Spatially resolved stellar populations of thick disks: insights from the GECKOS survey

Recent JWST high-redshift observations have shown that galaxies initially formed their thicker stellar disk components. Their properties already exhibited diversity, seeding the wide variety of properties observed in the local universe. Integral-field spectroscopy (IFS) studies have revealed diverse stellar populations in nearby thick disks of different types of galaxies, suggesting that they result from different galaxy evolution paths. However, deep IFS studies of edge-on galaxies have been limited so far, and only larger samples can unveil the driving factors for this diversity. I will show here the stellar population analysis of a first subsample of thick and thin disks from the GECKOS survey, a MUSE large program targeting 35 nearby edge-on Milky Way-mass galaxies. These results reveal a clear trend of thick-disk properties with galaxy star-formation rates. Galaxies on the star-formation main sequence or below display old, metal-poor and alpha-enhanced thick disks, suggesting an early and fast formation. In contrast, highly star-forming galaxies have much in common with galaxies observed at high redshift, suggesting a much slower evolution. Their thick and thin disks are both relatively young and metal-poor, and show extended star-formation histories. Numerical simulations suggest that mergers play a key role in driving these differences, contributing not only accreted stars, but also large amounts of gas to extend off-plane star formation over time.