Speaker
Description
As a key component of the Universe’s large-scale structure, the kinematics of cosmic filaments has long been debated. Classical theory holds that filaments exhibit multipolar flows without a net, coherent rotation. However, two independent studies in 2021—one simulation-based and one observational—reported detections of filament rotation, sparking broad discussion in the community.
(1) Using the TNG300 simulation, we introduce a side-by-side analysis of mass-weighted versus volume-weighted velocity fields( just momentum vs. velocity). We show for the first time that the mass-weighted field effectively captures the coherent rotation of filaments (with tangential speeds up to ~100 km s⁻¹), whereas the volume-weighted field predominantly exhibits a quadrupolar flow pattern. Accounting for halos embedded within filaments, our statistics indicate that more massive halos display a stronger tendency to orbit around the filament, and that more massive (denser) filaments rotate faster; rotation correlates positively with mass (density). Even after excising all halos with $M>10^{11}\,h^{-1}M_\odot$, the diffuse material within filaments retains substantial angular momentum (tangential speeds up to ~70 km s⁻¹). By systematically comparing prior methodologies, we further clarify why different statistics yield different filament–spin curves: in roughly 30–40% of cases the inner and outer regions of a filament rotate in opposite senses, and the two commonly used definitions of filament spin direction effectively emphasize the inner versus the outer rotation, respectively.
(2) Regarding baryons, we find that gas is more prevalent outside halos, while dark matter is more concentrated within halos. Because rotation correlates with density, dark matter rotates faster inside massive halos, whereas gas rotates faster outside halos. The dominant gas phase—the warm-hot component around $\sim 10^6\, K$—co-rotates with the dark matter, but the cold and hot phases exhibit markedly different spin behaviors: cold baryons ($T<2\times10^4\, K$) are distributed mainly in small central halos and in the diffuse outskirts, and kinematically show high rotation only near the filament core; hot baryons ($T>10^7\, K$) reside primarily in massive halos and display higher rotation in the outer regions. For both the cold- and hot-gas filaments, only about 60% share the same sense of rotation as the total gas filament. Moreover, analysis of halo (galaxy) spin orientations across the filament cross-section shows that within quadrupolar-flow regions, halos in the co-rotating sectors exhibit larger spin amplitudes and their spin axes are more closely aligned with the filament spine—features that provide new, independent evidence for filament rotation.
