Phase space structure in the solar neighbourhood

School of Physics & Astronomy, University of Nottingham, Nottingham NG7 2RD, UK e-mail: dalia.chakrbarty@nottingham.ac.uk

Received: 1 November 2006
Accepted: 6 March 2007

Abstract

Aims.We examine the idea that dynamical parameters can be estimated by identifying locations in the solar neighbourhood where velocity distributions recovered from test particle simulations and match the observed local distribution. Here, the dynamical influence of both the Galactic bar and the outer spiral pattern are taken into account.

Methods.The Milky Way disc is stirred by analytical potentials that are chosen to represent the two perturbations, the ratio of pattern speeds of which is explored, rather than held constant. The velocity structure of the final configuration is presented as heliocentric velocity distributions at different locations. These model velocity distributions are compared to the observed distribution in terms of a goodness-of-fit parameter that has been formulated here. We monitor the spatial distribution of the maximal value of this goodness-of-fit parameter, for a given simulation, in order to constrain the solar position from this model. Efficiency of a model is based on a study of this distribution as well as on other independent dynamical considerations.

Results.We reject the bar only and spiral only models and arrive at the following bar parameters from the bar+spiral simulations: bar pattern speed of  km s^-1 kpc^-1  and a bar angle in [ 0°, 30°] , where the error bands are ±1-σ. However, extracting information in this way is no longer viable when the dynamical influence of the spiral pattern does not succumb to that of the bar; an explanation for this is offered. Orbital analysis indicates that even though the basic bimodality in the local velocity distribution can be attributed to scattering off the Outer Lindblad Resonance of the bar, it is the interaction of irregular orbits and orbits of other resonant families, that is responsible for the other moving groups; it is realised that such interaction increases with the warmth of the background disk.