VINTERGATAN-GM: long-lived satellite planes induced by a massive GSE-like merger

Abstract

Satellite galaxies in the Local Group tend to be distributed in thin, planar configurations, with many sharing coherent orbital motion. Galaxy formation simulations in $Λ$CDM have historically struggled to produce similar structures, leading to the so-called "planes of satellites problem". In this work, we investigate whether the emergence of such structures is connected to the mass of a major merger at $z\sim2$, analogous to the Gaia-Sausage-Enceladus (GSE) event in the Milky Way. We use the VINTERGATAN-GM suite of high-resolution zoom-in simulations, comprising five realizations of the same Milky Way-mass halo generated through targeted genetic modifications of a GSE progenitor. The GSE-like merger mass ratio is systematically varied from 1:10 to 1:2.1, while keeping the final dynamical mass and large-scale environment fixed. We find a clear and consistent trend: more massive GSE-like mergers lead to satellite populations that are both more planar and more kinematically coherent. In particular, simulations with merger mass ratios larger than 1:6 develop Kinematic Persistent Planes (KPPs), in which at least 40% of satellites co-orbit around a common axis over extended periods, comparable to those observed in the Milky Way. These structures arise when sufficiently massive mergers, accreted along the direction of maximum compression of the Lagrangian volume, produce flattened host halos with anisotropic velocity dispersions aligned with the merger direction. The merger aligns the host halo's minor axis with the direction of flattening of the surrounding cosmic web, and planes of satellites then emerge through two complementary processes: (i) satellites preferentially infall along the host's equatorial plane, and (ii) anisotropic dynamical friction in the non-spherical halo gradually reshapes their orbits toward this plane, generating coherent and long-lived planar configurations.

Figures (20)

• Figure 1: Examples of four-galaxy-normal-density plots (4GND plots) at three different timesteps (7.5, 10.5 and 13.8 Gyr) for each of the simulations. Each column represents a different timestep and each row a different simulation. The main density peaks are labeled with numbers ordered by decreasing central bin density. The legend in each panel reports the total number of satellites at that timestep, $N_{\rm tot}$, and the peak strength of the main peak, $C_1$. The grayscale colorbar values are proportional to the normalized bin density of four-galaxy-normals. Overdensities in these plots indicate groups of satellites contributing to the same planar configuration and thus reflect an anisotropic satellite distribution, in contrast to the uniform density expected for an isotropic distribution.
• Figure 2: 4GND metrics evolution over cosmic time for each simulation. The first five columns show the temporal evolution of each metric for the five simulations, with green shaded regions indicating the time interval of Merger B. Thin lines show the full time evolution, while thick lines represent the median values computed in 0.5 Gyr bins. First row: Evolution of the peak strength ($C_1$), which quantifies the degree of anisotropy in the distribution of 4-galaxy-normal vectors. Second row: Evolution of the short-to-long axis ratio ($c/a$) of the plane associated with the main peak, computed using $f_{\rm sat} = 90\%$. Third row: Evolution of the root-mean-square thickness ($\Delta_{\rm RMS}$) of the plane associated with the main peak, computed considering $f_{\rm sat} = 90\%$. Fourth row: Evolution of the fraction of co-orbiting satellites relative to the normal of the plane used in the second and third rows. The sixth column shows, for each metric and simulation, the median value over the time evolution since Merger B, with vertical bars indicating the central 16th–84th percentile range. Black and grey dashed lines denote the present-day ($z=0$) values measured for the MW and M31, respectively. In the fourth row, the grey band shows the observed MW value and its uncertainty derived from proper motions uncertainties, while the yellow band corresponds to the expectation for an isotropic distribution of satellite angular momentum directions. The two rightmost columns display the fraction of time ($f_{\rm time}$), computed since Merger B, during which each simulation reaches or exceeds the levels of planarity measured for the MW and M31, according to each metric. A clear trend emerges across the suite, with more massive GSE-like mergers producing thinner and more coherent satellite planes throughout the post-merger evolution.
• Figure 3: Aitoff projections showing the evolution of the orbital poles of satellites along their orbits. Satellite identities are indicated by marker and color, and different points correspond to different output times between $z_{\mathrm{infall}}$ and $z=0$. Each row represents a different simulation. Crosses mark the (axial) direction of maximum satellite co-orbitation, $\hat{n}_{\mathrm{KPP}}$. First column: all surviving satellites at $z=0$. Second column: only the satellites that belong to the Kinematic Persistent Plane (KPP). Third column: satellites that are not members of the KPP. Fourth column: binned density map of the distribution of satellite orbital poles. Each satellite contributes at every output time step, and the grayscale intensity is proportional to the bin density normalized by the total number of counts, allowing comparison across simulations with different satellite populations. In this column, orbital poles are treated as axial vectors to enhance co-orbiting structures irrespective of the sense of rotation; therefore, the density map is restricted to one hemisphere (lon $\in [-90^\circ, 90^\circ]$). On the far right, the corresponding statistics indicate the population fraction of KPP members in each simulation. More massive GSE-like progenitors produce stronger clustering of satellite angular momentum poles and larger KPP populations across the simulation suite.
• Figure 4: Top panels: Fraction of satellites whose orbital poles, $\rm{\boldsymbol{J}_{sat}}$, lie within an angular distance $\alpha$ from the co-orbitation axes $\rm{\hat{n}_{KPP}}$, at given time steps for each simulation. The vertical green dashed line marks the co-orbitation criterion adopted in this work, $\alpha_{\rm co\hbox{-}orbit} = 36.87^\circ$. Bottom panel: Time evolution of the fraction of satellites co-orbiting around $\hat{n}_{\mathrm{KPP}}$ axes. In both panels, the orange solid lines and shaded regions show the results obtained for isotropized configurations of orbital poles. The black dashed line with grey shaded region indicates the corresponding measurements for the MW satellites, based on the data compilation presented in Taibi_portrait_2024. Across the simulation suite, satellite orbital poles transition from nearly isotropic in the Smallest run to strongly clustered around $\hat{n}_{\mathrm{KPP}}$ in the Largest run, reaching co-orbiting fractions of $40$–$50\%$, comparable to the MW.
• Figure 5: Time evolution of the alignment between the direction of maximum satellite co-orbitation, $\hat{n}_{\rm KPP}$, and the normals to the best-fitting positional planes, $\hat{n}_{\rm pos}$, identified in Section \ref{['sec:positional_planes']}. The horizontal gray dashed line marks as a reference $\rm{cos(\alpha_{\rm co-orbit})} = 0.8$. The shaded regions indicate epochs during which the fraction of co-orbiting satellites, $f_{\rm sat}^{\rm coorbit}$, around the positional-plane normal $\hat{n}_{\rm pos}$ is consistent with the values observed in MW (taking into account uncertainties), as identified in the bottom panel of Figure \ref{['fig:4GND_metrics_evolution']}. During these epochs, a clear alignment between $\hat{n}_{\rm KPP}$ and $\hat{n}_{\rm pos}$ is observed, highlighting the role of KPPs as long-lived structures that support transient high-quality positional planes.