Quantifying the (X/peanut)-shaped structure of the Milky Way - new constraints on the bar geometry

The nature, size and orientation of the Milky Way's bar and 'bulge' have been the subject of conflicting interpretations in the literature. Here, we present a novel approach to inferring the properties of the long bar, which extends beyond the inner 'bulge', by using 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 images, at 3.4 and 4.6 mu m, by measuring the deviations of the isophotes from pure ellipses and using the radial profile of their sixth-order Fourier harmonic (cosine term, B-6). In addition to the vertical height and integrated 'strength' of the observed X/P instability, we report an intrinsic radius R-Pi,R-int = 1.67 +/- 0.27 kpc, and an orientation angle of alpha = 37 degrees(+7 degrees)(-10 degrees) . with respect to our line of sight to the Galactic Centre. Based on X/P structures observed in other galaxies, we assume that the Milky Way's X/P structure is intrinsically symmetric, aligned with the long Galactic bar, and that its size is correlated with this bar. The implications for the Galactic bar are that it is oriented at a 37. angle and has a radius of approximate to 4.2 kpc, but possibly as low as approximate to 3.2 kpc. We have investigated how the Milky Way's X/P structure compares with analogues in other galaxies, and find that it is consistent with recently established scaling relations, though with a marginally stronger X/P 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 scalelength of h = 2.54 +/- 0.16 kpc.