Astronomers studying James Webb Space Telescope images have identified an object as a ‘dusty star-forming galaxy’ from nearly 1 billion years after the Big Bang. They have also discovered more than a dozen additional candidates, implying that these galaxies are three to ten times more common than previously thought. If that conclusion is correct, it indicates that the early universe was much dustier than previously thought.
It first appeared as a glowing blob in ground-based telescope images before disappearing completely in Hubble Space Telescope images. The ghostly object has reappeared as a faint but distinct galaxy in a James Webb Space Telescope (JWST) image.
The COSMOS-Web collaboration’s astronomers have identified the object AzTECC71 as a dusty star-forming galaxy. Alternatively, a galaxy that is busy forming many new stars but is shrouded in a dusty veil that is difficult to see through – nearly 1 billion years after the Big Bang. These galaxies were previously thought to be extremely rare in the early universe, but this discovery, along with more than a dozen additional candidates in the first half of COSMOS-Web data that have yet to be described in the scientific literature, suggests they could be three to ten times more common than previously thought.
This thing is a real monster. Even though it looks like a little blob, it’s actually forming hundreds of new stars every year. And the fact that even something that extreme is barely visible in the most sensitive imaging from our newest telescope is so exciting to me.
Jed McKinney
“This thing is a real monster,” said Jed McKinney, a postdoctoral researcher at The University of Texas at Austin. “Even though it looks like a little blob, it’s actually forming hundreds of new stars every year. And the fact that even something that extreme is barely visible in the most sensitive imaging from our newest telescope is so exciting to me. It’s potentially telling us there’s a whole population of galaxies that have been hiding from us.”
If that conclusion is correct, it indicates that the early universe was much dustier than previously thought. The team’s findings were published in The Astrophysical Journal.
The COSMOS-Web project, which is co-led by Caitlin Casey, an associate professor at UT Austin, aims to map up to 1 million galaxies from a region of the sky the size of three full moons. Part of the goal is to investigate the universe’s earliest structures. In JWST’s first year, the team of more than 50 researchers was awarded 250 hours of observing time and received the first batch of data in December 2022, with more coming in through January 2024.
A dusty star-forming galaxy is hard to see in optical light because much of the light from its stars is absorbed by a veil of dust and then re-emitted at redder (or longer) wavelengths. Before JWST, astronomers sometimes referred to them as “Hubble-dark galaxies,” in reference to the previously most-sensitive space telescope.
“Until now, the only way we’ve been able to see galaxies in the early universe is from an optical perspective with Hubble,” McKinney said. “That means our understanding of the history of galaxy evolution is biased because we’re only seeing the unobscured, less dusty galaxies.”
AzTECC71 was discovered as an indistinct blob of dust emission by a camera on the James Clerk Maxwell Telescope in Hawaii, which sees in wavelengths ranging from far infrared to microwave. The object was then discovered by the COSMOS-Web team in data collected by another team using the ALMA telescope in Chile, which has higher spatial resolution and can see in the infrared. This allowed them to narrow down the source’s location. When they examined the JWST data in the infrared at a wavelength of 4.44 microns, they discovered a faint galaxy in the same location. It was invisible at shorter wavelengths of light, less than 2.7 microns.
The team is now working to find more of these JWST-faint galaxies.
“With JWST, we can study for the first time the optical and infrared properties of this heavily dust-obscured, hidden population of galaxies,” he said, “because it’s so sensitive that not only can it stare back into the farthest reaches of the universe, but it can also pierce the thickest of dusty veils.”
The galaxy is being observed at a redshift of about 6, which corresponds to about 900 million years after the Big Bang, according to the team.
