Introducing galactic structure finder: the multiple stellar kinematic structures of a simulated Milky Way mass galaxy

TL;DR

This paper introduces a new method using Gaussian Mixture Models to identify and analyze multiple stellar kinematic structures in a simulated Milky Way galaxy, revealing detailed formation histories and properties.

Contribution

The paper presents the first application of Gaussian Mixture Models to stellar kinematic data in simulated galaxies, enabling detailed separation and analysis of galaxy components.

Findings

Identified multiple stellar structures consistent with observations.

Discovered different formation histories for bulges, discs, and halo.

Quantified angular momentum loss during galaxy evolution.

Abstract

We present the first results of applying Gaussian Mixture Models in the stellar kinematic space of normalized angular momentum and binding energy on NIHAO high resolution galaxies to separate the stars into multiple components. We exemplify this method using a simulated Milky Way analogue, whose stellar component hosts: thin and thick discs, classical and pseudo bulges, and a stellar halo. The properties of these stellar structures are in good agreement with observational expectations in terms of sizes, shapes and rotational support. Interestingly, the two kinematic discs show surface mass density profiles more centrally concentrated than exponentials, while the bulges and the stellar halo are purely exponential. We trace back in time the Lagrangian mass of each component separately to study their formation history. Between z~3 and the end of halo virialization, z~1.3, all components lose a fraction of their angular momentum. The classical bulge loses the most (~95%) and the thin disc the least (~60%). Both bulges formed their stars in-situ at high redshift, while the thin disc formed ~98% in-situ, but with a constant SFR~1.5M$_{\rm\odot}$/yr$^{\rm-1}$ over the last ~11 Gyr. Accreted stars (6% of total stellar mass) are mainly incorporated to the thick disc or the stellar halo, which formed ex-situ 8% and 45% of their respective masses. Our analysis pipeline is freely available at https://github.com/aobr/gsf.