  Bulges of edge-on galaxies are often boxy/peanut-shaped (B/PS), and unsharp
masks reveal the presence of an X shape. Simulations show that these shapes can
be produced by dynamical processes driven by a bar which vertically thickens
the centre. In the Milky Way, which contains such a B/PS bulge, the X-shaped
structure is traced by the metal-rich stars but not by the metal-poor ones.
Recently Debattista et al. (2016) interpreted this property as a result of the
varying effect of the bar on stellar populations with different starting
kinematics. This kinematic fractionation model predicts that cooler populations
at the time of bar formation go on to trace the X shape, whereas hotter
populations are more uniformly distributed. As this prediction is not specific
to the Milky Way, we test it with MUSE observations of the B/PS bulge in the
nearby galaxy NGC 4710. We show that the metallicity map is more peanut-shaped
than the density distribution itself, in good agreement with the prediction.
This result indicates that the X-shaped structure in B/PS bulges is formed of
relatively metal-rich stars that have been vertically redistributed by the bar,
whereas the metal-poor stars have a more uniform, box-shaped distribution.
