Revisiting the Great Attractor: The Local Group's streamline trajectory, cosmic velocity and dynamical fate

Abstract

We revisit the Great Attractor using the Manticore-Local suite of digital twins of the nearby Universe. The Great Attractor concept has been proposed as an answer to three distinct questions: what sources the Local Group velocity in the cosmic microwave background frame, where present-day velocity streamlines converge, and where the Local Group is moving to. Addressing the original motivation of the Great Attractor -- explaining the Local Group cosmic velocity -- we find that mass within $155~h^{-1}\mathrm{Mpc}$ accounts for only ${\sim}72\%$ of that velocity magnitude with ${\sim}38\,°$ directional offset. We show that even in the purely linear regime convergence within this volume is not guaranteed, particularly when also accounting for small-scale contributions to the observer velocity; no single structure, including the proposed Great Attractor, would be expected to dominate the velocity budget. Streamline convergence is smoothing-scale-dependent, transitioning from Virgo at small scales through the Hydra--Centaurus region at intermediate scales to Shapley at large scales; at intermediate smoothing the convergence point lies near Abell 3565 with an asymmetric basin of mass $\log( M / (h^{-1} \mathrm{M}_\odot)) = 16.4 \pm 0.1$ that excludes Norma. To address the third question, we evolve the Manticore-Local realisations to scale factor $a = 10$ in a new Beyond-Present-Time simulation suite and identify the asymptotic future location of the Local Group. We find that the dominant motion is towards Virgo, but even it contributes at most one third of the Local Group velocity. Our results demonstrate that the classical Great Attractor is not a dynamically dominant structure but an artifact of the instantaneous velocity field, and that no single attractor is likely to account for the Local Group motion in the cosmic rest frame.

Figures (10)

• Figure 1: Milky Way streamline convergence points in Galactic coordinates for different smoothing scales applied to the Manticore-Local velocity fields. Left and right panels show the Galactic longitude and latitude as functions of distance from the observer, respectively. Colours represent different smoothing scales; each line shows the streamline from a single Manticore-Local realisation. Three discrete endpoints emerge: without smoothing, streamlines converge to Virgo; at $\sigma_{\rm smooth}=3~h^{-1}\,\mathrm{Mpc}$ to the cGA; and at $\sigma_{\rm smooth}=8~h^{-1}\,\mathrm{Mpc}$ to Shapley. Black points indicate the approximate positions of these structures, with the cGA position adopted from Dupuy_2025.
• Figure 2: Enclosed mass profile around the Manticore-Local-inferred cGA position at $(\ell,\,b) = (313^{+1}_{-5},\,27^{+2}_{-4})\,\deg$ and distance $r=41^{+2}_{-5}~h^{-1}\,\mathrm{Mpc}$ identified from the Milky Way streamline at $\sigma_{\rm smooth}=3~h^{-1}\,\mathrm{Mpc}$, averaged over 80 Manticore-Local realisations. For comparison, we also show profiles centred on Norma, Centaurus, and random positions. The profile exhibits a $2-3\sigma$ overdensity out to $20~h^{-1}\,\mathrm{Mpc}$, confirming a significant mass concentration at this location.
• Figure 3: Depth of the inferred cGA basin as a function of sky position in Galactic coordinates, averaged over 80 Manticore-Local realisations. The basin is defined by selecting all voxels whose streamlines converge to the cGA position identified from the Milky Way streamline at $\sigma_{\rm smooth}=3~h^{-1}\,\mathrm{Mpc}$, and "cGA c.p." indicates its convergence point. For each sky direction, the depth is computed as the distance to the farthest voxel whose streamline converges to the cGA position. The basin is highly elongated, reaching six times farther toward the convergence point than away from it. Clusters that belong to the cGA basin in $>50\%$ of the realisations are plotted in cyan, whilst other clusters from \ref{['tab:cluster_catalogue']} within $100~h^{-1}\,\mathrm{Mpc}$ are shown in green. The inferred cGA convergence point is also indicated in cyan; the ZoA (white lines at $b = \pm 10\,\deg$) is shown for reference.
• Figure 4: Displacement vectors of particles within $R = 5~h^{-1}\,\mathrm{cMpc}$ of the observer position (centre of the box), tracked from $a = 1$ to $a = 10$, shown in Galactic coordinates (magnitude, longitude, and latitude, respectively) stacked over the simulations. Particles typically move towards the Virgo cluster but fall short of reaching it; Virgo's distance from the box centre at $a = 10$ is approximately $14~h^{-1}\,\mathrm{cMpc}$. Vertical lines mark Virgo at $(\ell,\,b) = (287.0,\,73.2)\,\deg$ (red) and the LG CMB-frame velocity at $(\ell,\,b) = (271.9,\,29.6)\,\deg$ (cyan) as reported by PlanckPlanck_2018. The $\ell$ and $b$ panels show the expected isotropic distribution as black dashed lines.
• Figure 5: Observer velocity sourced by matter within radius $R$ in Manticore-Local. Left: Amplitude $|\bm{V}_{\rm inner}(R)|$. Centre: Galactic longitude $\ell$ of $\bm{V}_{\rm inner}(R)$. Right: Galactic latitude $b$ of $\bm{V}_{\rm inner}(R)$. "Manticore (linear)" shows the linear velocity sourced by matter within radius $R$, computed using \ref{['eq:linear_velocity']}. "Manticore (obs)" denotes the observer velocity computed from the entire Manticore-Local simulation box, including the external velocity dipole of $90 \pm 8~\mathrm{km}\,\mathrm{s}^{-1}$. "Manticore (obs + $\sigma_v$)" includes $150~\mathrm{km}\,\mathrm{s}^{-1}$ scatter to mimic non-linear contributions. Horizontal red bands indicate the observed LG velocity in the CMB frame Planck_2018. Vertical grey bands indicate the 16th--84th percentile on the inferred distance to the cGA convergence point in Manticore-Local; mass within the cGA basin contributes ${\sim}75\%$ of the observer velocity amplitude, but the direction at this radius differs from the final direction by ${\sim}20\,\deg$, requiring contributions from larger scales to rotate the velocity vector. All error bars and shaded regions show $1\sigma$ uncertainties. The "Manticore (obs)" and "Manticore (obs + $\sigma_v$)" markers are offset horizontally for visual clarity.