The radial alignment of dark matter subhalos: from simulations to observations

TL;DR

This study investigates the radial alignment of dark matter subhalos in cosmological simulations, finding a stronger alignment signal than observed, which aligns with observations when focusing on the innermost subhalo regions.

Contribution

It provides the first detailed analysis of projected subhalo alignments in simulations, comparing directly with observational data and highlighting the importance of inner subhalo regions.

Findings

Projected subhalo major axes align with their host's center.

Alignment signal is stronger in simulations than observations.

Focusing on inner 10-20% of subhalo particles matches observed alignment.

Abstract

We explore the radial alignment of subhalos in 2-dimensional projections of cosmological simulations. While most other recent studies focussed on quantifying the signal utilizing the full 3-dimensional spatial information any comparison to observational data has to be done in projection along random lines-of-sight. We have a suite of well resolved host dark matter halos at our disposal ranging from 6 x 10^14 Msun/h down to 6 x 10^13Msun/h. For these host systems we do observe that the major axis of the projected 2D mass distribution of subhalos aligns with its (projected) distance vector to the host's centre. The signal is actually stronger than the observed alignment. However, when considering only the innermost 10-20% of the subhalo's particles for the 2D shape measurement we recover the observed correlation. We further acknowledge that this signal is independent of subhalo mass.