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HST/ACS observations along the major axis of M33 show that the mean age ofits stars decreases with increasing distance from the galaxy center. Such abehavior is consistent with an inside-out growth of the disc. However, in theoutermost observed field, at r$\simeq$11.6 kpc, a reversal of this gradient isdetected, with old stars found in high percentages beyond this radius. In thiswork we investigate the origin of such a reversal in stellar age gradient, byusing a simulated M33 analogue from the Constrained Local UniversE Simulations(CLUES). The simulated M33 is similar to the observed one in terms of mass,rotation velocity, surface brightness and, similar to what has been reported inobservations, shows a stellar age turnaround at large radii. We demonstratethat this reversal is mostly a result of stellar accretion from old satellitegalaxies and, to a lesser extent, of stellar migration of in-situ stars. Theold accreted stars, with formation times tf < 4 Gyrs, are kinematically hot andcan be differentiated from the in-situ stars by their high velocity dispersionand the fact that they do not have rotationally-supported orbits. In thefuture, obtaining kinematic information of the stars in the outskirt of M33will help to verify this scenario.

CLUES about M33: the reversed radial stellar age gradient in the outskirts of Triangulum galaxy