Context. Recent integral-field spectroscopy observations of edge-on galaxies have led to significant progress in our knowledge of the ages and chemical compositions of thick disks. However, the origin of thick disks and their evolutionary connection with thin disks is still a matter of debate.
Aims: We provide new insights into this topic by connecting the stellar populations of thick disks at redshift z = 0 with their past formation and growth in 24 Milky Way-mass galaxies from the AURIGA zoom-in cosmological simulations. We assess the role played by mergers of satellite galaxies in the mass assembly of geometrically defined thick disks.
Methods: We projected each galaxy edge on and decomposed it morphologically into two disk components in order to geometrically define the thin and thick disks, as is usually done in observations of external galaxies. We produced age, metallicity, and [Mg/Fe] edge-on maps of the 24 galaxies. We quantified the impact of satellite mergers by mapping the distribution of ex situ stars.
Results: Thick disks are on average ∼3 Gyr older, ∼0.25 dex more metal poor, and ∼0.06 dex more [Mg/Fe]-enhanced than thin disks. Their average ages range from ∼6 to ∼9 Gyr, metallicities from ∼ − 0.15 to ∼0.1 dex, and [Mg/Fe] from ∼0.12 to ∼0.16 dex. These properties are the result of an early initial in situ formation, followed by a later growth driven by the combination of direct accretion of stars, some in situ star formation fueled by mergers, and dynamical heating of stars. The balance between these processes varies from galaxy to galaxy and impacts thick-disk ages and metallicities. The oldest thick disks (older than 8 Gyr) are hosted by galaxies with a low mass fraction of accreted stars (below 8%), while the youngest thick disks (younger than 7 Gyr) are found in galaxies with higher accreted fractions (larger than 25%). Mergers play a key role in the mass assembly of thick disks, contributing an average accreted mass fraction of ∼22% in the analyzed thick-disk-dominated regions. In two galaxies, about half of the geometric thick-disk mass was directly accreted. The mass fraction of accreted stars is lower than 10% only in four thick disks. While primordial thick disks form at high redshifts in all galaxies, young metal-rich thin disks, with much lower [Mg/Fe] abundances, start to form later but at different times (at higher or lower redshifts) depending on the galaxy.
Conclusions: We conclude that thick disks, although mostly formed in situ, grow thanks to the significant contribution of satellite mergers, especially through the direct accretion of stars. They result from the interplay of external processes with the internal evolution of the galaxy.