Research Division Seminar
Properties and origin of thick disks in external galaxies

Abstract

The evolutionary connection between thick and thin disks is still a matter of debate, while observations of high-redshift galaxies have recently shown that stellar disks of various thicknesses emerged very early in the universe. Their diversity seeded the large variety of properties observed in the local universe. Zooming into the spatially resolved stellar populations of very nearby galactic disks traces the story of spiral and lenticular galaxies, from the early stages to recent times. I will present my contribution on this topic, combining high-quality integral-field spectroscopy (IFS) observations of edge-on galaxies with high-resolution numerical simulations.Our recent studies have revealed different stellar populations of nearby thick disks in different types of galaxies, with different star-formation rates (SFRs), suggesting that they result from different evolution histories. The sharp transition, in earlier-type disk galaxies, between old, metal-poor and alpha-enhanced thick disks, and younger metal-rich thin disks, suggests that they formed in two distinct evolution phases. Highly star-forming late-type galaxies, with little differences between relatively young and metal-poor thick and thin disks, suggest a slower upside-down formation. I will finally compare these observations with AURIGA zoom-in cosmological simulations of Milky Way-mass galaxies, revealing one other piece of the puzzle: the connection between the fraction of in-situ and ex-situ stars and stellar-population properties. In these simulations, younger thick disks are explained by later and more massive mergers. These not only contribute younger stars from accreted satellite galaxies, but also large amounts of gas to extend off-plane star formation in time.