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We introduce and apply a new approach to probe the response of galacticstellar haloes to the interplay between cosmological merger histories andgalaxy formation physics. We perform dark matter-only, zoomed simulations oftwo Milky Way-mass hosts and make targeted, controlled changes to theircosmological histories using the genetic modification technique. Populatingeach history's stellar halo with a semi-empirical, particle tagging approachthen enables a controlled study, with all instances converging to the samelarge-scale structure, dynamical and stellar mass at $z=0$ as their reference.These related merger scenarios alone generate an extended spread in stellarhalo mass fractions (1.5 dex) comparable to the observed population, with thelargest scatter achieved by growing late ($z\leq1$) major mergers that spreadout existing stars to create massive, in-situ dominated stellar haloes.Increasing a last major merger at $z\sim2$ brings more accreted stars into theinner regions, resulting in smaller scatter in the outskirts which arepredominantly built by subsequent minor events. Exploiting the flexibility ofour semi-empirical approach, we show that the diversity of stellar halo massesacross scenarios is reduced by allowing shallower slopes in the stellarmass--halo mass relation for dwarf galaxies, while it remains conserved whencentral stars are born with hotter kinematics across cosmic time. Themerger-dependent diversity of stellar haloes thus responds distinctly toassumptions in modelling the central and dwarf galaxies respectively, openingexciting prospects to constrain star formation and feedback at differentgalactic mass-scales with the coming generation of deep, photometricobservatories.

How cosmological merger histories shape the diversity of stellar haloes