Made-to-Measure models of the Galactic Box/Peanut bulge: stellar and total mass in the bulge region

TL;DR

This study constructs detailed dynamical models of the Milky Way's Box/Peanut bulge, accurately measuring its mass, stellar content, and kinematic properties, revealing insights into its structure, dark matter content, and rotation speed.

Contribution

It provides the first precise measurement of the bulge's total and stellar mass, and constrains the dark matter fraction, IMF, and pattern speed using Made-to-Measure modeling with new observational data.

Findings

Total bulge mass is 1.84 ± 0.07 × 10^10 Msun.

Stellar mass varies between 1.25-1.6 × 10^10 Msun depending on dark matter.

Pattern speed of the bulge is 25-30 km/s/kpc, indicating slow rotation.

Abstract

We construct dynamical models of the Milky Way's Box/Peanut (B/P) bulge, using the recently measured 3D density of Red Clump Giants (RCGs) as well as kinematic data from the BRAVA survey. We match these data using the NMAGIC Made-to-Measure method, starting with N-body models for barred discs in different dark matter haloes. We determine the total mass in the bulge volume of the RCGs measurement (+-2.2 x +- 1.4 x +- 1.2 kpc) with unprecedented accuracy and robustness to be 1.84 +- 0.07 x10^10 Msun. The stellar mass in this volume varies between 1.25-1.6 x10^10 Msun, depending on the amount of dark matter in the bulge. We evaluate the mass-to-light and mass-to-clump ratios in the bulge and compare them to theoretical predictions from population synthesis models. We find a mass-to-light ratio in the K-band in the range 0.8-1.1. The models are consistent with a Kroupa or Chabrier IMF, but a Salpeter IMF is ruled out for stellar ages of 10 Gyr. To match predictions from the Zoccali IMF derived from the bulge stellar luminosity function requires about 40% or 0.7 x10^10 Msun dark matter in the bulge region. The BRAVA data together with the RCGs 3D density imply a low pattern speed for the Galactic B/P bulge of 25-30 km.s-1.kpc-1. This would place the Galaxy among the slow rotators (R >= 1.5). Finally, we show that the Milky Way's B/P bulge has an off-centred X structure, and that the stellar mass involved in the peanut shape accounts for at least 20% of the stellar mass of the bulge, significantly larger than previously thought.