Figure. Two-stage accretion around a cosmic filament. Matter and galaxies first flow from the surrounding environment, including cosmic walls, into the filament in directions approximately perpendicular to its axis. Once inside the filament, satellite galaxies are transported toward galaxy groups or clusters through longitudinal and helical motions along the filament axis. Blue arrows indicate lateral infall, while golden arrows indicate transport along the filament.

A research team led by Prof. Wang Peng of the Shanghai Astronomical Observatory (SHAO), Chinese Academy of Sciences, has discovered that satellite galaxies retain a measurable “kinematic memory” of matter flowing into and along cosmic filaments.

Using a large spectroscopic sample from the Sloan Digital Sky Survey (SDSS), the team found that satellite galaxies preferentially orbit within planes containing the axes of nearby cosmic filaments. The orbital orientations deviate from a random distribution at a statistical significance of approximately 12.8σ.

More importantly, the preferred orbital orientation changes with the host galaxy system’s distance from the filament axis. Outside filaments, galaxies move predominantly toward the filaments in directions approximately perpendicular to their axes. Within filaments, the motion gradually shifts to a direction along the filament axes.

The result provides new observational evidence for a two-stage accretion process previously predicted mainly by theoretical studies and numerical simulations: matter first falls laterally from the surrounding environment into a cosmic filament and then flows along the filament toward galaxy groups and clusters.

“If cosmic filaments are highways that transport matter, satellite orbits are like tracks left by the traffic,” said Prof. Wang. “We found evidence not only of galaxies being transported along filaments, but also of the transition from lateral infall toward filaments to motion along their axes.”

According to Prof. Wang, the finding offers a new observational test of the two-stage accretion model. It may also provide dynamical clues to how angular momentum develops in cosmic filaments and to the physical origin of their characteristic radii.

Satellite galaxies orbit central galaxies, and their motions can preserve information about how they entered their host systems. The researchers combined the satellites’ projected positions and line-of-sight velocities to determine how their orbital orientations relate to nearby cosmic filaments.

Cosmic filaments are elongated structures in the cosmic web that connect galaxy groups and clusters. They are thought to channel matter continuously into these systems, but systematic observational evidence for this process has remained limited.

The study, entitled “Observational Evidence for the Kinematic Memory of Cosmic Filaments from Satellite Orbital Orientations,” is published in The Astrophysical Journal Letters. Prof. Wang is the first and corresponding author. The research was conducted in collaboration with Shanghai Jiao Tong University, Zhejiang University, and Nanjing Normal University.

The study was supported by the National Natural Science Foundation of China, including its Major Program and General Program; the dedicated scientific research funding for the China Space Survey Telescope under the Space Application System of the China Manned Space Program; and the Shanghai Rising-Star Program.

Figure. Left: Statistical correlation between satellite orbital planes and cosmic filament orientations. The observed distribution deviates significantly from random, indicating that satellite galaxies preferentially move within planes containing the filament axis. Right: Satellite orbital orientations as a function of distance from the cosmic filament axis. From the filament outskirts toward its interior, the preferred orbital orientation “flips” from perpendicular to parallel to the filament, corresponding to the two accretion stages of lateral infall and transport along the filament.

科学联系人：王鹏
电子邮箱：pwang@shao.ac.cn