On the Flaring of Thick Disc of Galaxies: Insights from Simulations

TL;DR

This study uses cosmological galaxy simulations to analyze how mono-age populations influence the formation and structure of thick galactic discs, revealing conditions that lead to flat or flared discs and their observable properties.

Contribution

It provides new insights into the formation of thick discs by linking the flaring of mono-age populations to galaxy merger history and mass, clarifying when thin and thick discs are distinct or continuous.

Findings

Flat thick discs form with minimally flaring populations and low surface density at flaring radius.

Flared thick discs are associated with high surface density at the flaring radius.

Galaxy mass and merger history influence the age gradients and structural distinction of discs.

Abstract

Using simulated galaxies in their cosmological context, we analyse how the flaring of mono-age populations (MAPs) influences the flaring and the age structure of geometrically-defined thick discs. We also explore under which circumstances the geometric thin and thick discs are meaningfully distinct components, or are part of a single continuous structure as in the Milky Way. We find that flat thick discs are created when MAPs barely flare or have low surface density at the radius where they start flaring. When looking at the vertical distribution of MAPs, these galaxies show a continuous thin/thick structure. They also have radial age gradients and tend to have quiescent merger histories. Those characteristics are consistent with what is observed in the Milky Way. Flared thick discs, on the other hand, are created when the MAPs that flare have a high surface density at the radius where they start flaring. The thick discs' scale-heights can either be dominated by multiple MAPs or just a few, depending on the mass and scale-height distribution of the MAPs. In a large fraction of these galaxies, thin and thick discs are clearly distinct structures. Finally, flared thick discs have diverse radial age gradients and merger histories, with galaxies that are more massive or that have undergone massive mergers showing flatter age radial gradients in their thick disc.