Direct Observation of the Filaments in the Cosmic Web
The cosmic web is among the largest structures in the universe, spanning a network of nodes interconnected by filaments. This is where the vast majority of matter resides, as the space between the web is composed of empty voids. However, not all matter is created equal as more complex structures, such as stars and galaxies, become bound by gravity and tend to collect into the nodes. Meanwhile, the filaments are composed of highly diffuse gasses flowing between the nodes, making direct observation of these structures challenging. Efforts to map the cosmic web identify these by considering the proximity between smaller structures and connecting these according to certain algorithms.
A new paper details the identification of an unusually straight and narrow filament, spanning ~5 Mpc, within our local universe. The team, led by Maryam Arabsalmani, stumbled across this structure while studying the surrounding environment of a particular galaxy. They noticed a peculiar alignment of HI sources, prompting them to investigate further. They subsequently confirmed the these as eight galaxies, by cross-referencing with the with the PAN-Starrs optical field. Initial investigations into these galaxies showed that they were confined to a narrow redshift range of z = 0.03609 - 0.03702, suggesting that they could be moving together.
To verify this, the study attempted to map a 1.3Mpc area and attempt to identify any structure. They leveraged the Jansky Very Large Array (JVLA) for a series of observations designed to identify and probe cold hydrogen gas clouds. This yielded 12 new HI sources along the suspected filament, with similar redshifts to the galaxies. Follow up measurements at the Canada-France-Hawaii Telescope (CFHT) only identified one of these within optical bands, suggesting the remaining 11 were not galaxies, but indeed dark gas clouds, a strong indicator in favour of the filamentary hypothesis.
Investigating their spectra, they found that the HI lines of these gas clouds were wider than the nearby galaxies, which is indicative of intense Doppler motions. This suggest that the gas has not yet virialised into a stable structure, which is consistent with the theoretical predications for flowing filamentary gas.
Finally, the study considers whether such linear structures should even exist according to our current understanding of cosmology. For this, they sourced the MillenniumTNG project, a large-scale simulation of the stellar, galactic and dark matter evolution throughout the history of our universe. The team catalogued the filaments therein, characterising these by a thinness parameter. They found that around 4% of all filaments equal or exceed the thinness of the observed structure. Hence, while rare, it is not impossible to observe such filaments within ΛCDM structure formation.
The study concludes that the observed structure is not a chance alignment of unrelated bodies but a direct detection of a filamentary structure. The researchers propose that the detected clouds represent density peaks within the filament, which explains why a continuous stream of gas is not visible. If confirmed, this would be the first observational evidence of such clouds, though the team acknowledges that further observations are needed for definitive confirmation.
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Journal Source: M. Arabsalmani et al, Pearls on a String: Dark and Bright Galaxies on a Strikingly Straight and Narrow Filament, The Astrophysical Journal Letters, Vol. 980, No. 1, (2025), DOI: 10.3847/2041-8213/ada779
Cover Image Credit: Richard G. Stevens