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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?

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 an...

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Bibliographic Details
Main Authors: Buder, Sven (Author) , Buck, Tobias (Author) , Skúladóttir, Ása (Author) , Ness, Melissa (Author) , McKenzie, Madeleine (Author) , Monty, Stephanie (Author)
Format: Article (Journal)
Language:English
Published: June 2026
In: Monthly notices of the Royal Astronomical Society
Year: 2026, Volume: 548, Issue: 4, Pages: 1-12
ISSN:1365-2966
DOI:10.1093/mnras/stag729
Online Access:Verlag, kostenfrei, Volltext: https://doi.org/10.1093/mnras/stag729
Verlag, kostenfrei, Volltext: https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag729/8659009
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Author Notes:Sven Buder, Tobias Buck, Ása Skúladóttir, Melissa Ness, Madeleine McKenzie and Stephanie Monty
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Summary: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 Splashlike 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 Vϕ = 73+74 −59 km s−1.
Item Description:Veröffentlicht: 17 April 2026
Gesehen am 10.07.2026
Physical Description:Online Resource
ISSN:1365-2966
DOI:10.1093/mnras/stag729

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