The James Webb Space Telescope didn't need much time to show us how wrong we were about the early Universe. Mere weeks after it began observations, it found galaxies in the very early Universe that were far more massive than our theories showed. These confounding images required an explanation.
Researchers working with the Atacama Large Millimetre/submillimetre Array (ALMA) are providing one. Their results are in research titled "A warm ultraluminous infrared galaxy just 600 million years after the big bang." It's published in the Monthly Notices of the Royal Astronomical Society and the lead author is Tom Bakx, a postdoc researcher at Chalmers University of Technology in Sweden.
This research is focused on a galaxy named simply Y1. It's at redshift 8.3, which means that we're seeing it only about 600 million years after the Big Bang. The light we're now sensing has been travelling for more than 13 billion years.
This galaxy is being called a superheated star factory because its star formation rate (SFR) is extremely high. Its SFR is about 180 times greater than the Milky Way's SFR: it forms about 180 solar masses per year while the Milky Way only forms 1. According to the researchers, this can explain why early galaxies were so much larger than thought. Our theories of star formation don't account for SFRs this great.
"Even though it's the first time we've seen a galaxy like this, we think that there could be many more out there." - Yoichi Tamura, Nagoya University, Japan.
This image shows two of the galaxies the JWST found in the very early Universe. They were much brighter than thought, which suggests that they're much more massive than researchers thought they could be. Image Credit: NASA, ESA, CSA, Tommaso Treu (UCLA); Image Processing: Zoltan Levay (STScI)
As with almost everything in astronomy, astrophysics, and cosmology, this comes down to light. What astronomers see in Y1 is red light from superheated dust that masks its high SFR.
"We're looking back to a time when the universe was making stars much faster than today," said lead author Bakx in a press release. "Previous observations revealed the presence of dust in this galaxy, making it the furthest away we've ever directly detected light from glowing dust."
"That made us suspect that this galaxy might be running a different, superheated kind of star factory. To be sure, we set out to measure its temperature," Bakx added.
Stars form in abundance inside complexes of massive gas clouds. Young stars are brilliant, as are the most massive stars. Their powerful light illuminates the clouds of gas and dust the stars reside in.
But some of this light is beyond the ability of the human eye to see. These stars light up the dust, which then emits light in the radio part of the electromagnetic spectrum. That's where ALMA comes in. It can see in what's called Band 9, or light at 0.44 mm wavelength. By observing in this wavelength, ALMA was able to take the galaxy's temperature.
"At wavelengths like this, the galaxy is lit up by billowing clouds of glowing dust grains. When we saw how bright this galaxy shines compared to other wavelengths, we immediately knew we were looking at something truly special," said Bakx.
It's called a super-heated galaxy because of its temperature relative to other galaxies. Y1's dust is about 90 Kelvin, or -180 Celsius (-292 F.) For comparison, the Milky Way's dust temperature is about 20 to 40 Kelvin. Both temperatures reflect the SFRs of their galaxies. The Milky Way's lower temperature reflects its relatively calm and stable SFR, while Y1's temperature reflects its much higher SFR.
"The temperature is certainly chilly compared to household dust on Earth, but it's much warmer than any other comparable galaxy we’ve seen," said co-researcher Yoichi Tamura, an astronomer at Nagoya University in Japan. "This confirmed that it really is an extreme star factory. Even though it's the first time we've seen a galaxy like this, we think that there could be many more out there. Star factories like Y1 could have been common in the early universe."
If these rapid bursts of star formation are common, they could explain why the JWST confounded us with its discovery of massive galaxies so early in the Universe's history. Even if these episodes are brief, their elevated SFRs could explain it.
"We don't know how common such phases might be in the early universe, so in the future we want to look for more examples of star factories like this. We also plan to use the high-resolution capabilities of ALMA to take a closer look at how this galaxy works," said Bakx.
But there's more to this research than just a potential answer to the early massive galaxies question posed by the JWST. Research shows that early galaxies can also have far more dust than they should. Astronomers know that older stars produce most galactic dust, especially evolved red giant stars, which are the major source. But if the dust is warmer than thought, it's just as luminous as a smaller amount of cooler dust. In that case, a large population of evolved stars isn't necessary to explain the dust observations.
"Galaxies in the early universe seem to be too young for the amount of dust they contain. That's strange, because they don't have enough old stars, around which most dust grains are created," said co-author Laura Sommovigo, of the Flatiron Institute and Columbia University. "But a small amount of warm dust can be just as bright as large amounts of cool dust, and that's exactly what we’re seeing in Y1. Even though these galaxies are still young and don't yet contain much heavy elements or dust, what they do have is both hot and bright."
"This source is an extreme example of dust-obscured star formation contributing strongly to the cosmic build-up of stellar mass, which can only be revealed through direct and comprehensive observations in the (sub)mm regime," the authors conclude.
