NIHAO VI. The hidden discs of simulated galaxies

TL;DR

This paper introduces a new method using Gaussian Mixture Models to define stellar kinematic discs in simulated galaxies, revealing differences between photometric and kinematic structures across galaxy masses.

Contribution

It provides a robust, dynamical-based approach for identifying galaxy components in simulations, improving comparison with observations.

Findings

Kinematic ratios are around 0.5 despite high photometric disc-to-total ratios.

Distinct properties of thin discs and spheroids emerge above log(M*)~9.5.

At lower masses, discs and spheroids are less distinct and tend to be thickened.

Abstract

Detailed studies of galaxy formation require clear definitions of the structural components of galaxies. Precisely defined components also enable better comparisons between observations and simulations. We use a subsample of eighteen cosmological zoom-in simulations from the NIHAO project to derive a robust method for defining stellar kinematic discs in galaxies. Our method uses Gaussian Mixture Models in a 3D space of dynamical variables. The NIHAO galaxies have the right stellar mass for their halo mass, and their angular momenta and S\'ersic indices match observations. While the photometric disc-to-total ratios are close to 1 for all the simulated galaxies, the kinematic ratios are around ~0.5. Thus, exponential structure does not imply a cold kinematic disc. Above log(M*)~9.5, the decomposition leads to thin discs and spheroids that have clearly different properties, in terms of angular momentum, rotational support, ellipticity, [Fe/H] and [O/Fe]. At log(M*)<9.5, the decomposition selects discs and spheroids with less distinct properties. At these low masses, both the discs and spheroids have exponential profiles with high minor-to-major axes ratios, i.e. thickened discs.