Browsing by Author "Debattista, Victor P."

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Co-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2
(Oxford University Press, 2020-12) Beraldo e Silva, Leandro; Debattista, Victor P.; Nidever, David; Amarante, João A S; Garver, Bethany
Since thin disc stars are younger than thick disc stars on average, the thin disc is predicted by some models to start forming after the thick disc had formed, around 10 Gyr ago. Accordingly, no significant old thin disc population should exist. Using 6D coordinates from Gaia-DR2 and age estimates from Sanders & Das, we select ∼24 000 old stars (${\tau \gt 10\mbox{$\, \mathrm{Gyr}$}}$, with uncertainties $\lesssim 15$) within $2\mbox{$\, \mathrm{kpc}$}$ from the Sun (full sample). A cross-match with APOGEE-DR16 (∼1000 stars) reveals comparable fractions of old chemically defined thin/thick disc stars. We show that the full sample pericentre radius (rper) distribution has three peaks, one associated with the stellar halo and the other two having contributions from the thin/thick discs. Using a high-resolution N-body + SPH simulation, we demonstrate that one peak, at $\mbox{$r_\mathrm{per}$}\approx 7.1\mbox{$\, \mathrm{kpc}$}$, is produced by stars from both discs that were born in the inner Galaxy and migrated to the Solar Neighbourhood. In the Solar Neighbourhood, ∼1/2 (∼1/3) of the old thin (thick) disc stars are classified as migrators. Our results suggest that thin/thick discs are affected differently by radial migration inasmuch as they have different eccentricity distributions, regardless of vertical scale heights. We interpret the existence of a significant old thin disc population as evidence for an early co-formation of thin/thick discs, arguing that clump instabilities in the early disc offer a compelling explanation for the observed trends.
The Imprint of Clump Formation at High Redshift. II. The Chemistry of the Bulge
(American Astronomical Society, 2023-04) Debattista, Victor P.; Liddicott, David J.; Gonzalez, Oscar A.; Beraldo e Silva, Leandro; Amarante, João A. S.; Lazar, Ilin; Zoccali, Manuela; Valenti, Elena; Fisher, Deanne B.; Khachaturyants, Tigran; Nidever, David L.; Quinn, Thomas R.; Du, Min; Kassin, Susan
In Paper I, we showed that clumps in high-redshift galaxies, having a high star formation rate density (ΣSFR), produce disks with two tracks in the [Fe/H]–[α/Fe] chemical space, similar to that of the Milky Way's (MW's) thin+thick disks. Here we investigate the effect of clumps on the bulge's chemistry. The chemistry of the MW's bulge is comprised of a single track with two density peaks separated by a trough. We show that the bulge chemistry of an N-body + smoothed particle hydrodynamics clumpy simulation also has a single track. Star formation within the bulge is itself in the high-ΣSFR clumpy mode, which ensures that the bulge's chemical track follows that of the thick disk at low [Fe/H] and then extends to high [Fe/H], where it peaks. The peak at low metallicity instead is comprised of a mixture of in situ stars and stars accreted via clumps. As a result, the trough between the peaks occurs at the end of the thick disk track. We find that the high-metallicity peak dominates near the mid-plane and declines in relative importance with height, as in the MW. The bulge is already rapidly rotating by the end of the clump epoch, with higher rotation at low [α/Fe]. Thus clumpy star formation is able to simultaneously explain the chemodynamic trends of the MW's bulge, thin+thick disks, and the splash.