March 1, 2024

JWST discovers a galaxy that shouldn’t exist

Astronomers working with JWST have found a dwarf galaxy they weren’t looking for. It is about 98 million years away, has no neighbors and was in the background of an image of other galaxies. This isolated galaxy shows a lack of star formation activity, which is very unusual for an isolated dwarf.

Most isolated dwarf galaxies form stars, according to a wealth of observations. What’s different about this one?

JWST’s PEARLS (Prime Extragalactic Areas for Reionization and Lensing Science) observation program aims to understand the timing of galaxy assembly, the growth of the active galactic nucleus (AGN), and First Light. As part of its work, it observed a galaxy cluster called CLG1212. The isolated dwarf galaxy, called PEARLSDG, was found by chance.

The discovery is in new research published in The Astrophysical Journal Letters. Its title is “PEARLS: A potentially isolated quiescent dwarf galaxy with a red giant branch tip at a distance of 30 Mpc”. The lead author is Tim Carleton, assistant research scientist at Arizona State University.

Dwarf galaxies contain far fewer stars than galaxies like our Milky Way. No one knows exactly how many stars there are in the Milky Way. But well-founded estimates point to a higher number of around 400 billion. In contrast, dwarf galaxies like PEARLSDG contain up to about 100 million stars.

In addition to its lack of star formation, PEARLSDG is unusual for another reason. JWST is able to discern individual red giant branch (RGB) stars in the dwarf galaxy because the stars are bright at the wavelengths observed by JWST. It’s almost too far away for JWST to see the stars, so PEARLSDG is one of the most distant galaxies where we can see individual stars.

JWST was able to discern individual stars in the dwarf galaxy, as shown in this survey image. Image credit: Carleton et al. 2024

Being able to see individual red giant branch (RGB) stars makes studying the dwarf galaxy much easier. RGB stars have a specific intrinsic brightness, and this means the astronomers behind the discovery can measure the galaxy’s distance: about 98 million light-years away. They can also measure the age of stars, showing that the PEARLSDG stellar population is older. If it were still forming stars, some stars would be much younger.

The researchers write that the dwarf galaxy has not formed a star for at least a billion years. Part of the evidence lies in the lack of UV energy in the galaxy. Young stars emit powerful UV rays, but PEARLSDG displays only low levels of UV radiation. “Consistent with its low level of UV emission and the lack of emission lines in its spectrum, we found a very low sSFR, suggesting that its star formation was interrupted more than 1 Gyr ago”, explain the researchers.

When a galaxy stops forming stars, it is called a quiescent galaxy. In a quiescent galaxy, the supply of gas used in star formation has been extinguished. It is usually caused by another neighboring galaxy that has interacted with the quiescent galaxy to stop star formation. Somehow, the interaction stripped gas from the quiescent galaxy or stopped the flow of gas.

But PEARLSDG has no close neighbors.

JWST's NIRCam instrument was photographing the regions in the green boxes when it also spotted PEARLSDG, the dwarf galaxy in the cyan box.  Image credit: Carleton et al.  2024
JWST’s NIRCam instrument was photographing the regions in the green boxes when it also spotted PEARLSDG, the dwarf galaxy in the cyan box. Image credit: Carleton et al. 2024

“These types of isolated quiescent dwarf galaxies have never been seen before except in relatively few cases. They are not expected to exist, given our current understanding of galaxy evolution, so seeing this object helps us improve our theories about galaxy formation,” said lead author Carleton. “Generally, dwarf galaxies that exist alone continue to form new stars.”

Interactions with other galaxies can cause extinction through tidal removal. The same can happen with other environmental effects, such as removal of ram pressure and throttling. But there are other causes as well, although astronomers are still working to understand them. “However, recent observations of a large number of ultra-diffuse galaxies have led to the development of internal extinction mechanisms, such as strong feedback,” the researchers say. In a strong feedback, the powerful energy of the biggest, brightest stars can eliminate the gas needed to form new stars.

Despite the fact that PEARLSDG has no close neighbors, the authors are cautious in their conclusions. “Regardless, we cannot completely rule out past interactions with other galaxies that may have affected their formation history,” they write. “However, PEARLSDG’s recessive velocity and luminosity distance are consistent with it being in the Hubble Stream, and there are no visible signatures of tidal interactions.”

The Hubble Stream is what causes galaxies to move away from each other as the Universe expands. Some galaxies interact and even merge despite expansion because other forces have acted on them. But there is no indication that anything interacted with the dwarf galaxy that could have quenched its star formation.

When galaxies interact with each other, tidal forces distort their shapes and can create tails and streams of stretched gas, dust and stars. But PEARLSDG does not show any of these symptoms. It is a normal-shaped and rather elusive dwarf galaxy.

A pair of interacting galaxies called Arp 273. The largest of the spiral galaxies, known as UGC 1810, has a disk that is distorted into a rose shape by the tidal gravitational pull of the companion galaxy below it, known as UGC 1813. Image credit : NASA, ESA and Hubble Heritage Team (STScI/AURA)
A pair of interacting galaxies called Arp 273. The largest of the spiral galaxies, known as UGC 1810, has a disk that is distorted into a rose shape by the tidal gravitational pull of the companion galaxy below it, known as UGC 1813. Image credit : NASA, ESA and Hubble Heritage Team (STScI/AURA)

Discoveries like this make astronomers pause and reconsider their models of galactic evolution. But JWST is likely to find more isolated, inactive dwarf galaxies. As more are observed, things will become clearer and eventually there will be an explanation.

“A more detailed analysis of the star formation history of PEARLSDG and the dynamics of PEARLSDG relative to its surroundings is needed to better understand its formation history, but this discovery suggests the possibility that many isolated quiescent galaxies are waiting to be identified and that JWST has the tools to do so,” the researchers write.

But for now, it’s just another mystery in the cosmos.

“This was absolutely against people’s expectations for a dwarf galaxy like this,” Carleton said.

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