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The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue II: Were Splash stars heated or already born hot?

Sven Buder, Tobias Buck, Ása Skúladóttir, Melissa Ness, Madeleine McKenzie, Stephanie Monty

TL;DR

Using the NIHAO-UHD Milky Way analogue and tracing birth positions, ages, and chemistry, the study tests whether Splash-like stars originate from merger heating or hot-born formation. The analysis shows protodisc stars were born on dynamically hot orbits and the disc transitioned to rotation support during or after the merger, with no strong evidence for large-scale splashing of in-situ stars, implying Splash-like stars may arise from a turbulent early disc rather than a distinct heated population. The Splash-like azimuthal-velocity distribution is broad and positively skewed around $V_\phi \approx 73$ km s$^{-1}$, consistent with an origin in a hot early disc and subsequent mixing with accreted components and merger-driven gas inflows. The results support a mixed formation scenario where Splash-like populations reflect both hot-born origins and merger-driven processes, underscoring the need for birth-position-aware cosmological simulations to disentangle heating versus hot-born pathways in the Milky Way’s early assembly.

Abstract

One of the most debated consequences of the Milky Way's last major merger is the so-called $Splash$: stars with disc-like chemistry but halo-like kinematics, often interpreted as evidence for the violent heating of an early protodisc. Using the same high-resolution NIHAO-UHD cosmological simulation analysed in Buder et al. (2025b, hereafter Paper I), we test whether, and if so how, a Splash-like population arises in the Milky Way analogue. By tracing stellar birth positions, ages, and present-day orbits, we find that protodisc stars were already born on dynamically hot orbits, with no evidence for significant additional dynamical $splashing$ of these particular in-situ stars despite a 1:5 stellar mass merger. The observed Splash may therefore reflect the already turbulent early disc, subsequently intermixed with accreted stars and those formed from merger-driven gas inflows, rather than a distinct merger-heated population. When selecting stars with similar chemistry and age as the Splash-like ones, we find their azimuthal velocity distribution to be broad and positively skewed, with $V_\varphi = 73_{-59}^{+74}\,\mathrm{km\,s^{-1}}$. The transition to a rotation-supported disc with large azimuthal velocities occurs only during or after the merger. Our results suggest an alternative to the proposed splashing scenario and highlight the need to disentangle the relative contributions of merger-induced heating and intrinsically hot disc formation to clarify the nature of Splash-like stars and their role in shaping the early Milky Way.

The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue II: Were Splash stars heated or already born hot?

TL;DR

Using the NIHAO-UHD Milky Way analogue and tracing birth positions, ages, and chemistry, the study tests whether Splash-like stars originate from merger heating or hot-born formation. The analysis shows protodisc stars were born on dynamically hot orbits and the disc transitioned to rotation support during or after the merger, with no strong evidence for large-scale splashing of in-situ stars, implying Splash-like stars may arise from a turbulent early disc rather than a distinct heated population. The Splash-like azimuthal-velocity distribution is broad and positively skewed around km s, consistent with an origin in a hot early disc and subsequent mixing with accreted components and merger-driven gas inflows. The results support a mixed formation scenario where Splash-like populations reflect both hot-born origins and merger-driven processes, underscoring the need for birth-position-aware cosmological simulations to disentangle heating versus hot-born pathways in the Milky Way’s early assembly.

Abstract

One of the most debated consequences of the Milky Way's last major merger is the so-called : stars with disc-like chemistry but halo-like kinematics, often interpreted as evidence for the violent heating of an early protodisc. Using the same high-resolution NIHAO-UHD cosmological simulation analysed in Buder et al. (2025b, hereafter Paper I), we test whether, and if so how, a Splash-like population arises in the Milky Way analogue. By tracing stellar birth positions, ages, and present-day orbits, we find that protodisc stars were already born on dynamically hot orbits, with no evidence for significant additional dynamical of these particular in-situ stars despite a 1:5 stellar mass merger. The observed Splash may therefore reflect the already turbulent early disc, subsequently intermixed with accreted stars and those formed from merger-driven gas inflows, rather than a distinct merger-heated population. When selecting stars with similar chemistry and age as the Splash-like ones, we find their azimuthal velocity distribution to be broad and positively skewed, with . The transition to a rotation-supported disc with large azimuthal velocities occurs only during or after the merger. Our results suggest an alternative to the proposed splashing scenario and highlight the need to disentangle the relative contributions of merger-induced heating and intrinsically hot disc formation to clarify the nature of Splash-like stars and their role in shaping the early Milky Way.
Paper Structure (9 sections, 6 figures)

This paper contains 9 sections, 6 figures.

Table of Contents

  1. Ages and chemistry of Splash-like stars
  2. Coeval counterparts of Splash-like stars
  3. Birth and present-day orbits of Splash-like stars
  4. Effect on newly forming stars
  5. Discussion
  6. The nature of Splash-like stars
  7. Implications for disc heating and angular momentum build-up
  8. Limitations and outlook
  9. Conclusions

Figures (6)

  • Figure 2: Age distribution of different samples of stars in the Solar neighbourhood as selected in the $\mathrm{[Fe/H]}$ vs. $V_\varphi$ plane of Fig. \ref{['fig:splash_feh_vphi']}. A grey bar indicates the time of the major merger around $8.6\,\mathrm{Gyr}$https://github.com/svenbuder/golden_thread_II/tree/main/figures.
  • Figure 3: Abundance distribution in [Al/Fe] vs. [Mg/Mn] of different samples of stars in the Solar neighbourhood as selected in the $\mathrm{[Fe/H]}$ vs. $V_\varphi$ plane of Fig. \ref{['fig:splash_feh_vphi']}. Contours correspond to the 68 % highest-density interval. An inset figure is showing the distribution of samples C and D with ages of $9-10\,\mathrm{Gyr}$, with contours showing the 40, 60, and 80 % highest-density intervals https://github.com/svenbuder/golden_thread_II/tree/main/figures.
  • Figure 4: Distributions of [Fe/H] and $V_\varphi$ for stars in the Solar neighbourhood at $-0.4 < \mathrm{[Fe/H]} < 0.1$. We show the stacked distribution (black lines) as well as 2-dimensional and 1-dimensional histograms of each distribution. https://github.com/svenbuder/golden_thread_II/tree/main/figures.
  • Figure 5: Density distribution of birth positions in galactocentric cylindrical coordinates $R_\mathrm{birth, 2D}$ and $Z_\mathrm{birth}$ for star particles of samples A-F (corresponding to panels a-f) that are currently in the Solar neighbourhood (black circles) of $2\,\mathrm{kpc}$ around $R_\mathrm{2D} = 8.2\,\mathrm{kpc}$. In panels e) and f) we note the imprint of our selection of in-situ vs. accreted stars via $\vert Z_\mathrm{birth} \vert > 5\,\mathrm{kpc}$ [Eqs. 4 and 5 of][]Buder2025c https://github.com/svenbuder/golden_thread_II/tree/main/figures.
  • Figure 6: Radial and vertical distribution of Splash stars. Panel a) shows birth radii $R_\mathrm{birth, 2D}$ and birth heights $Z_\mathrm{birth}$, whereas panel b) shows current radii $R_\mathrm{2D}$ and heights $Z$. Text insets show the median and 16th to 84th percentiles of each distribution https://github.com/svenbuder/golden_thread_II/tree/main/figures.
  • ...and 1 more figures