WEAVE data overlaid on a James Webb Space Telescope image of Stephan's Quintet, with green contours showing radio data from the Low Frequency Array (LOFAR) radio telescope. The orange and blue colours follow the brightness of Hydrogen-alpha obtained with the WEAVE LIFU, which trace where the intergalactic gas is ionised. The hexagon denotes the approximate coverage of the new WEAVE observations of the system, which is 36 kpc wide (similar in size to our own galaxy, the Milky Way). (Credit: University of Hertfordshire)
LONDON — Car crashes produce loud booms, but it is nothing compared to the shockwaves when two galaxies collide. That’s precisely what happened when galaxy NGC 7318b, traveling two million miles per hour, smashed into another galaxy.
The mesmerizing scene was captured thanks to one of Earth’s most powerful telescopes. The incident occurred in Stephan’s Quintet, a group of five galaxies first spotted 150 years ago. It acts as a galactic crossroads where galaxies bump into each other and leave behind piles of debris in their wake. In this case, the cosmic crash created a powerful shock similar to a sonic boom from a jet plane.
Scientists noticed the galactic crash through Stephan’s Quintet using the William Herschel Telescope Enhanced Area Velocity Explorer (WEAVE) wide-field spectrograph in La Palma, Spain. They recorded their observations in a study published in the Monthly Notices of the Royal Astronomical Society.Â
“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 Dr. Marina Arnaudova, a postdoctoral research fellow at the University of Hertfordshire and the study’s lead author, in a media release. “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.”
Astronomers say the shock from the collision moved through areas of cold gas, where it traveled at hypersonic speeds. For comparison, it moved several times faster than the speed of sound when passing through Stephan’s Quintet. The shockwave’s speed was powerful enough to remove electrons from atoms, resulting in a glowing trail of charged gas — captured by WEAVE.
While the galactic shock moved fast through cold gas, it slowed down and weakened when coming into contact with hot gases. The hotter gases compressed the weakened shockwave, which was seen using radiowaves from radio telescopes such as the Low Frequency Array.Â
WEAVE is a powerful, super-fast mapping device connected to the William Herschel Telescope. It examines the composition of stars and gas in the Milky Way and distant galaxies. This was possible thanks to a spectroscope, which shows the elements that create stars using a bar-code-style pattern within a color spectrum that makes up light.
Along with the current findings, astronomers hope WEAVE will uncover more information on how the galaxy was made, furthering our understanding of the universe.
“I’m excited to see that the data gathered at the WEAVE first light already provide a high-impact result, and I’m sure this is just an early example of the types of discoveries that will be made possible with WEAVE on the William Herschel Telescope in the coming years,” says Dr. Marc Balcells, the director of the Isaac Newton Group of Telescopes.
Paper Summary
Methodology
The researchers conducted a comprehensive analysis of the shock front in Stephan’s Quintet, using integral field spectroscopy with the WEAVE-LIFU instrument on the William Herschel Telescope. This device provided high-resolution spatial and spectral data of the shock region.
Additionally, they combined archival and new multiwavelength data, including radio, optical, and infrared observations, to create a detailed picture of the dynamics and properties of the gas and dust. Spectral fitting methods involving multiple Gaussian components were used to interpret emission lines, allowing the team to characterize gas properties like temperature, density, and motion. Techniques such as the Balmer decrement and emission-line ratio diagnostics were employed to estimate dust extinction and electron density.
Key Results
The study revealed that the shock front in Stephan’s Quintet spans approximately 45 kpc and shows complex interactions between multiple gas phases. Observations suggest that the shock is hypersonic in cold gas but relatively weak in hot plasma, which impacts its efficiency in accelerating particles and generating synchrotron emissions.
The researchers found significant emission from warm molecular hydrogen, which dominates over X-ray emissions, implying that shocks are effectively dissipating energy through molecular processes. High extinction regions correlate with active star formation, while the shock regions appear to have cleared dust and other materials, revealing a turbulent energy cascade across all gas phases.
Study Limitations
The study’s spatial and spectral resolution, while advanced, limited the ability to disentangle certain substructures within the shock region. The authors acknowledged difficulties in obtaining precise temperature measurements for hot gas due to the lack of direct detections of certain emission lines (e.g., [OIII] λ4363). Additionally, some analyses relied on stacked spectra, which may obscure spatial variations. Uncertainties in dust grain survival and molecular gas formation mechanisms within the shock regions also highlight gaps in understanding that require further investigation.
Discussion & Takeaways
This research underscores the complexity of shock-induced processes in interacting galaxy groups like Stephan’s Quintet. The findings highlight the role of shocks in driving energy dissipation through molecular emission, a process that could influence star formation and galaxy evolution.
The study advances understanding of how galaxy collisions reshape interstellar and intergalactic media by stripping gas, forming shocks, and altering chemical abundances. Future high-resolution spectroscopic observations, particularly with instruments like JWST, could refine these results and explore unanswered questions about dust and gas behavior in such extreme environments.
Funding & Disclosures
The WEAVE facility and its operations were supported by an extensive array of institutions and funding sources. Primary funding was provided by UKRI STFC, the University of Oxford, NOVA, NWO, Instituto de AstrofĂsica de Canarias (IAC), and the Isaac Newton Group partners, which include STFC, NWO, and Spain (led by the IAC).
Imagine what will happen when Andromeda finally manages to incorporate our smaller Milky Way galaxy?