These Galaxies are Colliding at Two Million Miles Per Hour in Deep Space

Imagine a cosmic battlefield. There are no weapons, but giant galaxies smash into each other, ripping apart interstellar mediums and creating shockwaves that run through the universe. This is pretty much what’s happening Stephan’s Quintet, a famous cluster of galaxies located roughly 94 million light-years away.

A new study used telescope observations to calculate the speed at which some of the galaxies collided, and came to a stunning conclusion: it’s over 2 million miles per hour. But this violent collision doesn’t just destroy anything in its path.

A galactic crossroads

Stephan’s Quintet is basically a group of five galaxies. Four of these galaxies actually form the first compact galaxy group ever discovered, almost 150 years ago. Think of Stephan’s Quintet as a galactic crossroad, where multiple galaxies converge, interact, and crash. In particular, one galaxy (NGC 7318b) is plunging into the group at high velocity, triggering large-scale shock waves.

The turbulence created here sparks star formation, compresses and destroys molecular clouds, and alters the structure of the galaxies themselves. Now, researchers have observed this crash using one of Earth’s most powerful telescopes: the new William Herschel Telescope Enhanced Area Velocity Explorer (WEAVE) in La Palma, Spain.

The WEAVE spectrograph, a state-of-the-art instrument with excellent resolution, allowed scientists to map the shock front with unprecedented detail. However, the astronomers didn’t just use this instrument. They combined data from multiple sources, including radio observations from the LOFAR Two-Metre Sky Survey (LoTSS), X-ray studies, and archival data from the JWST, to create a multiwavelength view of the Quintet.

With all this data, they calculated that NGC 7318b is traveling at a stunning speed of over 3.2 million km/h (2 million mph), colliding into its neighbors and producing powerful shock waves in the nearby galaxies.

“Since its discovery in 1877, Stephan’s Quintet has captivated astronomers, because it represents a galactic crossroad where past collisions between galaxies have left behind a complex field of debris,” says lead researcher Dr. Marina Arnaudova, of the University of Hertfordshire.

Dynamical activity in this galaxy group has now been reawakened by a galaxy smashing through it at an incredible speed of over 2 million mph (3.2 million km/h), leading to an immensely powerful shock, much like a sonic boom from a jet fighter.

The science of galactic shocks

Shocks in intergalactic mediums are like cosmic pressure cookers. They generate energy through turbulence, heating gas, and triggering the formation of stars or destruction of molecular clouds.

“As the shock moves through pockets of cold gas, it travels at hypersonic speeds — several times the speed of sound in the intergalactic medium of Stephan’s Quintet — powerful enough to rip apart electrons from atoms, leaving behind a glowing trail of charged gas, as seen with WEAVE,” Dr. Arnaudova said.

However, when the shock passes through the surrounding hot gas, it becomes much weaker, according to Ph.D. student Soumyadeep Das, of the University of Hertfordshire.

Instead of causing significant disruption, the weak shock compresses the hot gas, resulting in radio waves that are picked up by radio telescopes like the Low Frequency Array (LOFAR).

So, despite the violence of this collision, some molecular hydrogen and dust grains survive, likely forming the basis for post-shock cooling and possible new star formation. Additionally, the shock amplifies radio emissions, increasing luminosity tenfold.

Dense gas and dust pockets, protected from the shock, continue to form molecular hydrogen. Diffuse radio filaments and compact sources track the collision’s effects, including interactions with jets in NGC 7319.

This system showcases how galaxy collisions reshape structure and chemistry, offering insights into cosmic evolution. Thanks to advanced instruments like WEAVE and complementary observations across the electromagnetic spectrum, scientists are piecing together the complex story of these colliding galaxies.

Professor Gavin Dalton, WEAVE principal investigator at RAL Space and the University of Oxford, concludes:

“It’s fantastic to see the level of detail uncovered here by WEAVE. As well as the details of the shock and the unfolding collision that we see in Stephan’s Quintet, these observations provide a remarkable perspective on what may be happening in the formation and evolution of the barely resolved faint galaxies that we see at the limits of our current capabilities.”

Journal Reference: M I Arnaudova et al, WEAVE First Light Observations: Origin and Dynamics of the Shock Front in Stephan’s Quintet, Monthly Notices of the Royal Astronomical Society (2024). DOI: 10.1093/mnras/stae2235