CLUES about M33: the reversed radial stellar age gradient in the
  outskirts of Triangulum galaxy
  HST/ACS observations along the major axis of M33 show that the mean age of
its stars decreases with increasing distance from the galaxy center. Such a
behavior is consistent with an inside-out growth of the disc. However, in the
outermost observed field, at r$\simeq$11.6 kpc, a reversal of this gradient is
detected, with old stars found in high percentages beyond this radius. In this
work we investigate the origin of such a reversal in stellar age gradient, by
using 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 in
observations, shows a stellar age turnaround at large radii. We demonstrate
that this reversal is mostly a result of stellar accretion from old satellite
galaxies and, to a lesser extent, of stellar migration of in-situ stars. The
old accreted stars, with formation times tf < 4 Gyrs, are kinematically hot and
can be differentiated from the in-situ stars by their high velocity dispersion
and the fact that they do not have rotationally-supported orbits. In the
future, obtaining kinematic information of the stars in the outskirt of M33
will help to verify this scenario.
