ON THE FORMATION OF GALACTIC THICK DISKS

Abstract

Recent spectroscopic observations in the Milky Way suggest that the\nchemically defined thick disk (stars with high [alpha/Fe] ratios and thus old)\nhas a significantly smaller scale-length than the thin disk. This is in\napparent contradiction with observations of external edge-on galaxies, where\nthe thin and thick components have comparable scale-lengths. Moreover, while\nobserved disks do not flare (scale-height does not increase with radius),\nnumerical simulations suggest that disk flaring is unavoidable, resulting from\nboth environmental effects and secular evolution. Here we address these\nproblems by studying two different suites of simulated galactic disks formed in\nthe cosmological context. We show that the scale-heights of coeval populations\nalways increase with radius. However, the total population can be decomposed\nmorphologically into thin and thick disks, which do not flare. We relate this\nto the disk inside-out formation, where younger populations have increasingly\nlarger scale-lengths and flare at progressively larger radii. In this new\npicture, thick disks are composed of the imbedded flares of mono-age stellar\npopulations. Assuming that disks form inside out, we predict that\nmorphologically defined thick disks must show a decrease in age (or [alpha/Fe]\nratios) with radius and that coeval populations should always flare. This also\nexplains the observed inversion in the metallicity and [alpha/Fe] gradients for\nstars away from the disk midplane in the Milky Way. The results of this work\nare directly linked to, and can be seen as evidence of, inside-out disk growth.\n