In the cold dark matter model, hierarchical structure growth predicts not only halos but also their substructure, subhalos, which have long been recognized as a distinct population. For cosmic web filaments, a similar hierarchy was expected but not confirmed due to the lack of robust definitions. Most studies treated all filaments as a single population, which could introduce extra scatter in statistics. This work follows recent attempts to reconstruct the cosmic web hierarchically, alongside studies of filament density profiles and their connection to halo spin.
The authors used five independent cosmological N-body simulations with $1024^3$ particles in a cube of side 300 $h^{-1}$ Mpc, run with the Gadget-2 code. Halos were identified with the phase-space algorithm Rockstar. To extract filaments, they used the hierarchical algorithm Skeletor, which exploits the anisotropy of Voronoi cells and progressively lowers the tracer mass threshold, revealing increasingly smaller structures. Subfilaments were defined by geometric overlap: if more than 50% of a filament's length lies within the radius of a parent filament, it was considered a candidate; then repeated detections of the same structure were removed. After cleaning, 27789 filaments remained, divided into parent filaments and subfilaments.
Subfilaments are predominantly associated with more massive nodes and more often connect nodes with strongly differing masses, indicating their embedding in larger structures. When comparing radial profiles in the same node mass ranges, subfilaments showed 1.5 times denser outskirts, a deeper radial velocity dip (enhanced inflow), and 2 times larger angular momentum. Velocity dispersions (radial and tangential) are systematically higher for subfilaments in all mass ranges, indicating a dynamically hotter environment. These differences persist after controlling for node mass and filament radius, confirming the statistical distinctness of the population.
This work demonstrates for the first time that subfilaments are not a random sample of all filaments but a separate class of objects. Unlike neighboring studies that either constructed universal profiles for all filaments or studied hierarchy only at the halo level, here the hierarchy within filaments themselves is directly identified. This changes the approach to finding universality: perhaps universal profiles exist separately for parent filaments and subfilaments. It also opens the way to studying the evolution of subfilaments and their role in matter transport.
In the future, one could look for universal profiles separately in each population, which would reduce scatter and improve physical interpretation. It is important to trace the evolution of subfilaments over time: whether they form independently and then become embedded in larger filaments, or are born as secondary branches. Also, we need to find out how subfilaments affect mass transfer and the properties of galaxies within them.
The results will affect models of large-scale structure formation, the connection between galaxies and the cosmic web, and the interpretation of observational data (e.g., weak lensing).
A detailed analysis of parent filament–subfilament pairs is needed to test the hypothesis of shared nodes, as well as tracking the evolution of subfilaments in high-resolution simulations.
For the cosmology of large-scale structure, the question of the universality of filament profiles remains open, as does how the environment (tidal fields, matter flows) affects their evolution. This work shows that ignoring hierarchy may be one reason for the large scatter in previous measurements, and offers a path forward.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors applied the hierarchical Skeletor algorithm to five cosmological N-body simulations to identify subfilaments within larger parent filaments. They compared radial density and velocity profiles and found that subfilaments are denser, rotate faster, and have higher velocity dispersions than parent filaments. This indicates that subfilaments form a distinct population rather than a random sample of all filaments.
Where its strength is
This work is the first to directly demonstrate the hierarchy within cosmic filaments, singling out subfilaments as a separate class of objects. It opens the way to searching for universal profiles separately for parent filaments and subfilaments, which could reduce scatter in statistics.
Where it could go next
Worth trying
Why this may matter
Identifying subfilaments as a distinct population could improve modeling of the cosmic web and our understanding of matter and angular momentum transfer in the large-scale structure, which is important for interpreting observations and refining cosmological models.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.52, against a usual 0.64. It looks like few have drilled nearby.
The filament radius is determined by the position of the minimum radial velocity, the place where matter stops falling onto the filament and starts moving along it.