In What Detail Can We Represent the Milky Way in a Conventional N-Body Model?

Abstract

Abstract. After a brief review of past N -body models of the Milky Way, I consider some of the difficulties that are inherent in the N -body approach to modelling any disk galaxy. Past N -Body Models of the Milky WayModern efforts at modelling the Milky Way (MW) take into account that it is barred. The first large effort in this regard was carried out by Fux (1997), who ran simulations of initially axisymmetric disk+bulge+halo systems. He then compared regularly spaced outputs with a de-reddened COBE K-band bulge map. He scaled the kinematics of his model by requiring that the velocity dispersion matches that of Baade's window. A number of models provided reasonable fits to the data. His best fit bar angle to the sun-center line, ψ bar = 28 • ± 7 • . Later he added SPH gas to his simulations and was able to fit a number of features in the CO gas l − V diagram Fux (1999). He argued that the high-velocity connecting arm is due to the shocked gas within the near side of the bar. The gas distribution is also sensitive to the pattern speed of the bar, and he constrained this parameter to Ω b ∼ 50 km s −1 kpc −1 . Widrow et al., (2008) modelled the MilkyWay by matching observations (including the rotation curve, local force field, Oort's constants, local and bulge velocity dispersions, surface density and total mass within 100 kpc) to a suite of axisymmetric models. Nbody models of these then all produced bars. The time of bar formation depended on the Toomre Q and X parameters; in all cases Ω p started declining after the bar formed and a dynamically young bar is required if Ω p = 50 km s −1 kpc −1 . A problem with this approach is, however, that bar formation leads to the model departing from the observations. N -body models are also very useful for testing models constructing using other methods. Zhao (1996) tested his Schwarzschild model of the MW bulge using N -body simulations, finding that its shape and mass distribution is stable.