  We investigate the evolution of stellar population gradients from $z=2$ to
$z=0$ in massive galaxies at large radii ($r > 2R_{\mathrm{eff}}$) using ten
cosmological zoom simulations of halos with $6 \times 10^{12} M_{\odot} <
M_{\mathrm{halo}} < 2 \times 10^{13}M_{\odot}$. The simulations follow metal
cooling and enrichment from SNII, SNIa and AGB winds. We explore the
differential impact of an empirical model for galactic winds that reproduces
the mass-metallicity relation and its evolution with redshift. At larger radii
the galaxies, for both models, become more dominated by stars accreted from
satellite galaxies in major and minor mergers. In the wind model, fewer stars
are accreted, but they are significantly more metal poor resulting in steep
global metallicity ($\langle \nabla Z_{\mathrm{stars}} \rangle= -0.35$ dex/dex)
and color (e.g. $\langle \nabla g-r \rangle = -0.13$ dex/dex) gradients in
agreement with observations. In contrast, colour and metallicity gradients of
the models without winds are inconsistent with observations. Age gradients are
in general mildly positive at $z=0$ ($\langle \nabla Age_{\mathrm{stars}}
\rangle= 0.04$ dex/dex) with significant differences between the models at
higher redshift. We demonstrate that for the wind model, stellar accretion is
steepening existing in-situ metallicity gradients by about 0.2 dex by the
present day and helps to match observed gradients of massive early-type
galaxies at large radii. Colour and metallicity gradients are significantly
steeper for systems which have accreted stars in minor mergers, while galaxies
with major mergers have relatively flat gradients, confirming previous results.
This study highlights the importance of stellar accretion for stellar
population properties of massive galaxies at large radii, which can provide
important constraints for formation models.
