Astronomers Uncover an Anomaly: The Galaxy That Defies Expectations

February 2, 2024

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Assistant Research Scientist Tim Carleton, along with a team of astronomers from Arizona State University, have discovered a dwarf galaxy via the imagery of James Webb Space Telescope, even though it wasn't the primary observation target.

Galaxies, which are bound by gravity, consist of star and planets, vast dust and gas clouds, but also dark matter. What we know as dwarf galaxies are the most abundant galaxies in the universe. They are relatively small and dim with less than 100 million stars, compared to their Milky Way counterpart that has around 200 billion stars.

The recent observations of dwarf galaxies suggest incomplete understanding of the 'ultra-diffuse galaxies' profile that is beyond the survey reach of large spectroscopic.

While looking at a galaxy cluster through the JWST Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) project, Carleton and his team unexpectedly discovered the dwarf galaxy, PEARLSDG, in the observational images received from JWST. It was slightly away from the main observation field, in an area where no presence was expected.

Their findings have been published in the Astrophysical Journal Letters.

PEARLSDG did not bear the usual characteristics of a dwarf galaxy. It was neither interacting with any nearby galaxy, nor was it forming new stars, making it an isolated quiescent galaxy.

'These types of isolated quiescent dwarf galaxies haven't really been seen before except for relatively few cases. They are not 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,' said Carleton.

On previous understanding, an isolated galaxy would keep forming young stars or would interact with a larger galaxy. However, PEARLSDG, chaotic and old, did not conform to the theory.

Fascinatingly, individual stars could also be seen in the team's JWST images. These stars, brighter in JWST wavelengths, allowed the astronomers to measure a distance of 98 million light-years making it one of the farthest galaxies observable with this level of detail.

In addition to the data from JWST's Near-InfraRed Camera (NIRCam), Carleton and his team also used a slew of wide range data sets from different sources, including the DeVeney Optical Spectrograph at the Lowell Discovery Telescope, imaging from NASA's Galex and Spitzer space telescopes, and images from the Sloan Digital Sky Survey and Dark Energy Camera Legacy Survey.

The NIRCam from JWST with its high angular resolution and sensitivity allowed the team to identify individual stars in this distant galaxy and bring the elements of PEARLSDG into sharp focus, almost like observing cells under a microscope.

Key to determining its distance was the ability to identify specific stars. These stars, which have a particular intrinsic brightness, could be measured for their apparent brightness with JWST, allowing the team to calculate their distance. These were among the most distant stars of their type to be observed.

The color of PEARLSDG was able to be studied through the assimilation of the archival imaging data observed in ultraviolet, optical and infrared wavelengths. The absence of a particular color signature, usually indicative of newly formed stars, showed that PEARLSDG was not forming new stars.

Lastly, the DeVeney Spectrograph at the Lowell Discovery Telescope was able to break down the light from this astronomical object enabling astronomers to study its detailed properties. Much like a radar gun measures speed, the specific wavelength shift observed in spectroscopic data helped gather information about the motion of PEARLSDG.

This was key to show that PEARLSDG is not associated with any other galaxy and is truly isolated.

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

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

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

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

Provided by Arizona State University