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

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 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 only limited additional dynamical $splashing$ of these particular in-situ stars despite a 1:5 stellar mass merger. A subset of stars, particularly those that end up in the Solar neighbourhood, shows evidence for merger-driven angular-momentum redistribution, but the overall kinematic distribution of stars with $Splash$-like chemistry remains largely unchanged. The observed $Splash$ may therefore primarily 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 $V_φ= 73_{-59}^{+74}\,\mathrm{km\,s^{-1}}$. 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.