Quantifying the (X/peanut)-shaped Structure of the Milky Way - New
  Constraints on the Bar Geometry
  The nature, size and orientation of the dominant structural components in the
Milky Way's inner ~4 kpc - specifically the bulge and bar - have been the
subject of conflicting interpretations in the literature. We present a
different approach to inferring the properties of the long bar which extends
beyond the inner bulge, via the information encoded in the Galaxy's X/peanut
(X/P)-shaped structure. We perform a quantitative analysis of the X/P feature
seen in WISE wide-field imaging at 3.4 $\mu$m and 4.6 $\mu$m. We measure the
deviations of the isophotes from pure ellipses, and quantify the X/P structure
via the radial profile of the Fourier n=6 harmonic (cosine term $B_6$). In
addition to the vertical height and integrated `strength' of the X/P
instability, we report an intrinsic radius of $R_{\Pi;{\rm int}} = 1.67\pm0.27$
kpc, and an orientation angle of $\alpha = {37^\circ}^{+7^\circ}_{-10^\circ}$
with respect to our line-of-sight to the Galactic Centre. Based on X/P
structures observed in other galaxies, we make three assumptions: (i) the
peanut is intrinsically symmetric, (ii) the peanut is aligned with the long
Galactic bar, and (iii) their sizes are correlated. Thus the implication for
the Galactic bar is that it is oriented at the same $37^\circ$ angle and has an
expected radius of ~4.2 kpc, but possibly as low as ~3.2 kpc. We further
investigate how the Milky Way's X/P structure compares with other analogues,
and find that the Galaxy is broadly consistent with our recently established
scaling relations, though with a moderately stronger peanut instability than
expected. We additionally perform a photometric decomposition of the Milky
Way's major axis surface brightness profile, accounting for spiral structure,
and determine an average disc scale length of $h = 2.54\pm0.16$ kpc in the WISE
bands, in good agreement with the literature.
