A parametric description of the 3D structure of the Galactic Bar/Bulge using the VVV survey

TL;DR

This paper models the 3D structure of the Galactic Bulge using VVV survey data, revealing a boxy shape, a rotation angle of at least 20 degrees, and estimating its mass with detailed residual analysis.

Contribution

It introduces a parametric model of the Galactic Bulge based on VVV data, including a dust map and detailed density distribution, with new insights into its shape and mass.

Findings

Best-fit model describes the Bulge with 5% residuals

Bulge has a boxy shape with axis ratio 1:0.44:0.31

Estimated Bulge mass is 2.36 x 10^10 solar masses

Abstract

We study the structure of the inner Milky Way using the latest data release of the Vista Variables in Via Lactea (VVV) survey. The VVV is a deep near-infrared, multi-colour photometric survey with a coverage of 300 square degrees towards the Bulge/Bar. We use Red Clump (RC) stars to produce a high-resolution dust map of the VVV's field of view. From de-reddened colour-magnitude diagrams we select Red Giant Branch stars to investigate their 3D density distribution within the central 4 kpc. We demonstrate that our best-fit parametric model of the Bulge density provides a good description of the VVV data, with a median percentage residual of 5$\%$ over the fitted region. The strongest of the otherwise low-level residuals are overdensities associated with a low-latitude structure as well as the so-called X-shape previously identified using the split RC. These additional components contribute only $\sim5\%$ and $\sim7\%$ respectively to the Bulge mass budget. The best-fit Bulge is `boxy' with an axis ratio of [1:0.44:0.31] and is rotated with respect to the Sun-Galactic Centre line by at least $20^{\circ}$. We provide an estimate of the total, full sky, mass of the Bulge of $M_\mathrm{Bulge}^{\mathrm{Chabrier}} = 2.36 \times 10^{10} M_{\odot}$ for a Chabrier initial mass function. We show there exists a strong degeneracy between the viewing angle and the dispersion of the Red Clump absolute magnitude distribution. The value of the latter is strongly dependent on the assumptions made about the intrinsic luminosity function of the Bulge.