Astronomers from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS), in collaboration with researchers from Shanghai Jiao Tong University, have directly detected, for the first time, matter flowing in opposite directions along cosmic filaments toward the galaxy clusters at either end.

Using galaxy spectroscopic data from the Sloan Digital Sky Survey (SDSS), the team measured the velocity field in galaxy cluster–filament–cluster systems.

The study was published in Science China Physics, Mechanics & Astronomy.

AI-generated illustration: Due to the gravity of the two clusters, the filament in between can not maintain stability under its own gravity, and flows into the two clusters. This velocity can be measured by subtracting cosmic expansion and stacking multiple similar systems. (Image by SHAO)

Cosmic filaments are channels of matter connecting galaxy clusters. Their velocity fields reveal how matter flows into clusters and how gravity shapes the cosmic web. Directly observing this flow has been difficult because observed redshifts reflect both cosmic expansion and the galaxies' peculiar velocities. The new method avoids relying on linear assumptions used in traditional redshift-space distortion analyses, allowing the researchers to directly measure matter flows on quasi-linear scales.

By selecting cluster pairs separated by 2–6 Mpc and extracting spectroscopic galaxies in the filament region between them, the researchers removed the Hubble flow and rigid-body motion through linear interpolation between the two clusters. The remaining excess redshift revealed the filament’s own infall motion.

The velocity profile was found to reverse sign near the midpoint: filament segments closer to one cluster flowed toward that cluster, while those closer to the other flowed toward the other. This split infall pattern was detected at a significance level of more than 5σ across a broad range of cluster and filament selections.

For clusters of about 10^14.3 solar masses, the maximum infall velocity is about 30 km/s, or about 20 km/s when projected along the line of sight. The results were validated through density–velocity correlation analysis, mass-dependence tests, and comparisons with cosmological simulations.

"Our key idea is to use the geometry of cluster pairs to break the redshift degeneracy," said YAO Ji, first author of the study and a researcher at SHAO. "After subtracting the rigid background, the residual excess redshift directly traces the filament's own inflow."

"This work opens a new window on quasi-linear velocity fields in the cosmic web," said SHAN Huanyuan, corresponding author and researcher at SHAO. "The split infall pattern is a direct manifestation of gravity in large-scale structure and offers a promising probe for mass measurements, gravity tests, and velocity-field reconstruction with upcoming large spectroscopic surveys."

The study was supported by the National Natural Science Foundation of China and the CAS. Other collaborators included ZHANG Pengjie, YANG Xiaohu, ZHOU Jiale, and HAN Jiaxin from Shanghai Jiao Tong University, and WANG Peng and XU Haojie from SHAO.