N-body Simulation of the Milky Way


The Milky Way, like most other spiral galaxies, is a barred galaxy. Its bar has a semi-major axis length of about half the galactocentric distance of the Sun and is rapidly rotating. In this project, the outputs of a very high-resolution N-body simulation (14 million particles) designed to mimic the Milky Way have been analysed to determine how the bar affects the stellar kinematics in the surrounding disc and whether the induced perturbations may explain some of the "streams" observed in the velocity distribution of stars in the Solar neighbourhood. The Mass Data Storage System was useful to store and have efficient access to the large files produced by the simulation. The present report will focus on the most outstanding results obtained.


Principal Investigator

Roger Fux
Astronomy
RSAA
ANU

Project

b103

Facilities Used

MDSS

Co-Investigator

Ken Freeman
Astronomy
RSAA
ANU

RFCD Codes

240101


Significant Achievements, Anticipated Outcomes and Future Work

Figure (a) shows the velocity distribution of predominantly old stars in the Solar neighbourhood, based on the Hipparcos input (radial velocities) and output (tangential velocities) catalogues. The horizontal (v) and vertical (u) axes represent the azimuthal and radial heliocentric velocities respectively, positive for motion towards galactic rotation and towards the Galactic anti-centre. These data for the first time clearly delineate the "Hercules" stream, characterised by a low angular momentum and a net u>0 average velocity. Such a net outward motion cannot be understood in terms of axisymmetric models of the Milky Way. Moreover, the involved stars are essentially older than 2 billion years, suggesting a dynamical origin of the stream.

The three-dimensional N-body simulation, started from bar-unstable axisymmetric initial conditions, as well as other test particle simulations within a rotating barred potential, have shown that a bar is able to induce Hercules-like streams at realistic space positions for the Sun, i.e. at roughly twice the galactocentric distance of the bar ends (close to the outer Lindblad resonance, hereafter OLR) and a trailing angle relative to the bar major axis of 10-50 degrees. Figure (b) gives an example taken from the N-body simulation, with the u-component of the velocity relative to the Galactic centre.

Figure in pdf format

A detailed orbital study of the simulations has revealed two possible bar-related origins for the Hercules-like streams occuring in them. The first one, which corresponds to an older idea introduced by Agris Kalnajs (RSAA) to explain the Sirius and Hyades streams and recently revived by Walter Dehnen in the Hercules context, is that near the OLR of a barred potential, there exists two distinct families of elongated and perpendicularly-oriented regular orbits, resulting in a bimodality of the velocity distribution. The second origin, which is a completely new interpretation, is that at high orbital energies (in terms of Jacobi's integral), the velocity distribution is dominated by stars on chaotic orbits covering essentially the whole disc, including the Galactic bar region. According to this interpretation, the stars in the Hercules stream follow such orbits, and the u-asymmetry of the stream is due to the fact that its u<0 counterpart in the local velocity plane coincides with a regular orbit region that chaotic orbits are forced to avoid.

Most of the study was restricted to the two-dimensional motion of stars within the Galactic plane, i.e. to the u-v velocity components. A possible extension of this work would be to also take into account the vertical velocity component.

 

Publications, Awards and External Funding

External funding:

Visiting fellow grant from the Swiss National Foundation.

Publications:

R. Fux, Order and chaos in the local disc stellar kinematics induced by the Galactic bar, Astronomy & Astrophysics, 373, 2001, 511-535

R. Fux, Stellar streams in the Solar neighbourhood from high resolution N-body simulations, in "Galaxy Dynamics: from the Early Universe to the Present", eds. F. Combes, G.A. Mamon, V. Charmandaris, ASP conference series vol. 197, 2000, 27-30

R. Fux, Order and chaos in the local disc stellar kinematics, in "Dynamics of Star Clusters and the Milky Way", eds. S. Deiters, B. Fuchs, A. Just, R. Spurzem, R. Wielen, ASP Conference series vol. 228, 2001, 283-289

R. Fux, Order and chaos in the local disc stellar kinematics, in "Stellar Dynamics: from Classical to Modern", eds. L.P. Ossipkov, I.I. Nikiforov, Saint Petersburg University, 2001, 209-212