Stellar populations across galaxy bars in the MUSE TIMER project
  Stellar populations in barred galaxies save an imprint of the influence of
the bar on the host galaxy's evolution. We present a detailed analysis of star
formation histories (SFHs) and chemical enrichment of stellar populations in
nine nearby barred galaxies from the TIMER project. We use integral field
observations with the MUSE instrument to derive unprecedented spatially
resolved maps of stellar ages, metallicities, [Mg/Fe] abundances and SFHs, as
well as H$\alpha$ as a tracer of ongoing star formation. We find a
characteristic V-shaped signature in the SFH perpendicular to the bar major
axis which supports the scenario where intermediate age stars ($\sim 2$-$6\
\mathrm{Gyr}$) are trapped on more elongated orbits shaping a thinner part of
the bar, while older stars ($> 8\ \mathrm{Gyr}$) are trapped on less elongated
orbits shaping a rounder and thicker part of the bar. We compare our data to
state-of-the-art cosmological magneto-hydrodynamical simulations of barred
galaxies and show that such V-shaped SFHs arise naturally due to the dynamical
influence of the bar on stellar populations with different ages and kinematic
properties. Additionally, we find an excess of very young stars ($< 2\
\mathrm{Gyr}$) on the edges of the bars, predominantly on the leading side,
confirming typical star formation patterns in bars. Furthermore, mass-weighted
age and metallicity gradients are slightly shallower along the bar than in the
disc likely due to orbital mixing in the bar. Finally, we find that bars are
mostly more metal-rich and less [Mg/Fe]-enhanced than the surrounding discs. We
interpret this as a signature that the bar quenches star formation in the inner
region of discs, usually referred to as star formation deserts. We discuss
these results and their implications on two different scenarios of bar
formation and evolution.
