Radius-Dependent Spin Transition of Dark Matter Halos

TL;DR

This study detects a radius-dependent transition in the orientation of dark matter halo spins within simulations, revealing how inner and outer regions align differently with cosmic web structures and how vorticity influences these alignments.

Contribution

It introduces the first numerical evidence of radius-dependent spin orientation transitions in dark matter halos using IllustrisTNG data, highlighting the role of smoothing scales and vorticity.

Findings

Halo spins transition from Tweb intermediate to major axes at smaller radii.

The transition radius depends on the smoothing scale, increasing as the scale decreases.

Vorticity vectors are always perpendicular to the Tweb and Vweb major axes, regardless of radius.

Abstract

A numerical detection of the radius-dependent spin transition of dark matter halos is reported. Analyzing the data from the IllustrisTNG simulations, we measure the halo spin vectors at several inner radii within the virial boundaries and investigate their orientations in the principal frames of the tidal and velocity shear fields, called the Tweb and Vweb, respectively. The halo spin vectors in the high-mass section exhibit a transition from the Tweb intermediate to major principal axes as they are measured at more inner radii, which holds for both of the dark matter and baryonic components. The radius threshold at which the transition occurs depends on the smoothing scale, $R_{f}$, becoming larger as $R_{f}$ decreases. For the case of the Vweb, the occurrence of the radius-dependent spin transition is witnessed only when $R_{f}\ge 1\, h^{-1}$Mpc. Repeating the same analysis but with the vorticity vectors, we reveal a critical difference from the spins. The vorticity vectors are always perpendicular to the Tweb (Vweb) major principal axes, regardless of $R_{f}$, which indicates that the halo inner spins are not strongly affected by the generation of vorticity. It is also shown that the halo spins, as well as the Tweb (Vweb) principal axes, have more directional coherence over a wide range of radial distances in the regions where the vorticity vectors have higher magnitudes. The physical interpretations and implications of our results are discussed.