The nature , size and orientation of the dominant structural components in the Milky Way ’ s inner \sim 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 _ { { \it \Pi } , { int } } = 1.67 \pm 0.27 kpc , and an orientation angle of \alpha = { 37 \degree } ^ { +7 \degree } _ { -10 \degree } 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 \degree angle and has an expected radius of \approx 4.2 kpc , but possibly as low as \approx 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 \pm 0.16 kpc in the wise bands , in good agreement with the literature .