Way back in the earliest ages of the universe, the first galaxies were born. Astronomers want to know more about them. They’re especially interested to know exactly when these distant galaxies formed and what their stars were like. Now that JWST is a working observatory, astronomers are excited to use its data to explore those early epochs. They’re eager to see the most distant objects and — as seems likely — do a rejiggering of the cosmic timeline after the Big Bang.
July 13, 2022, was a momentous day. It marked the first images released from the telescope; a set called the Early Release Observations. Many call it the first day in a new era of astronomy. They’re not wrong, even if it does sound grandiose. Since then, astronomers have been digging into the images and data to learn more about the universe.
JWST will answer a lot of questions astronomers have been asking about the earliest epochs of the universe. In particular, they want to know more about the distant galaxies that exist “out there.” Thanks to its infrared sensitivity, the telescope will look beyond what the venerable and highly productive Hubble Space Telescope reveals about the early universe. And a group of astronomers in the U.S., Israel, and China proposes to use JWST data to search for early galaxies. They want to look out at objects existing at redshifts beyond z~11. That’s when the newborn universe was about 420 million years old.
How the Webb Telescope studies distant galaxies
Their instrument of choice onboard JWST is the Near-infrared Camera (NIRCam) and the images of very distant objects that it produces. It should be able to extend our view out to a time only tens of millions of years after the Big Bang. That would be when the first galaxies (if they existed) started to take shape. They’d look as they did when the Epoch of Reionization began. That’s a period after the Cosmic Dark Ages when light could travel freely through the infant universe.
Of course, NIRCam can’t do this observing all by itself. It gets cosmic assistance. In particular, the telescope relied on gravitational lensing to capture images of the earliest possible galaxies. That target is the nearby galaxy cluster SMACS 0723-73, and it’s part of the ongoing Early Release Observations program on JWST. This cluster is massive. Thanks to the gravitational influence of its high mass, it’s recognized as a good cosmic “magnifying glass.” It’s a gravitational lens that amplifies the view of the faraway galaxies populating the distant Universe. Luckily, the NIRCam field-of-view is large enough that it was able to study both the cluster and a flanking field not boosted by gravitational lensing. It’s so sensitive that the flanking field also sees far beyond what HST could do.
The international team has searched out a field of candidate galaxies, using ERO data from the SMACS
0723-73 observations. They’ve identified 88 candidate galaxies at very great distances (redshift z>11). They are hoping that some could lie as far as z~20. That could be a time less than 100 million years after the Big Bang. If these galaxies are confirmed to lie at such early epochs of cosmic time, that would be amazing. It would mean that the timeline of the Universe after the Big Bang may have to be changed. For one thing, it would mean that the beginning of the Epoch of Reionization would be much earlier than we expected.
Currently, astronomers think it began about 370,000 years after the Big Bang. Before that, the Universe was in a hot, dense state, populated with a soup of ionized gas. Eventually, it cooled enough for protons and neutrons to combine and form neutral atoms. And that’s when light from the earliest galaxies and their stars was finally able to move freely across the expanding Universe.
However, if, as expected, the first galaxies can be seen only a few tens of millions of years after the Big Bang, then perhaps the Cosmic Dark Ages didn’t last as long as everyone thought. NIRCam and spectroscopic observations of those early galaxies will eventually confirm their ages, which will help further refine the timeline of the early Universe.