James Webb Space Telescope detects a sonic boom bigger than Milky Way

Galactic shock is changing Stephan's Quintet in enigmatic ways, according to ALMA and JWST.
Ayesha Gulzar
Galactic shock shaping Stephan’s Quintet

New observations made with the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA) allowed researchers to see the complex interactions within the multi-galaxy collision event known as Stephan's Quintet.

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.

According to the observations, one of the galaxies, NGC 7318b, is forcing its way into the five within Stephan's Quintet at approximately 1.8 million miles per hour (800 kilometers per second) - a speed that would take you from Earth to the Moon in just eight minutes.

Galactic invasion in Stephan's Quintet

This violent invasion is triggering a shockwave several times as large as the Milky Way that is rippling through the interstellar plasma and kickstarting a "recycling plant" for warm and cold molecular hydrogen gas between the five galaxies.

Typically galaxy collisions and mergers trigger a burst of star formation; that's not the case in Stephan's Quintet. Instead, this violent activity is taking place in the intergalactic medium, away from the galaxies, in places where there is little to no star formation to obstruct the view.

"As the shockwave passes through this clumpy streamer, it is creating a highly turbulent, or unsteady, cooling layer, and it's in the regions affected by this violent activity that we're seeing unexpected structures and the recycling of molecular hydrogen gas," said Philip Appleton, an astronomer and senior scientist at Caltech's Infrared Processing and Analysis Center (IPAC), and lead investigator on the project.

"This is important because 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," said Appleton.

Three key regions of Stephan's Quintet

The discovery allowed scientists to zoom into three key regions of Stephan's Quintet in extreme detail and, for the first time, build a clear picture of how the hydrogen gas is moving and being shaped continuously.

At the heart of the main shockwave, a region designated Field 6, a giant cloud of cold molecules is being broken down and reshaped into a tail of warm molecular hydrogen.

In a region called Field 5, the team spotted two cold clouds of gas linked by a warm molecular hydrogen gas stream. One of the clouds has a bullet-like shape and is punching through this filament, giving rise to a ring-like structure.

Field 4 seems to be the most ordinary and serene of the regions investigated by the team, hosting a less turbulent environment that has seen hydrogen gas collapse to trigger the creation of a disk of stars. The team believes that this marks the beginning of a small dwarf galaxy forming in Field 4.

"These new observations have given us some answers but ultimately showed us just how much we don't yet know. Essentially, we've got one side of the story. Now it's time to get the other," said Appleton.

Researchers believe that these new observations have significant implications for theoretical models of the impact of turbulence in the universe. However, additional work will be needed to understand the effect of high-level turbulence and how hot and cold gas mix.

The observations were presented at the 241st meeting of the American Astronomical Society (AAS) in Seattle, Washington.

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