A number of 3-D N-body simulations of barred disc galaxies have been reported in recent years (Combes & Sanders 1981, Combes et al. 1990 and Raha et al. 1991) in which the bar always acquired a box-peanut shape when viewed from the side. Here, I compare a model which exhibits this behaviour with the COBE surface photometry and recent kinematic data from the Milky Way bulge.
The initial bar unstable model contains 50K particles chosen so as to create a disc with a Kuz'min-Toomre density profile in the plane and to have a small but finite thickness. This component contains 70% of the total mass, the remaining 30% is in a rigid Plummer sphere which has half the scale length of that of the disc. (Raha et al. showed that the behaviour studied here is not greatly affected by this rigid bulge/halo approximation.) The disc particles were given initial velocities to create an equilibrium disc. The model was evolved using a 3-D Cartesian particle-mesh code having cubic grid cells (double the resolution in each dimension of that used by Raha et al.).
Figure 1: Three orthogonal projections of the particle distribution at equal time intervals throughout the simulation. The boundaries represent the grid edges and the length units are those of the KT disc. Notice the formation of the peanut shape sometime after the bar has formed.
Figure 1 shows that the model undergoes a normal bar instability, as expected, which saturates before time 80, though the disc continues to display weak spiral activity and the bar to intensify further up to time 120. The initial bar rotation period is 20 time units, but slows to about 26 units by the end. The bar gradually bends out of the plane, reaching a maximum around time 140. After this time, the bar abruptly regains symmetry about the disc plane, but is significantly fatter normal to the plane and noticeably shorter. The z-thickness of the bar is greatest at its ends, giving it a pronounced peanut shape when viewed from the side. The thickness of the disc at larger radii is not much affected by the bending of the bar, however. This new model confirms much of the behaviour seen previously using codes of lower spatial resolution. The peanut shape of the object is visible from viewing angles in the plane which are greater than about 30 to the bar major axis, but at even smaller angles, the object still appears boxy (e.g. time 160).
Figure 2: The projected surface brightness as ``observed'' towards the inner galaxy from the position marked in the last frame of Figure 1. The contours levels are spaced by factors of 3.