Astronomers have discovered an incredibly rare system in which at least five galaxies from the early universe are merging — just 800 million years after the Big Bang. The remarkable discovery was made using data from the James Webb Space Telescope (JWST) and the Hubble Space Telescope.
Galaxy mergers play a key role in galaxy formation in the early universe. While not commonly seen, merging systems do occur, typically involving two galaxies. However, the newly identified merger, nicknamed JWST’s Quintet, contains at least five galaxies and 17 galaxy clumps.
“Finding such a system with five physically linked galaxies is exceptionally rare, both in current simulations and in observations,” said study lead author Weida Hu, a postdoctoral researcher at Texas A&M University. “The probability of detecting even one [multiple-galaxy merger] is quite low, which raises the possibility that we may have been ‘lucky’ in identifying this system so early,” Hu told Live Science in an email.
These galaxies are called emission-line galaxies as they have prominent signatures in their light, particularly those emitted by hydrogen and oxygen, which are telltale signs of new stars forming.
The power of two
The research, published Aug. 15 in the journal Nature Astronomy, used a combination of JWST and Hubble data.
JWST’s Near-Infrared Camera (NIRCam)hinted at a large halo of gas around the group of galaxies, which meant that the five galaxies are not independent but are instead physically connected and embedded in the same system, Hu explained.
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While some of these galaxies were previously detected using Hubble, “only JWST data tell us that the five galaxies have the same redshift and are interacting with each other,” Hu added. (Redshift is a measure of cosmic distance, with higher redshifts corresponding to more distant, ancient objects. Redshift occurs as the light emitted by distant objects stretches into longer, redder wavelengths while crossing the expanding universe.)
Hu suggested that there could be other faint or hidden galaxies linked to JWST’s Quintet that have not yet been detected. But discovering these galaxies may require advanced multi-wavelength observations.
Early universe mergers involving more than two galaxies are extremely rare, said Christopher Conselice, a professor of extragalactic astronomy at the University of Manchester who was not involved in the study.
“If you look at all galaxies, then 20-30% of them will be in a merger. This will be just two galaxies. The fraction of these multiple merger systems will be much, much lower, and we don’t have stats on it quite yet, but certainly lower than 1%,” Conselice told Live Science.
The team found that the two main galaxies in the system appear to be separated by a distance of 43,300 light-years, and the most distant pair among all the galaxies in the system appear to be 60,700 light-years apart. (For comparison, our Milky Way galaxy is about 100,000 light-years end to end.)
“The fact that the galaxies are spatially close together is the indication that they probably will merge,” Conselice said. “There is some room for interpretation regarding whether some objects might be parts of the same galaxy,” he added.
The distant cousin
This system is similar to its local universe counterpart, Stephan’s Quintet, which is a merger of four galaxies, with a fifth galaxy that appears in the same part of the sky but isn’t merging.
“A striking similarity is the presence of a bridge of material connecting two galaxies in JWST’s Quintet — a feature also seen in Stephan’s Quintet, indicative of tidal tails produced by the galaxy interaction,” Hu said. “However, the star formation rate of JWST’s Quintet is much higher.”
While all the galaxies in Stephan’s Quintet are much older systems in the nearby universe, and therefore are less active, the galaxies in JWST’s Quintet are rich in gas and are vigorously forming new stars at a rate higher than expected for that period.
JWST’s Quintet, with at least five galaxies and 17 galaxy clumps, has a total stellar mass of 10 billion suns. The study suggests that the high mass and star formation rate indicate that the galaxies in the merger may evolve into a massive quiescent galaxy, occurring approximately 1 billion to 1.5 billion years after the Big Bang. Quiescent galaxies are those that stop forming new stars. Previous JWST studies have detected several of them in the early universe, which raised questions about how galaxies could become “dead” so early in the universe.
Conselice said that the future of merging galaxies is a big question. They might end up as star-forming galaxies but with less activity, or they could just become “dead” or passive over time. The future of the system will also depend on whether the galaxies host actively feeding black holes, which may nudge the system to extinguish star formation very quickly.
If the merging galaxies turn into a dead system, JWST’s Quintet could potentially explain how massive quiescent galaxies can form rapidly through the merger of smaller, starbursting galaxies in the early universe.
Hu noted that JWST’s NIRCam images show clear details of shapes and structures of the objects, but they do not offer precise information like the intensity of spectral lines. Without these spectroscopic details, it’s hard to accurately measure properties such as metallicity, motion and dynamics of the system, or the nature of the gas inside these galaxies and clumps.
If more systems like JWST’s Quintet are found in future JWST surveys, researchers can study how often these merging groups of galaxies appear, their nature, and examine the conditions in which they form. This will enable researchers to verify whether these systems belong to a rare class that the current standard model of the universe predicts, or if they suggest new mechanisms in action.
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