In a surprising twist, a team of astronomers led by Assistant Research Scientist Tim Carleton from Arizona State University has discovered a dwarf galaxy that wasn't their primary observation target. This unexpected find, made using the James Webb Space Telescope (JWST), challenges our current understanding of galaxy formation and evolution.
Galaxies, bound together by gravity, are composed of stars, planets, vast clouds of dust and gas, and the elusive dark matter. Dwarf galaxies, small and low in luminosity, are the most abundant type of galaxies in the universe. They typically contain fewer than 100 million stars, a stark contrast to the nearly 200 billion stars of the Milky Way.
Recent observations of "ultra-diffuse galaxies" suggest that our understanding of the dwarf galaxy population may be incomplete. These galaxies, abundant beyond the reach of previous large spectroscopic surveys, have prompted astronomers to revisit their theories.
In a study recently published in the Astrophysical Journal Letters, Carleton and his team were initially observing a cluster of galaxies as part of the JWST Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) project. However, the dwarf galaxy, named PEARLSDG, appeared unexpectedly in some of the team's JWST imaging.
PEARLSDG is an anomaly. Unlike typical dwarf galaxies, it isn't interacting with a nearby galaxy, nor is it forming new stars. Instead, it presents as an isolated quiescent galaxy, a type of galaxy that hasn't been observed frequently.
"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.
Until now, astronomers believed that an isolated galaxy would continue to form young stars or interact with a more massive companion galaxy. However, PEARLSDG defies this theory. It presents as an old stellar population, not forming new stars, and keeping to itself.
In a further surprise, individual stars can be observed in the team's JWST images. These stars are brighter in JWST wavelengths, making it one of the farthest galaxies where we can see these stars with this level of detail. The brightness of these stars allows astronomers to measure its distance—98 million light-years.
For this study, Carleton and the team used a wide range of data, including 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 imaging from NASA's Galex and Spitzer space telescopes; and ground-based imaging from the Sloan Digital Sky Survey and the Dark Energy Camera Legacy Survey.
JWST's NIRCam, with its high angular resolution and sensitivity, allowed the team to identify individual stars in this distant galaxy. Identifying specific stars in the imaging provided a key clue to its distance—these stars have a specific intrinsic brightness, so by measuring their apparent brightness with JWST, the team was able to determine how far away they are.
All of the archival imaging data, observed at ultraviolet, optical, and infrared wavelengths, was pulled 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 DeVeney Spectrograph at the Lowell Discovery Telescope spreads the light astronomical objects into its distinct components, allowing astronomers to study its properties in detail. This was key to show that PEARLSDG is not associated with any other galaxy and is truly isolated.
"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.