The structure of the Milky Way's bar outside the bulge

While it is incontrovertible that the inner Galaxy contains a bar, its structure near the Galactic plane has remained uncertain, where extinction from intervening dust is greatest. We investigate here the Galactic bar outside the bulge, the long bar, using red clump giant (RCG) stars from United Kingdom Infrared Deep Sky Survey, Two Micron All Sky Survey, Vista Variables in the Via Lactea and Galactic Legacy Infrared Midplane Survey Extraordinaire. We match and combine these surveys to investigate a wide area in latitude and longitude, |b| ≤ 9° and |l| ≤ 40°. We find (i) the bar extends to l ∼ 25° at |b| ∼ 5° from the Galactic plane, and to l ∼ 30° at lower latitudes; (ii) the long bar has an angle to the line-of-sight in the range (28°–33°), consistent with studies of the bulge at |l| < 10°; (iii) the scale height of RCG stars smoothly transitions from the bulge to the thinner long bar; (iv) there is evidence for two scale heights in the long bar; we find a ∼180 pc thin bar component reminiscent of the old thin disc near the Sun, and a ∼45 pc superthin bar components which exist predominantly towards the bar end; (v) constructing parametric models for the red clump magnitude distributions, we find a bar half-length of 5.0 ± 0.2 kpc for the two-component bar, and 4.6 ± 0.3 kpc for the thin bar component alone. We conclude that the Milky Way contains a central box/peanut bulge which is the vertical extension of a longer, flatter bar, similar as seen in both external galaxies and N-body models.