ALMA and Webb Reveal Galactic Shock is Shaping Stephan’s Quintet in Mysterious Ways

Shockwaves resulting from the violent collision between an intruder galaxy and Stephan’s Quintet are helping astronomers to understand how turbulence influences gas in the intergalactic medium. New observations with the Atacama Large Millimeter/submillimeter Array (

Astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST) to uncover just what is going on in Stephan’s Quintet, where hot, warm, and cold molecular gas are acting a little strange. This animated video highlights observational fields 4, 5, and 6, the areas where the team discovered that turbulence caused by a giant shockwave has created a recycling plant for warm and cold molecular gas, and is enabling the Quintet’s strange structural behaviors. Field 6 revealed the first indications of a recycling plant, with the area stretching a giant cloud of cold molecules into a tail of warm molecular hydrogen gas on repeat. Field 5 shockingly revealed a high-speed collision where a bullet of gas struck through a molecular cloud, creating a ring and connecting two cold gas clouds together. Field 4, the most normal, is a relatively steady environment, allowing for the growth of what may be a small dwarf galaxy. Credit: ALMA (

“The power of ALMA is obvious in these observations, providing astronomers new insights and better understanding of these previously unknown processes,” said Joe Pesce, Program Officer for ALMA at the U.S. National Science Foundation (NSF).

Stephan’s Quintet is a group of five galaxies—NGC 7317, NGC 7318a, NGC 7318b, NGC 7319, and NGC 7320— generally located about 270 million light-years from Earth in the constellation Pegasus. Credit: IAU/Sky & Telescope

The region at the center of the main shock wave, dubbed Field 6, revealed a giant cloud of cold molecules that is being broken apart and stretched out into a long tail of warm molecular hydrogen and repeatedly recycled through these same phases. “What we’re seeing is the disintegration of a giant cloud of cold molecules in super-hot gas, and interestingly, the gas doesn’t survive the shock, it just cycles through warm and cold phases,” said Appleton. “We don’t yet fully understand these cycles, but we know the gas is being recycled because the length of the tail is longer than the time it takes for the clouds it is made from to be destroyed.”

This intergalactic recycling plant isn’t the only strange activity resulting from the shockwaves. In the region dubbed Field 5, scientists observed two cold gas clouds connected by a stream of warm molecular hydrogen gas. Curiously, one of the clouds— which resembles a high-speed bullet of cold hydrogen gas colliding with a large thread-like filament of spread out gas— created a ring in the structure as it punched through. The energy caused by this collision is feeding the warm envelope of gas around the region, but scientists aren’t quite sure what that means because they don’t yet have detailed observational data for the warm gas. “A molecular cloud piercing through intergalactic gas, and leaving havoc in its wake, may be rare and not yet fully understood,” said Bjorn Emonts, an astronomer at NRAO and a co-investigator on the project. “??But our data show that we have taken the next step in understanding the shocking behavior and turbulent life-cycle of molecular gas clouds in Stephan’s Quintet.”

Perhaps the most “normal” of the bunch is the region dubbed Field 4, where scientists found a steadier, less turbulent environment that allowed hydrogen gas to collapse into a disk of stars and what scientists believe is a small dwarf galaxy in formation. “In field 4, it is likely that pre-existing large clouds of dense gas have become unstable because of the shock, and have collapsed to form new stars as we expect, ” said Pierre Guillard, a researcher at the Institut d’Astrophysique de Paris and a co-investigator on the project, adding that all of the new observations have significant implications for theoretical models of the impact of turbulence in the Universe. “The shock wave in the intergalactic medium of Stephan’s Quintet has formed as much cold molecular gas as we have in our own Milky Way, and yet, it forms stars at a much slower rate than expected. Understanding why this material is sterile is a real challenge for theorists. Additional work is needed to understand the role of high levels of turbulence and efficient mixing between the cold and hot gas.”

Prior to the ALMA observations, scientists had little idea all of this was playing out in the Quintet’s intergalactic medium, but it wasn’t for lack of trying. In 2010, the team used