The Density Parity Model for the Evolution of the Subhalo Inner Spin Alignments with the Cosmic Web

TL;DR

This paper introduces a new analytical model explaining how subhalo inner spin alignments with the cosmic web evolve, considering the competition between internal pressure and external tidal forces, and validates it against high-resolution simulations.

Contribution

The paper presents a novel density parity model that predicts the radius-dependent transition of subhalo spin alignments with the cosmic web, incorporating effects of mass, redshift, and smoothing scale.

Findings

Model accurately predicts the transition radius $r_{th}$ across various parameters.

Excellent agreement between model predictions and N-body simulation results.

The model naturally explains the evolution of spin alignments from high redshift to present.

Abstract

We develop a new model within which the radius-dependent transition of the subhalo inner spins with respect to the cosmic web and the variation of the transition threshold radius ($r_{\rm th}$) with subhalo mass ($M_{\rm vir}$), smoothing scale ($r_{f}$), and redshift ($z$) can be coherently explained. The key tenet of this model is that the competition between the pressure effect of the inner mass and the compression effect of the local tidal field determines which principal direction of the tidal field the inner spins are aligned with. If the former predominates, then only the tidal torques turn on, resulting in the alignments of the inner spins with the intermediate principal axes of the tidal field. Otherwise, the subhalo spins acquire a tendency to be aligned with the shortest axes of the subhalo shapes, which is in the major principal directions of the tidal field. Quantifying the two effects in terms of the densities, we make a purely analytical prediction for $r_{\rm th}(M_{\rm vir}, z, r_{f})$. Testing this model against the numerical results from a high-resolution dark matter only N-body simulation in the redshift range of $0\le z\le 3$ on the galactic mass scale of $11.8\le \log[M_{\rm vir}/(h^{-1}M_{\odot})]\le 12.6$ for two different cases of $r_{f}/(h^{-1}{\rm Mpc})=0.5$ and $1$, we find excellent agreements of the model predictions with the numerical results. It is also shown that this model naturally predicts the alignments between the inner spins of the present subhalos with the principal axes of the high-$z$ tidal field at the progenitors' locations.