The Structure and Dynamical Evolution of the Stellar Disk of a Simulated Milky Way-Mass Galaxy

TL;DR

This study uses a cosmological simulation to analyze the structure, age, and metallicity gradients of a Milky Way-like galaxy's stellar disk, revealing how formation history and dynamical heating shape observed properties.

Contribution

It provides a detailed analysis of the formation and evolution of the stellar disk in a Milky Way-mass galaxy within a cosmological context, highlighting the impact of star formation modes and feedback.

Findings

Older stars have larger vertical scale heights.

Vertical and radial age and metallicity gradients are consistent with Milky Way observations.

A two-component vertical density profile effectively describes the stellar disk.

Abstract

We study the structure, age and metallicity gradients, and dynamical evolution using a cosmological zoom-in simulation of a Milky Way-mass galaxy from the Feedback in Realistic Environments project. In the simulation, stars older than 6 Gyr were formed in a chaotic, bursty mode and have the largest vertical scale heights (1.5-2.5 kpc) by z=0, while stars younger than 6 Gyr were formed in a relatively calm, stable disk. The vertical scale height increases with stellar age at all radii, because (1) stars that formed earlier were thicker "at birth", and (2) stars were kinematically heated to an even thicker distribution after formation. Stars of the same age are thicker in the outer disk than in the inner disk (flaring). These lead to positive vertical age gradients and negative radial age gradients. The radial metallicity gradient is neg- ative at the mid-plane, flattens at larger disk height |Z|, and turns positive above |Z|~1.5kpc. The vertical metallicity gradient is negative at all radii, but is steeper at smaller radii. These trends broadly agree with observations in the Milky Way and can be naturally understood from the age gradients. The vertical stellar density profile can be well-described by two components, with scale heights 200-500 pc and 1-1.5 kpc, respectively. The thick component is a mix of stars older than 4 Gyr which formed through a combination of several mechanisms. Our results also demonstrate that it is possible to form a thin disk in cosmological simulations even with strong stellar feedback.