Modelling the Milky Way as a dry Galaxy

Abstract

We construct a model for the Milky Way Galaxy composed of a stellar disc and\nbulge embedded in a dark-matter halo. All components are modelled as $N$-body\nsystems with up to 8 billion equal-mass particles and integrated up to an age\nof 10\\,Gyr. We find that net angular-momentum of the dark-matter halo with a\nspin parameter of $\\lambda=0.06$ is required to form a relatively short bar\n($\\sim 4$\\,kpc) with a high pattern speed (40--50\\,km\\,s$^{-1}$). By comparing\nour model with observations of the Milky Way Galaxy, we conclude that a disc\nmass of $\\sim 3.7\\times10^{10}M_{\\odot}$ and an initial bulge scale length and\nvelocity of $\\sim 1$\\,kpc and $\\sim 300$\\,km\\,s$^{-1}$, respectively, fit best\nto the observations. The disc-to-total mass fraction ($f_{\\rm d}$) appears to\nbe an important parameter for the evolution of the Galaxy and models with\n$f_{\\rm d}\\sim 0.45$ are most similar to the Milky Way Galaxy. In addition, we\ncompare the velocity distribution in the solar neighbourhood in our simulations\nwith observations in the Milky Way Galaxy. In our simulations the observed gap\nin the velocity distribution, which is expected to be caused by the outer\nLindblad resonance (the so-called Hercules stream), appears to be a\ntime-dependent structure. The velocity distribution changes on a time scale of\n20--30\\,Myr and therefore it is difficult to estimate the pattern speed of the\nbar from the shape of the local velocity distribution alone.\n