Mapping the Three-Dimensional Density of the Galactic Bulge with VVV Red Clump Stars

TL;DR

This study maps the three-dimensional density of the Milky Way's bulge using VVV survey data, revealing a highly elongated bar with a boxy/peanut shape and an X-structure, advancing understanding of Galactic structure.

Contribution

It provides the first detailed 3D density map of the Galactic bulge using red clump stars, assuming triaxial symmetry, and measures the bar's orientation and shape.

Findings

Bar angle to line-of-sight: 27±2 degrees

Density falls off exponentially along the bar

Prominent X-structure observed above the Galactic plane

Abstract

The inner Milky Way is dominated by a boxy, triaxial bulge which is believed to have formed through disk instability processes. Despite its proximity, its large-scale properties are still not very well known, due to our position in the obscuring Galactic disk. Here we make a measurement of the three-dimensional density distribution of the Galactic bulge using red clump giants identified in DR1 of the VVV survey. Our density map covers the inner (2.2x1.4x1.1)kpc of the bulge/bar. Line-of-sight density distributions are estimated by deconvolving extinction and completeness corrected K-band magnitude distributions. In constructing our measurement, we assume that the three-dimensional bulge is 8-fold mirror triaxially symmetric. In doing so we measure the angle of the bar-bulge to the line-of-sight to be (27+- 2)deg, where the dominant error is systematic arising from the details of the deconvolution process. The resulting density distribution shows a highly elongated bar with projected axis ratios ~(1:2.1) for isophotes reaching ~2kpc along the major axis. Along the bar axes the density falls off roughly exponentially, with axis ratios (10:6.3:2.6) and exponential scale-lengths (0.70:0.44:0.18)kpc. From about 400pc above the Galactic plane, the bulge density distribution displays a prominent X-structure. Overall, the density distribution of the Galactic bulge is characteristic for a strongly boxy/peanut shaped bulge within a barred galaxy.