06 February 2025A necklace of pearls on a cosmic filament

Filaments are slender, thread-like structures of dark matter, gas and galaxies, forming a complex network known as the cosmic web. Theoretical models predict that they attract and channel cold gas to fuel star formation in galaxies. An international team including two researchers from Strasbourg astronomical Observatory has now, for the first time, discovered a dozen massive hydrogen clouds along a filament. The filament is also very unusual: it consists of an exceptionally narrow and straight chain of eight galaxies.

Credits: Arabsalmani et al (2025, 2022)/ORIGINS

Background: A cosmic filament with eight galaxies (yellow squares) in a strikingly straight line. The background image comes from the publicly accessible optical sky survey Pan-STARRS of the University of Hawaii. Lower right: Positions of the massive dark gas clouds along the filament (blue squares). Upper left: Galaxy with butterfly-shaped gas distribution (in green) and spiral-shaped star distribution (in purple). Image: Arabsalmani et al (2025, 2022)/ORIGINS.

Supercomputer simulations show that the architecture of the Universe on the largest scales – where galaxies are mere points – has a kind of honeycomb structure. Dark matter, gas and galaxies are distributed over a huge network of filaments, walls and nodes. In between there are large voids, i.e. areas that are comparatively empty. The thread-like filaments are the largest known structures in the cosmos. They consist primarily of diffuse, low-density gas and are only sparsely inhabited by galaxies. In simulations they appear like cosmic highways between the nodes of galaxy clusters: gas can flow into them and galaxies can move within them. A team led by Maryam Arabsalmani, Vera Rubin Fellow at the ORIGINS Cluster, has now discovered a surprisingly narrow and straight filament of eight galaxies and detected huge dark gas clouds in it. The work has been published in the Astrophysical Journal Letters.

Tracing the backbone of the Universe

Observations of the distribution of galaxies in the nearby Universe show a large number of filaments with lengths ranging from ten million to more than 300 million lightyears. The discovery of the eight galaxies, which are arranged in an unusually narrow, straight line at a redshift of z = 0.037, was a happy coincidence. The team originally analysed the strange properties of a spiral galaxy with a butterfly-shaped gas distribution and found that it is in fact part of a filament of eight consecutive galaxies.

Using the Very Large Array (VLA), a configuration of 28 radio telescopes in New Mexico (USA), the researchers were also able to identify cold gas in a segment of the filament based on measurements of the 21 cm hydrogen line. “These are colossal dark gas clouds with billions of solar masses. These are gas masses like those found in large galaxies that actively form stars,” reports Maryam Arabsalmani. However, follow-up optical observations with the MegaCAM instrument on the Canada-France-Hawaii (CFH) Telescope on Mauna Kea in Hawaii, performed by Pierre-Alain Duc from Strasbourg observatory, showed no signs that these gas clouds have ever formed stars. These massive gas clouds without stars are probably located in halos of dark matter in the filament. The existence of such dark gas clouds within filaments, as predicted from simulations, has thus been confirmed for the first time by direct observations.

Rare and strange

With a length of around 16 million lightyears and a diameter of a few hundred thousand lightyears, the newly discovered filament is one of the shortest and narrowest filaments known to date. In addition, the scientists observed a segment of the filament with the VLA and found, compared to similar volumes in the sky, an above-average number of hydrogen-gas clouds. However, its atypical geometry and morphology is compatible with today’s theoretical understanding of the Universe. Florent Renaud from Strasbourg observatory used his expertise in numerical simulations to observe that such systems are unusual and rare, but are still predicted by theoretical models, and can be found in the Millenium TNG simulation of the Universe.

The research team suspects that it is not a coincidence that a filament with such unusual properties features a number of intriguing objects. For instance could a recent passage of a filament cloud through the spiral galaxy with its butterfly-shaped gas distribution explain the mismatch between the location of gas and stars. New observations coupled with numerical simulations shall clarify whether and how the observed peculiarities are related to the special geometry of this unusual filament.

Publication:
M. Arabsalmani, S. Roychowdhury, B. Schneider, V. Springel et al, “Pearls on a string: Dark and bright galaxies on a strikingly straight and narrow filament“, ApJL 2025

Contacts:
P.-A. Duc, pierre-alain.duc@astro.unistra.fr
F. Renaud, florent.renaud@astro.unistra.fr

More information:
– ORIGINS press release.