2020MNRAS.498.3334D

We introduce the study of box/peanut (B/P) bulges in the action space of the initial axisymmetric system. We explore where populations with different actions end up once a bar forms and a B/P bulge develops. We find that the density bimodality due to the B/P bulge (the X-shape) is better traced by populations with low radial, J_R,0, or vertical, J_z,0, actions, or high azimuthal action, J_φ,0_. Generally, populations separated by J_R,0 have a greater variation in bar strength and vertical heating than those separated by J_z,0. While the bar substantially weakens the initial vertical gradient of J_z,0, it also drives a strikingly monotonic vertical profile of J_R,0. We then use these results to guide us in assigning metallicity to star particles in a pure N-body model. Because stellar metallicity in unbarred galaxies depends on age as well as radial and vertical positions, the initial actions are particularly well suited for assigning metallicities. We argue that assigning metallicities based on single actions, or on positions, results in metallicity distributions inconsistent with those observed in real galaxies. We therefore use all three actions to assign metallicity to an N-body model by comparing with the actions of a star-forming, unbarred simulation. The resulting metallicity distribution is pinched on the vertical axis, has a realistic vertical gradient, and has a stronger X-shape in metal-rich populations, as found in real galaxies.