Box/peanut-shaped bulges in action space

Abstract

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, $\rm J_{R,0}$, or vertical, $\rm J_{z,0}$, actions, or high azimuthal action, $\rm J_{\phi ,0}$. Generally populations separated by $\rm J_{R,0}$ have a greater variation in bar strength and vertical heating than those separated by $\rm J_{z,0}$. While the bar substantially weakens the initial vertical gradient of $\rm J_{z,0}$, it also drives a strikingly monotonic vertical profile of $\rm J_{R,0}$. 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.

Predicted trends in Milky Way bulge proper motion rotation curves: future prospects for HST and LSST

“…However the single chemical track of the bulge implies that separating populations by the α−abundance is similar to separating by metallicity. In summary, these observational results point to stronger barred streaming motions in metal-rich stars, which is borne out by models and simulations (Portail et al 2017;Debattista et al 2020). However the origin of each component within the bulge is still a matter of debate.…”

“…This led Di Matteo et al (2019) to argue that, in addition to the radial velocity dispersions, the vertical dispersion also played a key role in the vertical thickening of populations. However, Debattista et al (2020) showed vertical thickening is a monotonic function of the initial radial action of a given stellar population. Consequently, a thick disc can produce a vertical gradient largely because it has a higher radial velocity dispersion.…”

“…The correlation between the two components, as measured by vertex deviation, indicates the presence of elongated stellar orbits (Babusiaux et al 2010;Hill et al 2011;Vásquez et al 2013). Measurements of the vertex deviation in Baade's Window show clear non-zero values in metal-rich stars, indicating their stronger bar structure (Portail et al 2017;Debattista et al 2020). The dependence of bulge kinematics on chemistry is also seen in the radial velocity dispersion.…”

Predicted Trends in Milky Way Bulge Proper Motion Rotation Curves: future Prospects for HST and LSST

“…In summary, these observational results point to stronger barred streaming motions in metal-rich stars, which is borne out by models and simulations (Portail et al 2017;Debattista et al 2020). However, the origin of each component within the bulge is still a matter of debate.…”

Chemodynamical Signatures of Bar Resonances in the Galactic Disk: Current Data and Future Prospects

“…Appendix C Alternate Metallicity Assignment Debattista et al (2020) found, in the context of the Galactic bulge that metallicities assigned to star particles in simulations are more accurate when painted on according to initial axisymmetric action. Figure 12 shows our proposed signatures when metallicities are assigned according to (2020) (left column).…”