies in the universe receding from a giant dark cloud of hydrogen gas.
Three faint patches of red light in a recent batch of JWST data traveled more than 13 billion light-years through space to reach the telescope’s mirrors. This ancient light carries a snapshot of what galaxies looked like between 400 and 600 million years after the Big Bang, when the universe was essentially a cosmic toddler. And all three of these early galaxies are shrouded in dense hydrogen gas that slowly falls into the galaxies’ gravitational wells—where they eventually help form new stars.
University of Copenhagen astrophysicist Kasper Heintz and his colleagues published their work in the journal Science.
This artist’s illustration shows a young galaxy, just a few hundred million years after the Big Bang, still trying to pull itself together from the surrounding cold hydrogen gas.
NASA, ESA, CSA, Joseph Olmsted (STScI)
The reionization epoch will Pour
The JWST instruments usefully separate the light from distant galaxies into the individual wavelengths that make them up. The spectrum of light emanating from an object such as a galaxy is like a fingerprint of the chemicals that make it up, as each chemical compound absorbs, emits, and reflects its own very specific wavelengths of light. Around three distant galaxies, Heintz and his colleagues noticed something that seemed to absorb the same wavelengths of light as cold hydrogen gas—and lots of that.
“These galaxies are like sparkling islands in a sea of ​​otherwise neutral, opaque gas,” Heintz says in a recent statement.
Hydrogen gas, when cold and not electrically charged (or ionized), absorbs light but does not emit it. This neutral gas filled the early universe, preventing light from traveling very far until several hundred million years after the Big Bang: a period called the cosmic dark ages.
It took powerful bursts of radiation from the first stars in the first galaxies to remove the electrons from all those hydrogen atoms and create an ionized gas (also called plasma) that is translucent instead of opaque. The epoch of reionization has begun—and the three galaxies in Heintz and a recent study by his colleagues are just beginning to illuminate it.
Brand new galaxy, some assembly required
Sometime between 13.2 billion and 13.4 billion years ago—when the light that had just reached JWST began its long journey through space—these three early galaxies were still in the process of being assembled from the surrounding gas.
“[The data] suggests that we are seeing the assembly of neutral hydrogen into galaxies,” says University of Copenhagen astrophysicist Darach Watson, co-author of the recent study. And that’s a phase of galaxy formation that astronomers haven’t seen before, especially in the very early universe.
Galaxies, in their infancy, are still surrounded by a cloud of cold, dark, neutral hydrogen gas—the same thing that caused the cosmic dark ages. Most of this gas is heated as it falls into galaxies and is pulled in by their relentless gravity. And then it will slowly cool and form lumps like solidified oatmeal, and some of those lumps will be so heavy that they will collapse on each other and form new stars.
Right now (or as we see them right now, which actually happened billions of years ago), the stars that these early galaxies contain are mostly young and newly formed.
“The fact that we see large gas reservoirs also suggests that the galaxies haven’t had enough time to form most of their stars yet.” But most likely they will get there.
The data reveals not only a never-before-seen moment in a galaxy’s life, but also a glimpse of what the early universe was like before the expansion of space slowed everything down and turned most galaxies into lonely beacons or, at most, isolated clusters of light. in the void.
“We are moving away from the image of galaxies as isolated ecosystems,” University of Copenhagen astrophysicist Simone Nielsen said in a recent statement. “At this stage in the history of the universe, all galaxies are intimately connected to the intergalactic medium with its filaments and structures of pristine gas.”
In the very early universe, no galaxy was an island (yet).