A team of astronomers discovers a galaxy that should not exist

A team of astronomers, led by Tim Carleton, an assistant research scientist at Arizona State University, has discovered a dwarf galaxy that appeared in James Webb Space Telescope imagery and was not the primary target of ‘observation.

Galaxies are held together by gravity and made up of stars and planets, with vast clouds of dust and gas as well as dark matter. Dwarf galaxies are the most abundant galaxies in the universe and are by definition small and of low luminosity. They have fewer than 100 million stars, while the Milky Way, for example, has nearly 200 billion.

Recent dwarf galaxy observations of the abundance of “ultra-diffuse galaxies” beyond the reach of previous large spectroscopic studies suggest that our understanding of the dwarf galaxy population may be incomplete.

In a recently published study, Carleton and the team were initially studying a galaxy cluster as part of the JWST Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) project.

The dwarf galaxy, PEARLSDG, appeared by chance in some of the team’s JWST images. It wasn’t the target at all, just a little away from the main field of observation, in the area of ​​space where they didn’t expect to see anything.

Their results were published in the Letters from astrophysical journals.

PEARLSDG did not exhibit the usual characteristics of a dwarf galaxy that one might expect to see. It does not interact with a nearby galaxy, but it does not form new stars either. It turns out to be an interesting case of an isolated, quiescent galaxy.

“These types of isolated, quiet dwarf galaxies haven’t really been observed before, except in relatively rare cases. They aren’t really expected to exist given our current understanding of galaxy evolution, So the fact that we see this object helps us improve our theories for galaxy formation,” Carleton said. “In general, dwarf galaxies that exist alone continue to form new stars.”

Until now, astronomers’ understanding of galaxy evolution showed an isolated galaxy continuing to form young stars or interacting with a more massive companion galaxy. This theory did not apply to PEARLSDG, which appears as an ancient stellar population, not forming new stars and remaining isolated.

Another surprise is that individual stars can be seen in the team’s JWST images. These stars are brightest in JWST wavelengths; it is one of the most distant galaxies where we can see these stars in this level of detail. The brightness of these stars allows astronomers to measure its distance: 98 million light years.

For this study, Carleton – who is an assistant research scientist in the Beus Center for Cosmic Foundations in ASU’s School of Earth and Space Exploration – and the team used a wide range of data.

This includes imaging data from JWST’s Near Infrared Camera (NIRCam); spectroscopic data from the DeVeney Optical Spectrograph on the Lowell Discovery Telescope in Flagstaff, Arizona; archival imagery from NASA’s Galex and Spitzer space telescopes; and ground-based imagery from the Sloan Digital Sky Survey and Dark Energy Camera Legacy Survey.

JWST’s NIRCam has very high angular resolution and sensitivity, allowing the team to identify individual stars in this distant galaxy. Just like individual cells brought into focus under a microscope, these observations revealed the components of PEARLSDG.

Importantly, identifying specific stars in the imagery provided a key clue to its distance: these stars have a specific intrinsic luminosity, so by measuring their apparent luminosity with JWST, the team was able to determine how how far away they are. It turns out that these stars were among the most distant stars of their type to be observed.

All archival imaging data, observed at ultraviolet, optical and infrared wavelengths, were brought together to study the color of PEARLSDG. Newly formed stars have a specific color signature, so the absence of such a signature was used to show that PEARLSDG was not forming new stars.

The Lowell Discovery Telescope’s DeVeney spectrograph breaks down light from astronomical objects into its distinct components, allowing astronomers to study its properties in detail. For example, the specific wavelength shift observed in the spectroscopic data features encodes information about the movement of PEARLSDG, using the same Doppler effect that radar guns use to measure the speed of drivers on roadways. Arizona.

This was essential in showing that PEARLSDG is not associated with any other galaxy and is truly isolated.

Additionally, particular features of the spectrum are sensitive to the presence of young stars, so the absence of these features further corroborates measurements of the absence of young stars in the imaging data.

“It was absolutely contrary to people’s expectations for a dwarf galaxy like this,” Carleton said.

This discovery changes astronomers’ understanding of how galaxies form and evolve. This suggests the possibility that many isolated quiet galaxies are waiting to be identified and that JWST has the tools to do so.

This research was presented at the 243rd AAS press conference in January: Oddities in the Sky,