Stellar populations of thick disks in Milky Way-mass galaxies: observations from the GECKOS survey and simulations

Abstract

GECKOS is a new survey targeting 35 edge-on disk galaxies with an ESO VLT/MUSE large program of 317 hours, aiming at unveiling the key physical processes of disk formation in nearby galaxies. Edge-on galaxies are ideal for this task: they allow us to disentangle the assembly history imprinted in the vertical structure of thin and thick disks. The GECKOS sample was designed to trace properties of galaxies across a large range of star formation rates, bulge-to-total ratios, and boxy and non-boxy bulges. GECKOS is delivering spatially resolved measurements, at a spatial resolution better than 200 pc out to solar environments. After introducing the survey, I focus here on the study of the stellar populations and origin of thick disks in the GECKOS sample. I present the first results for 6 galaxies, which have a clear thick and thin disk structure, observed during the survey’s first semester. I show maps of stellar ages, metallicity and [Mg/Fe] abundance and investigate trends of thick disk properties with the galaxy star formation rate (SFR). Galaxies with different star-formation rates show different properties, indicating different evolutionary paths. Galaxies above the star-formation main sequence do not show sharp differences between thick and thin disks. The former are relatively young, while older than their thin disks. On the other hand, galaxies around the main sequence and slightly below clearly display an old, metal-poor and α -enhanced thick disk, and a young, metal-rich thin disk. Finally, I compare the results with mock stellar-population maps from AURIGA zoom-in cosmological simulations of Milky Way-mass galaxies at redshift z = 0. Going back in time to previous snapshots of the simulations reveals that galaxy evolution at early times and the later merger history are key to determining properties of thick disks at z = 0.