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Abstract:
We investigate the stellar kinematics of a sample of galaxies extracted from the hydrodynamic cosmological Magneticum Pathfinder simulations out to five half-mass radii. We construct differential radial stellar spin profiles quantified by the observationally widely used λ and the closely related (V/σ) parameters. We find three characteristic profile shapes: profiles exhibiting a (i) peak within 2.5 half-mass radii and a subsequent decrease; (ii) continuous increase that plateaus at larger radii typically with a high amplitude; (iii) completely flat behaviour typically with low amplitude, in agreement with observations. This shows that the kinematic state of the stellar component can vary significantly with radius, suggesting a distinct interplay between in-situ star formation and ex-situ accretion of stars. Following the evolution of our sample through time, we provide evidence that the accretion history of galaxies with decreasing profiles is dominated by the anisotropic accretion of low-mass satellites that get disrupted beyond ∼2.0 half-mass radii, building up a stellar halo with non-ordered motion while maintaining the central rotation already present at z = 2. In fact, at z = 2 decreasing profiles are the predominant profile class. Hence, we can predict a distinct formation pathway for galaxies with a decreasing profile and show that the centre resembles an old embedded disc. Furthermore, we show that the radius of the kinematic transition provides a good estimation for the transition radius from in-situ stars in the centre to accreted stars in the halo.
