The chemical and spatial variations of the bulge's velocity ellipsoids

TL;DR

This study analyzes the velocity ellipsoids in a simulated barred galaxy's bulge, comparing the results with Milky Way data, revealing how stellar age influences kinematic properties and bar imprints.

Contribution

It provides a detailed analysis of velocity ellipsoids in a simulated bulge, highlighting the impact of stellar age and bar structure, and compares these findings with observational data from the Milky Way.

Findings

Quadrupole patterns are stronger in young stellar populations.

Velocity anisotropy and correlation vary with stellar age and position.

Model results qualitatively agree with Milky Way observations.

Abstract

We study the velocity ellipsoids in an $N$-body$+$SPH simulation of a barred galaxy which forms a bar with a BP bulge. We focus on the 2D kinematics, and quantify the velocity ellipses by the anisotropy, $\beta_{ij}$, the correlation, $\rho_{ij}$, and the vertex deviation, $l_{\rm v}$. We explore the variations in these quantities based on stellar age within the bulge and compare these results with the Milky Way's bulge using data from APOGEE DR16 and {\it Gaia} DR3. We first explore the variation of the model's velocity ellipses in galactocentric velocities, $v_R$ and $v_\phi$, for two bulge populations, a (relatively) young one and an old one. The bar imprints quadrupoles on the distribution of ellipse properties, which are stronger in the young population, as expected from their stronger bar. The quadrupoles are distorted if we use heliocentric velocities $v_r$ and $v_l$. We then project these kinematics along the line of sight onto the $(l,b)$-plane. Along the minor axis $\beta_{rl}$ changes from positive at low $|b|$ to negative at large $|b|$, crossing over at lower $|b|$ in the young stars. Consequently the vertex deviation peaks at lower $|b|$ in the young population, but reaches similar peak values in the old. The $\rho_{rl}$ is much stronger in the young stars, and traces the bar strength. The APOGEE stars split by the median [Fe/H] follow the same trends. Lastly we explore the velocity ellipses across the entire bulge region in $(l,b)$ space, finding good qualitative agreement between the model and observations.