The Origin of Spin-Alignment of Dark Matter Subhalos

TL;DR

The paper investigates how subhalo spins are acquired in host halos and what governs their alignment, with implications for intrinsic alignments in weak lensing. Using the Shin-Uchuu $N$-body simulation, it measures angles between subhalo spin $\vec{J}_{\rm sub}$, host spin $\vec{J}_{\rm host}$, and orbital angular momentum $\vec{J}_{\rm orb}$ as functions of $r/R_{\rm vir}$, mass ratio $M_{\rm sub}/M_{\rm host}$, and accretion redshift $z_{\rm acc}$. It finds that spin–host alignment strengthens toward smaller $r/R_{\rm vir}$ and that subhalo spins align with $\vec{J}_{\rm orb}$, which frequently aligns with $\vec{J}_{\rm host}$, yielding a global $\cos\theta$ signal; prograde orbits enhance this alignment, while polar/retrograde orbits produce perpendicular or anti-parallel spins in the inner regions. These results support a tidal-torque origin for subhalo spin and provide constraints for IA models, highlighting the need for hydrodynamic follow-ups to connect dark matter spin to the baryonic components of satellites.

Abstract

Subhalo spin is essential for modeling galaxy formation and controlling systematic uncertainties in intrinsic alignment (IA) studies. However, the physical mechanisms governing subhalo spin acquisition within the tidal environments of host halos remain poorly understood. In this work, we investigate the alignment between subhalo and host halo spins using the high-resolution cosmological $N$-body simulation, Shin-Uchuu. We find that the spin alignment between subhalos and host halos becomes increasingly pronounced toward the central regions. Our analysis reveals that subhalos typically acquire spin in the same direction as their orbital angular momentum. Since the orbital angular momentum of most subhalos is aligned with the host halo spin, an overall alignment between subhalo and host spins emerges. When classified by orbital orientation, however, subhalo spins in the inner regions are found to be oriented perpendicularly or anti-parallel to the host spin for polar and retrograde orbits, respectively. These results provide strong evidence that subhalo spins are acquired through torques exerted by the tidal field of the host halo. Furthermore, we demonstrate that the mass ratio and the radial distance from the host center are the primary parameters governing subhalo spin alignment, while the dependence on the accretion redshift is found to be negligible.

Figures (6)

• Figure 1: Schematic illustration of the vectors and the alignments of subhalo and host halo used for this study.
• Figure 2: Left : A 2D histogram of the angle between the spin of the subhalos and the host halos, $\cos\theta$, and the distance from the center of the host halos, $r/R_{\rm vir}$. The red lines indicate the median, and the 25th and 75th percentiles. Right : PDF of $\cos\theta$, y-axis shows the deviation from a uniform probability distribution. Data are the full sample (black line) and subsample by $r/R_{\rm vir}$ (blue lines).
• Figure 3: Upper panels: 2D histograms of $\cos\theta$ and $r/R_{\rm vir}$. Lower panels: Corresponding PDFs of $\cos\theta$. Each column represents a subsample restricted by mass ratio: $10^{-3} \le M_{\rm sub}/M_{\rm host} < 10^{-2}$ (left), $10^{-2} \le M_{\rm sub}/M_{\rm host} < 10^{-1}$ (middle), and $10^{-1} \le M_{\rm sub}/M_{\rm host} < 1$ (right).
• Figure 4: Same as Figure \ref{['fig:SAbasic']}, but for the alignment between subhalo spin and orbital angular momentum, $\cos\phi$.
• Figure 5: Upper panels: 2D histograms of $\cos\theta$ versus $r/R_{\rm vir}$. Lower panels: Corresponding PDFs of $\cos\theta$. The sample is restricted to subhalos located near the equatorial plane of the host halo. The columns show results for different orbital orientation: retrograde (left, $-1 \le \cos\eta \le -1/3$), polar (middle, $-1/3 \le \cos\eta < 1/3$) and prograde (right, $1/3 \le \cos\eta < 1$). Here, $\cos\eta$ is the alignment between the subhalo orbital angular momentum $\vec{J}_{\rm orb}$ and the host halo spin $\vec{J}_{\rm host}$.