Volume 645, January 2021
|Number of page(s)||4|
|Section||Letters to the Editor|
|Published online||26 January 2021|
Letter to the Editor
A prediction about the age of thick discs as a function of the stellar mass of the host galaxy
Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
2 Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
Accepted: 12 January 2021
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) ∼ 109.5 ℳ⊙ and by checking whether they contain ∼5 Gyr-old stars.
Key words: galaxies: evolution / galaxies: spiral / galaxies: structure
© ESO 2021
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