Letter to the Editor

A prediction about the age of thick discs as a function of the stellar mass of the host galaxy

¹ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
e-mail: seb.comeron@gmail.com
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain

Received: 18 December 2020
Accepted: 12 January 2021

Abstract

One of the suggested thick disc formation mechanisms is that they were born quickly and in situ from a turbulent clumpy disc. Subsequently, thin discs formed slowly within them from leftovers of the turbulent phase and from material accreted through cold flows and minor mergers. In this Letter, I propose an observational test to verify this hypothesis. By combining thick disc and total stellar masses of edge-on galaxies with galaxy stellar mass functions calculated in the redshift range of z ≤ 3.0, I derived a positive correlation between the age of the youngest stars in thick discs and the stellar mass of the host galaxy; galaxies with a present-day stellar mass of ℳ_⋆(z = 0) < 10¹⁰ ℳ_⊙ have thick disc stars as young as 4 − 6 Gyr, whereas the youngest stars in the thick discs of Milky-Way-like galaxies are ∼10 Gyr old. I tested this prediction against the scarcely available thick disc age estimates, all of them are from galaxies with ℳ_⋆(z = 0) ≳ 10¹⁰ ℳ_⊙, and I find that field spiral galaxies seem to follow the expectation. On the other hand, my derivation predicts ages that are too low for the thick discs in lenticular galaxies, indicating a fast early evolution for S0 galaxies. I propose the idea of conclusively testing whether thick discs formed quickly and in situ by obtaining the ages of thick discs in field galaxies with masses of ℳ_⋆(z = 0) ∼ 10^9.5 ℳ_⊙ and by checking whether they contain ∼5 Gyr-old stars.