Stephan’s Quintet is a group of five galaxies (NGC 7317, NGC 7318a, NGC 7318b, NGC 7319, and NGC 7320) located about 270 million light-years from Earth, in the constellation Pegasus. This group serves as an ideal laboratory to study galactic collisions and their consequences on the environment. In general, galaxy collisions and mergers trigger a burst of star formation (the formation of many stars in an area of the cosmos in a much shorter period of time than normal), something that does not happen in Stephan’s Quintet. Instead, there is turbulent activity in the intergalactic medium, far from galaxies, in places where the rate of star formation is very low or even zero, which favors astronomical observation.
By having such a clear view of that area, the astronomical community has been able to observe what happens while one of the galaxies, NGC 7318b, violently joins the group, at a relative speed of about 800 kilometers per second. At that speed, a trip from Earth to the Moon would take just 8 minutes. “While colliding with the group, this interloper collides with an ancient plume of gas, probably left behind by the interaction between two other galaxies, and generates a huge shock wave,” said Philip Appleton, an astronomer at the Center for Infrared Processing and Analysis. (IPAC) of the California Institute of Technology (Caltech) in the United States, who led the research project. “Passing through this dense plume, the shock wave forms a turbulent, or unstable, layer of cooling, and it is in the areas affected by this violent activity that we see unexpected structures and a process of recycling molecular hydrogen gas. This is important because molecular hydrogen creates the raw material that ultimately allows stars to form, hence knowing its evolution allows us to better understand the evolution of Stephan’s Quintet and galaxies in general.”
The new observations, made with Band 6 of the ALMA astronomical observatory, specifically the 1.3mm wavelength receiver developed by the National Radio Astronomical Observatory (NRAO) of the National Science Foundation (NSF) of the United States, allowed the scientific community to gain an extremely detailed view of three key regions and, for the first time, clearly understand how hydrogen gas continuously moves and structures itself.
In the region known as Camp 4, something very special is happening. Here the scientists observed a more stable and less turbulent environment that allows hydrogen gas to concentrate in a disk of stars, and what is believed to be a dwarf galaxy in the process of formation. “In Field 4, it is likely that pre-existing large clouds of dense gas have become unstable due to the collision and have collapsed to form new stars, as would be expected,” explains Pierre Guillard, a researcher at the Institut d’Astrophysique de Paris and co-investigator. of the project.
In this area (Field 4) of Stephan’s Quintet, a more stable and less turbulent environment has been observed, where a large mass of hydrogen has concentrated in what appears to be a dwarf galaxy in the process of formation. (Photo: ALMA (ESO / NAOJ / NRAO) / JWST / P. Appleton (Caltech) / B. Saxton (NRAO / AUI / NSF. CC BY)
At the center of the main shock wave, a region known as Field 6, a huge cloud of gas made of cold molecules was observed being shredded and stretched to form a long plume of hot molecular hydrogen, a process that repeats itself over and over again. time. “What we see is the disintegration of a huge cloud of cold molecules that form a superhot gas, and, interestingly, the gas does not survive the collision, but repeatedly goes through hot and cold stages. We still do not fully understand these cycles, but we know that the gas is being recycled, because the length of the plume is greater than the time it takes to destroy the clouds from which it is formed”, says Philip Appleton.
This intergalactic recycling plant isn’t the only strange activity caused by shock waves. In the region known as Field 5, two gas clouds connected by a flow of hot molecular hydrogen gas were observed. Interestingly, one of the clouds, which looks like a bullet of cold hydrogen gas colliding at high speed with a large filamentary structure of gas, created a ring in the structure as it passed through it. The energy released by this collision is feeding the envelope of hot gas that surrounds the region, although it is not known for sure what happens there, since there are still no detailed observational data for that gas. “The existence of a molecular cloud traversing intergalactic gas and creating chaos in its path may be a rare phenomenon, and we still don’t fully understand it,” says Bjorn Emonts, an NRAO astronomer and co-investigator on the project. “But our data shows that we took an important step in understanding the strange behavior and turbulent cycles of molecular gas clouds in Stephan’s Quintet.”
Before the ALMA observations, very little was known about all these phenomena that occur in the intergalactic milieu of Stephan’s Quintet, but it was not for lack of interest. In 2010, the team used the Spitzer Space Telescope to look at Stephan’s Quintet and discovered large clouds of hot molecular hydrogen — estimated to be between about 170 degrees Celsius below zero and about 100 degrees above zero. These clouds should have been destroyed by the large shock wave traveling through the group, but they were not. Why have they survived?
To solve this mystery, the team needed different and more powerful technological capabilities. More than a year later, at the end of 2011, ALMA picked up its first signal. In 2022, the James Webb Telescope captured its first images. Combining these powerful resources has enabled stunning infrared images of Stephan’s Quintet and a much broader, if incomplete, understanding of the relationship between the cold, hot molecular and ionized hydrogen gases behind the huge shock wave.
“These new observations gave us some answers, but most of all, they showed us how little we still know,” says Philip Appleton. “Although we now better understand the gas structures and how turbulence affects their creation and maintenance, we need more spectroscopic observations to determine the movements of the gas through the Doppler effect, to know how fast the gas is moving, to measure the temperature of the hot gas and see how the gas is being cooled or heated by the shock waves. In short, we have seen one side of the coin. Now we have to see the other one.” (Source: NRAO)