Why do some galaxies spin like giant tops, while others appear stationary? This question has puzzled astronomers for decades, and a recent discovery might provide unexpected answers.
An international team has identified a thin chain of galaxies located 140 million light-years away, within a vast cosmic filament. This structure, described in the
Monthly Notices of the Royal Astronomical Society, shows that fourteen hydrogen-rich galaxies are aligned over a length of 5.5 million light-years. Most surprisingly, their rotation appears to follow that of the filament itself, a rare phenomenon that challenges our understanding of the origin of galactic motion.
Illustration showing the rotation of neutral hydrogen in the galaxies of an extended filament, where they exhibit coherent motion following the large-scale cosmic web.
Credit: Lyla Jung Cosmic filaments are the largest known structures in the Universe, forming a network of galaxies and dark matter. They guide matter and energy toward neighboring galaxies, influencing their growth. In this specific case, researchers observed that the galaxies on either side of the filament's axis are moving in opposite directions, indicating an overall rotation of the entire structure. This configuration provides a unique opportunity to study how large cosmic structures transmit their motion to the galaxies they host (to learn more about these structures, see below).
Scientists used dynamic models to estimate the filament's rotation speed at about 110 kilometers per second (nearly 68 miles per second). Its dense central region extends for nearly 163,000 light-years. This young and relatively undisturbed structure, described as "dynamically cold," contains many gaseous galaxies, a sign that it is still forming. The hydrogen present serves as raw material for star creation, making it a prime observation site for understanding galactic evolution.
The implications are numerous: this study helps better understand how galaxies acquire their rotation from surrounding structures. It could also assist future missions like the European Space Agency's Euclid or the Vera C. Rubin Observatory in Chile by identifying intrinsic alignments that could bias cosmological measurements. Understanding these mechanisms opens perspectives on the formation and evolution of the Universe on a large scale.
Cosmic filaments: Matter's highways in the Universe
Cosmic filaments are immense structures that connect galaxy clusters, forming a branched web across the Universe. They are primarily composed of dark matter, an invisible substance that makes up most of the cosmic mass, and hot gas. These filaments stretch for hundreds of millions of light-years and act as conduits for matter, guiding gas toward galaxies where it can fuel star formation.
The discovery of rotation in a filament indicates that these structures are not static but dynamic. Their movement can influence how galaxies form and evolve by transferring energy and matter. This challenges the idea that galaxies develop their rotational motion in isolation, instead pointing to continuous interaction with their large-scale environment.
Current cosmological models predict the existence of these filaments, but directly observing their rotation is exceptional. This observation allows for testing and refining theories about the formation of cosmic structures. It shows how the Universe organizes matter from the vastest scales down to individual galaxies, linking cosmology to galactic astrophysics.
Understanding these filaments is essential for tracing the history of the Universe. They act as fossil archives of primordial matter flows, offering clues about the initial conditions after the Big Bang. By studying their rotation, scientists can better grasp how the Universe evolved to give rise to the galaxies we observe today.