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Dynamical Modelling of the Galactic Bulge and Bar: Pattern Speed, Stellar, and Dark Matter Mass Distributions

Surface density of the best fitting model

Dark matter density profile

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Matthieu Portail, Ortwin Gerhard, Christopher Wegg and Melissa Ness, 2017, MNRAS, 465, 1621

We construct a large set of dynamical models of the galactic bulge, bar and inner disk using the Made-to-Measure method. Our models are constrained to match the red clump giant density from a combination of the VVV, UKIDSS and 2MASS infrared surveys together with stellar kinematics in the bulge from the BRAVA and OGLE surveys, and in the entire bar region from the ARGOS survey. We are able to recover the bar pattern speed and the stellar and dark matter mass distributions in the bar region, thus recovering the entire galactic effective potential. We find a bar pattern speed of 39.0 ± 3.5 km s-1 kpc-1, placing the bar corotation radius at 6.1 ± 0.5 kpc and making the Milky Way a typical fast rotator. We evaluate the mass of the long bar and bulge structure to be Mbar/bulge = 1.88 ± 0.12 x 1010 M, larger than the mass of disk in the bar region of only Minner disk = 1.29 ± 0.12 x 1010 M. The total dynamical mass in the bulge volume is 1.85 ± 0.05 x 1010 M.Thanks to more extended kinematic datasets and recent measurement of the bulge IMF we obtain a low dark matter fraction in the bulge of 17% ± 2%. We find a dark matter density profile which flattens to a shallow cusp or core in the bulge region. Finally, we find dynamical evidence for an extra central mass of 2∼109 M, probably in a nuclear disk or disky pseudobulge.