Analysis of the plane of satellites around Milky Way-like galaxies in $Λ$CDM cosmology

TL;DR

This work uses IllustrisTNG simulations to quantify the planar and kinematic properties of satellite systems around Milky Way–like hosts in two mass ranges, showing that MW-like PoS configurations occur at the percent level but are compatible with ΛCDM. By measuring $c/a$, $\Delta_k$, $\Delta_{k7}$, $G$, and $V_{PoS}$ for the 11 most luminous satellites, the authors demonstrate that the MW PoS is a rare but natural outcome, often linked to recent infall and the presence of LMC/SMC–like satellites. The analysis reveals that PoS formation is typically transient and environmentally driven, with satellite accretion along filaments and spin-up of massive satellites serving as key indicators; the MW–like PoS can arise through multiple channels and does not require a special cosmology. The findings align with observational results (e.g., SAGA) that MC-like satellites influence PoS properties and suggest that the MW’s PoS reflects recent dynamical history rather than a persistent structural anomaly, potentially enabling diagnostic use of PoS features as tracers of large-scale structure and accretion history in MW analogs.

Abstract

It has been suggested that the Plane of Satellites (PoS) phenomenon may imply a tension with current $Λ$CDM cosmology since a Milky-Way (MW)-like PoS is very rare in simulations. In this study, we analyze a large sample of satellite systems of MW-like galaxies in the IllustrisTNG simulations. We analyze their spatial aspect ratio, orbital pole dispersion, Gini coefficient, radial distribution, and bulk satellite velocity relative to the host galaxy. These are compared to the observed Milky~Way PoS. We identified galaxy samples in two mass ranges ($0.1 - 0.8 \times 10^{12} $ M$_\odot$ and $0.8 - 3.0 \times 10^{12}$ M$_\odot$). We find for both mass ranges that only $\sim$ 1 percent of MW-like galaxies contain a PoS similar to that of the MW. Nevertheless, these outliers occur naturally in $Λ$CDM cosmology. We analyze the formation, environment, and evolution of the PoS for nine systems that are most MW-like. We suggest that a PoS can form from one or more of at least five different processes. A massive Magellanic~Cloud (MC)-like satellite is found in 1/3 of the systems and probably plays an important role in the PoS formation. We find a tendency for about half of the satellites to have recently arrived at $z < 0.5$, indicating that a MW-like PoS is a recent and transient phenomenon. We also find that a spin up of the angular momentum amplitude of the most massive satellites is an indicator of the recent in-fall of the PoS satellites.

Figures (11)

• Figure 1: Probability distribution of the $c/a$ aspect ratio for the plane of the 11 selected satellites (red line), the total subhalos these satellites are embedded in (green line), and the dark matter halo of the main galaxy (black line). The dashed arrow shows the adopted ratio for the MW PoS, $c/a = 0.182$.
• Figure 2: The two orbital pole dispersions ($\Delta_k$ and $\Delta_{k7}$) as a function of the spatial distribution aspect ratio ($c/a$) for the MW-like satellite systems at redshift $z=0$. The color of each symbol indicates the Gini coefficient of the PoS system. The 11 most luminous satellites are shown in the top panels; a subsample of the 7 most aligned satellites is shown in the second row. The left and right columns correspond to high and low mass ranges, respectively. The MW is represented by the star symbol, color coded to indicate its high Gini coefficient. Satellite systems with a MW-like PoS are labeled by group number in the TNG50 simulations. The two bottom rows are the same as the top two, except the group numbers identify systems with Gini coefficients higher than that of the MW.
• Figure 3: Distance of the 11 most luminous satellites to the host galaxy center in MW-like systems. Results are shown for the high-mass (left column) and low-mass (right column) ranges, ordered from the closest ($n=1$) to the most distant ($n=11$) satellite. Black asterisks show the satellite distance distribution for all MW-like systems. The red dashed line marks the 11 luminous satellites of the Milky Way PoS. Colored solid lines identify satellite systems with a MW-like PoS (top row) or with a Gini coefficient higher than that of the MW (bottom row). The black solid line is the mean distance of the $n$th satellite, and the black dashed lines indicate the standard deviation.
• Figure 4: Orbital pole dispersion ($\Delta_k$) versus spatial distribution aspect ratio ($c/a$) for MW-like satellite systems at $z=0$. Symbols are color-coded by the PoS velocity, $V_{\mathrm{PoS}}$, relative to the main galaxy (km s$^{-1}$). The MW is shown by a star symbol, color-coded to indicate a moderate $V_{\mathrm{PoS}}$. Other aspects match the first two rows of Figure \ref{['fig_delta_k_gini']}.
• Figure 5: Edge-on views of the plane of the nine MW-like PoS systems in the TNG50 simulation on a 300 kpc scale. The stellar component is shown as 2D column density; color represents stellar age (blue to red/yellow for younger to older). The larger circle indicates the system’s virial radius. Small circles mark each of the 11 most luminous satellites. Each system is oriented so the central galaxy is at the origin with its stellar disk in the $xy$ plane; the fitted PoS is edge-on and marked by a dashed line.