The Structure of the Milky Way's Bar Outside the Bulge

TL;DR

This study maps the structure of the Milky Way's long bar using red clump giant stars, revealing its extent, orientation, and multiple scale heights, and linking it to the galaxy's bulge and disk components.

Contribution

It provides detailed measurements of the long bar's extent, orientation, and scale heights, and models its structure, enhancing understanding of the Milky Way's central regions.

Findings

The bar extends to l~25deg at |b|~5deg and l~30deg at lower latitudes.

The bar's angle to the line-of-sight is between 28-33 degrees.

Two scale heights are identified: ~180pc and ~45pc.

Abstract

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 UKIDSS, 2MASS, VVV, and GLIMPSE. We match and combine these surveys to investigate a wide area in latitude and longitude, |b|<9deg and |l|<40deg. We find: (1) The bar extends to l~25deg at |b|~5deg from the Galactic plane, and to l~30deg at lower latitudes. (2) The long bar has an angle to the line-of-sight in the range (28-33)deg, consistent with studies of the bulge at |l|<10deg. (3) The scale-height of RCG stars smoothly transitions from the bulge to the thinner long bar. (4) There is evidence for two scale heights in the long bar. We find a ~180pc thin bar component reminiscent of the old thin disk near the sun, and a ~45pc super-thin bar component which exists predominantly towards the bar end. (5) Constructing parametric models for the RC magnitude distributions, we find a bar half length of 5.0+-0.2kpc for the 2-component bar, and 4.6+-0.3kpc 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.