Angular momentum evolution in the CIELO simulations. I. Temporal evolution of gas-stellar misalignments and baryonic merger timing
Gas-stellar kinematic misalignments trace how galaxies assemble and reorient their angular momentum. Although well documented locally, their continuous evolution remains largely unexplored. We aim to characterise the pathways linking aligned, misaligned, and counter-rotating phases, assessing how gas accretion channels and mergers drive reorientation. We analysed 44 central galaxies from the CIELO simulations from z=3.5 to z=0. We defined kinematic episodes using the intrinsic angle, psi, between the star-forming (SF) gas and stellar angular momentum vectors, interpreting these histories by tracking accreted gas origins and evaluating paired statistical contrasts. Nearly 80% of the simulated galaxies are aligned at z=0, yet 86% experience at least one non-aligned episode. Although 19% of non-aligned episodes last >2 Gyr, their durations do not differ significantly from aligned episodes. Abrupt changes in psi coincide with intervals where accreted gas dominates the pre-existing SF reservoir and is highly tilted relative to pre-existing stars. During transitions, the median mass ratio of accreted to pre-existing SF gas rises from 0.57 to 2.14, and the median angular offset increases from 21.2 to 64.6 deg. While relevant mergers cluster near these boundaries, they can drive either alignment or misalignment regardless of mass ratios or orbits. Instead, mergers triggering abrupt transitions typically encounter hosts that are already partially decoupled. In the simulated sample, gas-stellar misalignment is fundamentally driven by reservoir competition. Mergers act as conditional triggers, but a galaxy's ultimate kinematic fate depends strictly on how newly accreted gas couples to, replaces, or mixes with the pre-existing material as it joins the central SF reservoir.