The Diversity of Thick Galactic Discs

TL;DR

This study investigates the stellar populations of thick galactic discs in three edge-on galaxies across different environments, revealing diverse formation histories and ages, from intermediate to old, indicating multiple formation scenarios.

Contribution

It provides new spectroscopic data on thick disc stellar populations in three galaxies, highlighting the diversity of their formation processes and ages.

Findings

NGC4111 and NGC4710 have intermediate age, metal-rich thick discs.

NGC5422 has an old, thick disc with no young stars.

The formation epochs of these discs vary from a few Gyr to high redshift.

Abstract

Although thick stellar discs are detected in nearly all edge-on disc galaxies, their formation scenarios still remain a matter of debate. Due to observational difficulties, there is a lack of information about their stellar populations. Using the Russian 6-m telescope BTA we collected deep spectra of thick discs in three edge-on S0-a disc galaxies located in different environments: NGC4111 in a dense group, NGC4710 in the Virgo cluster, and NGC5422 in a sparse group. We see intermediate age (4-5 Gyr) metal rich ([Fe/H] $\sim$ -0.2 - 0.0 dex) stellar populations in NGC4111 and NGC4710. On the other hand, NGC5422 does not harbour young stars, its disc is thick and old (10 Gyr), without evidence for a second component, and its $\alpha$-element abundance suggests a 1.5-2 Gyr long formation epoch implying its formation at high redshift. Our results suggest the diversity of thick disc formation scenarios.