James Webb observes an intergalactic shock wave

The James Webb telescope observes an interesting shock wave generated by the intruding galaxy NGC 731b in Stephan's Quintet.
A team of astronomers used ALMA to confirm images from the James Webb Space Telescope that discovered a hot and cold molecular hydrogen recycling facility in Stephan’s Quintet. Credit ALMA

The renowned James Webb Space Telescope observes a shock wave caused by an intergalactic collision in the famous Stephan’s Quintet. Latest observations confirmed also by the ALMA (Atacama Large Millimeter Array) have provided astronomers with a view of an intruding galaxy, NGC 731b, as it traverses this crowded space at an incredible speed of 800km/s.

The violent invasion of NGC 731b has triggered a shockwave larger than the entire Milky Way. The ripples that form in the interstellar plasma have triggered a re-stirring of the cold and warm hydrogen in this tumultuous region of the cosmos.

Galaxy NGC 731b

The discovery of these phenomena is very important in understanding how these violent phenomena can influence both star formation and the evolution of galaxies. “As NGC 731b crashes into the group, it’s passing through an old gas streamer probably caused by a previous interaction between the galaxies. All this is generating a huge shock wave” says in a note (ref.) Philip Appleton, chief astronomer of the research.

“James Webb has captured a shock wave passing through a gaseous streamer, creating a layer of highly turbulent and unstable gas cooling. In the regions affected by this violent activity, we are observing unexpected structures and the recycling of hydrogen. Molecular hydrogen forms the raw material that forms stars. Understanding its fate will tell us more about the evolution of Stephan’s Quintet and galaxies in general” added.

Located about 270 million light-years from Earth in the constellation Pegasus, Stephan’s Quintet includes the galaxies NGC 7317, NGC 7318a, NGC 7318b, NGC 7319, and NGC 7320. The five galaxies have always been an ideal laboratory for studying galactic interactions, including violent collisions and how these interactions influence their environments.

Stephan’s Quintet: the ideal lab

Despite Stephan’s Quintet being an ideal laboratory for studying these phenomena, it has a peculiar characteristic. In the five galaxies that make it up, there is no intense star formation despite constant collisions between the present galaxies. And it is precisely this peculiarity that allows astronomers to observe the turbulence without the mask generated by star formation.

Taking advantage of this opportunity, Appleton and his team enlarged three key regions in Stephan’s Quintet using ALMA, a 66-radio telescope astronomical interferometer in the Atacama desert region of northern Chile. The observations allowed astronomers to build the first clear picture of how gaseous hydrogen is continuously moved and shaped.

A picture of the array of 66 ALMA radio telescopes in Chile
A picture of the array of 66 ALMA radio telescopes in Chile. Credit: ALMA

“The power of ALMA is evident in these observations. It has provided astronomers with new insights and a better understanding of these previously unknown processes” said Joe Pesce, head of the ALMA program at the US National Science Foundation, in the same statement.

The investigated regions

Center of the shock wave is called FIELD6. The region is a huge cloud of cold molecules reshaped by a mass of warm molecular hydrogen. Each time the processes repeat, the hydrogen is recycled through the same temperature phases. “We are observing the disintegration of a cold molecular cloud into super-hot gas. The interesting thing is that the gas simply goes through hot and cold phases” said Appleton. “We still don’t understand these cycles, but we know that the gas goes through these phases because the sizes are bigger than the time it would take for the clouds to be destroyed”

This hydrogen recycling plant is not the only bizarre phenomenon that the shock waves are generating. In another region called FIELD5, the team found cold gas clouds connected by a warm molecular hydrogen flow. One of the clouds is bullet-shaped and is piercing the filament, forming a ring-like structure. Of the investigated regions, FIELD4 seems to be the most normal. It hosts a less turbulent environment and the collapsed hydrogen has triggered the creation of a star disk. The team thinks this is the beginning of the formation of a small dwarf galaxy.

“In field 4, the pre-existing large gas clouds are likely to have become unstable due to the shock and collapsed to form new stars” said Pierre Guillard, researcher at the Institut d’Astrophysique de Paris. “The intergalactic shock wave in the middle of Stephan’s Quintet has formed as much cold molecular gas as we have in our Milky Way. Yet, star formation is slow”. Guillard believes that these observations have significant implications for theoretical models. However, further work will be needed to understand the effect of turbulence and how hot and cold gas mix.

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