Free Astronomy Magazine March-April 2023

28 MARCH-APRIL 2023 ASTRO PUBLISHING collision is feeding the warm enve- lope 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 pierc- ing 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 under- standing the shocking behavior and turbulent life-cycle of molecular gas clouds in Stephan’s Quintet.” Per- haps the most “normal” of the bunch is the region dubbed Field 4, where scientists found a steadier, less turbulent environment that al- lowed 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 observa- tions 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. Under- standing why this material is sterile is a real challenge for theorists. Ad- ditional work is needed to under- stand the role of high levels of turbulence and efficient mixing be- tween the cold and hot gas.” Prior to the ALMA observations, sci- entists had little idea all of this was playing out in the Quintet’s inter- galactic medium, but it wasn’t for lack of trying. In 2010, the team T his 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 Quin- tet’s strange structural behaviors. Field 6 revealed the first indications of a re- cycling 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. [ALMA (ESO/NAOJ/NRAO)/JWST/P. Appleton (Caltech), B.Saxton (NRAO/AUI/NSF)] used NASA’s Spitzer Space Telescope to observe Stephan’s Quintet and discovered large clouds of warm — estimated to be between 100 to 400 kelvin, or roughly -280° to 260° Fahrenheit— molecular hydrogen mixed in with the super-hot gas. “These clouds should have been de- stroyed by the large-scale shock- wave moving through the group, but weren’t. And we wanted to know, and still want to know, how did they survive?” said Appleton. To solve the mystery, the team needed more and different technological power and capability. ALMA’s first light occurred more than a year later, in late 2011 and JWST cap- tured its first images last year. The combination of these powerful re- sources has provided strikingly beau- tiful infrared images of Stephan’s Quintet, and a tantalizing, though incomplete, understanding of the relationship between the cold, warm molecular, and ionized hydro- gen gases in the wake of the giant shockwave. The team now needs spectroscopic data to unlock the secrets of the warm molecular hydrogen gas. “These new observations have given us some answers, but ultimately showed us just how much we don’t yet know,” said Appleton. “While we now have a better understanding of the gas structures and the role of turbulence in creating and sustain- ing them, future spectroscopic obser- vations will trace the motions of the gas through the doppler effect, tell us how fast the warm gas is moving, allow us to measure the temperature of the warm gas, and see how the gas is being cooled or warmed by the shockwaves. Essentially, we’ve got one side of the story. Now it’s time to get the other.” !