The diversity of thick galactic discs

Abstract

Although thick stellar discs are detected in nearly all edge-on disc galaxies, their formationscenarios still remain a matter of debate. Due to observational difficulties, there is a lackof information about their stellar populations. Using the Russian 6-m telescope BTA wecollected deep spectra of thick discs in three edge-on S0-a disc galaxies located in differentenvironments: NGC 4111 in a dense group, NGC 4710 in the Virgo cluster, and NGC 5422 in asparse group.We see intermediate age (4-5 Gyr) metal rich ([Fe/H] ~- 0.2. . . 0.0 dex) stellarpopulations in NGC 4111 and NGC 4710. On the other hand, NGC 5422 does not harbouryoung stars, its disc is thick and old (10 Gyr), without evidence for a second component, andits a-element abundance suggests a 1.5-2 Gyr long formation epoch implying its formation athigh redshift. Our results suggest the diversity of thick disc formation scenarios.