Title: On the orbits that generate the X-shape in the Milky Way bulge,
Abstract: The Milky Way bulge shows a box/peanut or X-shaped bulge (hereafter BP/X)
when viewed in infrared or microwave bands. We examine orbits in an N-body
model of a barred disk galaxy that is scaled to match the kinematics of the
Milky Way (MW) bulge. We generate maps of projected stellar surface density,
unsharp masked images, 3D excess-mass distributions (showing mass outside
ellipsoids), line-of-sight number count distributions, and 2D line-of-sight
kinematics for the simulation as well as co-added orbit families, in order to
identify the orbits primarily responsible for the BP/X shape. We estimate that
between 19-23\% of the mass of the bar is associated with the BP/X shape and
that most bar orbits contribute to this shape which is clearly seen in
projected surface density maps and 3D excess mass for non-resonant box orbits,
"banana" orbits, "fish/pretzel" orbits and "brezel" orbits. {We find that
nearly all bar orbit families contribute some mass to the 3D BP/X-shape. All
co-added orbit families show a bifurcation in stellar number count distribution
with heliocentric distance that resembles the bifurcation observed in red clump
stars in the MW. However, only the box orbit family shows an increasing
separation of peaks with increasing galactic latitude $|b|$, similar to that
observed.} Our analysis shows that no single orbit family fully explains all
the observed features associated with the MW's BP/X shaped bulge, but
collectively the non-resonant boxes and various resonant boxlet orbits
contribute at different distances from the center to produce this feature. We
propose that since box orbits have three incommensurable orbital fundamental
frequencies, their 3-dimensional shapes are highly flexible and, like Lissajous
figures, this family of orbits is most easily able to adapt to evolution in the
shape of the underlying potential.