Using the James Webb Space Telescope (JWST), an international team of astronomers observed a galaxy with a glowing ring of newborn stars in the early universe. The ring galaxy, GS18660, is the most distant such structure ever found, according to a study published September 8 in the journal Astronomy & Astrophysics.
Researchers led by Michele Perna of the Spanish Astrobiology Center initially selected GS18660 for observation because it appeared stretched in older Hubble Space Telescope images. JWST's detailed data revealed the prominent ring structure, allowing the team to analyse the galaxy in three dimensions. They tracked the motions of hot gas, studied chemical properties, and mapped the star formation history.
The preferred explanation is that a small companion galaxy plunged through the centre of a larger galaxy, creating a ripple of dense gas that triggered a burst of star formation. The team spotted the companion moving away at about 264 miles per second (425 kilometres per second) and detected a bridge of gas linking the two. The change in velocity along the bridge suggests interaction, and the companion's position is consistent with a head-on plunge through the galaxy's disk. Using the companion's speed and distance, the team estimated the collision occurred between 17 million and 42 million years earlier in the galaxy's history.
"It is really the combination of several pieces of evidence rather than one single observation," Perna told Live Science. "So we regard the collisional interpretation as our preferred scenario rather than a definitive conclusion."
Calculations indicate that 10% to 20% of the stellar mass in the ring formed in the past tens of millions of years, suggesting the collision and the burst of new stars were likely caused by the same event.
Christopher Conselice, a professor of extragalactic astronomy at the University of Manchester who was not involved in the study, said the evidence is convincing. "It looks to me like the merger hypothesis is more likely, given the nearby galaxy intruder and the stability of the intruder galaxy and the main galaxy itself," Conselice said in an email.
An alternative explanation involves a stellar bar, a structure running north to south through GS18660 that could funnel gas into a ring without an intruder. Perna said the team cannot yet tell whether it is a true stellar bar or a result of the interaction. A bar-driven ring would also be a significant finding, as no clear example has been found so early in the universe.
The discovery may also revise estimates of how common such collisions were in the early universe. Previous estimates suggested collisional rings occur in about 1 in 10,000 galaxies, but GS18660 hints that head-on collisions may have been more frequent.