[Paper Review] The Debris of the "Last Major Merger" is Dynamically Young

Thomas Donlon, Heidi Jo Newberg+5 more|arXiv (Cornell University)|Oct 13, 2023

Diverse Scientific and Economic StudiesEconomics, Econometrics and Finance3 citations

TL;DR

This paper argues that the Milky Way's inner stellar halo component, traditionally attributed to the ancient Gaia-Sausage/Enceladus (GSE) merger, is instead the result of a recent radial merger—designated the Virgo Radial Merger (VRM)—that occurred 1–3 Gyr ago. Using phase-mixing models and 2D causticality metrics applied to Gaia DR3 data and FIRE-2 simulations, the authors show the observed phase-space folds are dynamically young and inconsistent with an 8–11 Gyr ago collision, challenging the long-held GSE age paradigm.

ABSTRACT

The Milky Way's (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the "last major merger." Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor collided with the MW proto-disk 8-11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the MW disk within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space, because the morphology of debris depends on how long it has had to phase mix. The recently-identified phase-space folds in Gaia DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations at late times. Roughly 20\% of the stars in the prograde local stellar halo are associated with the observed caustics. Based on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1--2 Gyr ago. We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data 1--2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the "last major merger" did not collide with the MW proto-disk at early times, as is thought for the GSE, but instead collided with the MW disk within the last few Gyr, consistent with the body of work surrounding the VRM.

Motivation & Objective

• To re-evaluate the timing of the Milky Way's 'last major merger' using phase-space structure in Gaia DR3 data.
• To test whether the observed phase-space folds in the local stellar halo are consistent with a recent merger or an ancient one like GSE.
• To develop and apply a 2D causticality metric to quantify phase mixing in r–vr phase space, comparing observations with cosmological simulations.
• To determine whether the observed dynamical structure can be explained by the Gaia-Sausage/Enceladus scenario or instead by the Virgo Radial Merger (VRM) model.

Proposed method

• Applied a simple phase-mixing model to estimate the age of the merger based on the number and morphology of observed phase-space folds.
• Used the FIRE-2 m12f Latte cosmological zoom-in simulation to model a GSE-like merger and track phase mixing over time.
• Developed a 2D causticality metric as an extension of the 1D phase-mixing metric from Donlon et al. (2020), quantifying the degree of phase mixing in radial velocity and radius space.
• Compared the observed causticality in Gaia DR3 data with simulated causticality at different times post-collision to find the best match.
• Evaluated the impact of dynamical disequilibrium features such as a large bar on phase-mixing estimates.
• Analyzed the radial density profile evolution in simulations to test consistency with the observed MW halo profile.

Figure 1: A simple semi-analytical model for estimating the time of a radial collision given the number of observed caustics in a range of the Galaxy. The top panel illustrates phase mixing in $r$ - $v_{r}$ phase space; an initial Gaussian perturbation to the phase space density between $r_{\textrm{

Experimental results

Research questions

• RQ1 Is the observed phase-space structure in the Milky Way's inner stellar halo consistent with a merger that occurred 8–11 Gyr ago, as assumed for the Gaia-Sausage/Enceladus event?
• RQ2 Can the morphology of the observed phase-space folds—characterized by positive caustic velocities—be explained by a phase-mixed debris from an ancient merger?
• RQ3 What is the inferred age of the merger that produced the observed phase-space folds, based on phase-mixing dynamics?
• RQ4 Does the observed level of phase mixing in r–vr space match simulations of a GSE-like merger at different times after collision?
• RQ5 Could the observed substructure instead be explained by a more recent radial merger, such as the Virgo Radial Merger (VRM), occurring within the last 3 Gyr?

Key findings

• The observed phase-space folds in Gaia DR3 have positive caustic velocities, indicating they are dynamically young and fundamentally different from the phase-mixed chevrons seen in late-time simulations.
• The number of observed caustics is consistent with a merger that occurred 1–2 Gyr ago, not 8–11 Gyr ago as assumed for the GSE.
• The 2D causticality metric shows the observed data best matches simulated data 1.5 Gyr after a merger, with a reasonable match up to 3 Gyr post-collision.
• Simulations show that phase mixing proceeds rapidly: the radial density profile becomes smooth and monotonically decreasing within 1.3 Gyr after collision, consistent with the observed MW halo profile.
• The observed level of phase mixing is significantly less than expected for a merger 8–11 Gyr ago, making the GSE scenario dynamically inconsistent with the data.
• The results are consistent with the Virgo Radial Merger (VRM) model, where a metal-rich progenitor collided with the MW disk 1.5–3 Gyr ago, producing the observed substructure.

Figure 2: Sample of local stars from Gaia DR3 with high quality phase-space measurements and $|L_{z}|<500$ kpc kms -1 . The top left panel shows the observed distribution of the data in $r$ - $v_{r}$ phase space; the phase-space folds are barely visible as faint wrinkles in the density of stars, par