Free Astronomy Magazine March-April 2023

27 MARCH-APRIL 2023 A team of astronomers using the Atacama Large Millimeter/ submillimeter Array (ALMA) and the James Webb Space Telescope (JWST) discovered a re- cycling plant for warm and cold molecular hydrogen gas in Stephan’s Quintet, and it’s causing mysterious things to happen. At left: Field 6, which sits at the center of the main shock wave, is recycling warm and cold hydrogen gas as a giant cloud of cold molecules is stretched out into a warm tail of molecular hy- drogen over and over again. At center: Field 5 unveiled two cold gas clouds con- nected by a stream of warm molecular hydrogen gas characterized by a high-speed collision that is feeding the warm envelope of gas around the region. At right: Field 4 revealed a steadier, less turbulent environment where hydrogen gas collapsed, forming what scientists believe to be a small dwarf galaxy in formation. [ALMA (ESO/NAOJ/NRAO)/JWST/P. Appleton (Caltech), B.Saxton (NRAO/AUI/NSF)] watch what’s happening as one of the galaxies, NGC 7318b, violently intrudes into the group at a relative speed of roughly 800 km/second. At that speed, a trip from Earth to the Moon would take just eight min- utes. “As this intruder crashes into the group, it is colliding with an old gas streamer that likely was caused by a previous interaction between two of the other galaxies, and is causing a giant shockwave to form,” said Philip Appleton, an astronomer and senior scientist at Caltech’s IPAC, and lead investigator on the project. “As the shockwave passes through this clumpy streamer, it is creating a highly turbulent, or unsteady, cool- ing layer, and it’s in the regions af- fected by this violent activity that we’re seeing unexpected structures and the recycling of molecular hy- drogen gas. This is important be- cause molecular hydrogen forms the raw material that may ultimately form stars, so understanding its fate will tell us more about the evolution of Stephan’s Quintet and galaxies in general.“ The new observations using ALMA’s Band 6 (1.3mm wavelength) receiver —developed by NSF’s National Ra- dio Astronomy Observatory (NRAO)— allowed scientists to zoom into three key regions in extreme detail, and for the first time, build a clear picture of how the hydrogen gas is moving and being shaped on a con- tinuous basis. “The power of ALMA and the formation of a new galaxy. 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. The group provides a pristine labo- ratory for the study of galaxy colli- sions and their impact on the surrounding environment. Typically galaxy collisions and mergers trigger a burst of star formation; that’s not the case in Stephan’s Quintet. In- stead, this violent activity is taking place in the intergalactic medium, away from the galaxies in places where there is little to no star for- mation to obstruct the view. That clean window into the Uni- verse has allowed astronomers to 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). The region at the center of the main shock wave, dubbed Field 6, re- vealed a giant cloud of cold mole- cules that is being broken apart and stretched out into a long tail of warm molecular hydrogen and re- peatedly 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 be- cause 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 result- ing from the shockwaves. In the re- gion dubbed Field 5, scientists ob- served two cold gas clouds con- nected by a stream of warm molec- ular hydrogen gas. Curiously, one of the clouds —which resembles a high- speed bullet of cold hydrogen gas colliding with a large thread-like fil- ament of spread out gas— created a ring in the structure as it punched through. The energy caused by this