科研之友
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摘要
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 Bulge Radial Velocity Assay (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 x 10(10)M(circle dot). The stellar mass in this volume varies between 1.25 and 1.6 x 10(10)M(circle dot), 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 initial mass 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 similar to 40 per cent or similar to 0.7 x 10(10)M(circle dot) 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 Omega(p) = 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 per cent of the stellar mass of the bulge, significantly larger than previously thought.
- 出版日期2015-3-21
