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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 730095.0, MPI für Astronomie / Publikationen_mpia
On the formation of galactic thick disks
Authors:Minchev, I.; Martig, M.; Streich, D.; Scannapieco, C.; de Jong, R. S.; Steinmetz, M.
Date of Publication (YYYY-MM-DD):2015
Title of Journal:The Astrophysical Journal Letters
Issue / Number:1
Start Page:id. L9 (5 pp)
Audience:Not Specified
Abstract / Description:Recent spectroscopic observations in the Milky Way suggest that the chemically defined thick disk (stars that have high [α/Fe] ratios and are thus old) has a significantly smaller scale-length than the thin disk. This is in apparent contradiction with observations of external edge-on galaxies, where the thin and thick components have comparable scale-lengths. Moreover, while observed disks do not flare (scale-height does not increase with radius), numerical simulations suggest that disk flaring is unavoidable, resulting from both environmental effects and secular evolution. Here we address these problems by studying two different suites of simulated galactic disks formed in the cosmological context. We show that the scale-heights of coeval populations always increase with radius. However, the total population can be decomposed morphologically into thin and thick disks, which do not flare. We relate this to the disk inside-out formation, where younger populations have increasingly larger scale-lengths and flare at progressively larger radii. In this new picture, thick disks are composed of the imbedded flares of mono-age stellar populations. Assuming that disks form inside out, we predict that morphologically defined thick disks must show a decrease in age (or [α/Fe] ratios) with radius and that coeval populations should always flare. This also explains the observed inversion in the metallicity and [α/Fe] gradients for stars away from the disk midplane in the Milky Way. The results of this work are directly linked to, and can be seen as evidence of, inside-out disk growth.
Free Keywords:galaxies: evolution; galaxies: formation; galaxies: kinematics and dynamics; galaxies: structure; Galaxy: disk; Galaxy: formation
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
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