Formation, vertex deviation, and age of the Milky Way's bulge: input from a cosmological simulation with a late-forming bar. Issue 4 (15th March 2019)

• Record Type: Journal Article
• Authors: Debattista, Victor P; Gonzalez, Oscar A; Sanderson, Robyn E; El-Badry, Kareem; Garrison-Kimmel, Shea; Wetzel, Andrew; Faucher-Giguère, Claude-André; Hopkins, Philip F
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 485:Issue 4(2019)
• Page Start: 5073
• Page End: 5085
• Publication Date: 2019-03-15
• Subjects: Galaxy: bulge -- Galaxy: evolution -- Galaxy: formation -- Galaxy: kinematics and dynamics -- Galaxy: structure; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz746
• Languages: English
• ISSNs: 0035-8711

Abstract

Abstract: We present the late-time evolution ofm12m, a cosmological simulation of a Milky Way-like galaxy from the FIRE project. The simulation forms a bar after redshift z = 0.2. We show that the evolution of the model exhibits behaviours typical of kinematic fractionation, with a bar weaker in older populations, an X-shape traced by the younger, metal-rich populations, and a prominent X-shape in the edge-on mean metallicity map. Because of the late formation of the bar inm12m, stars forming after$10\mbox{$\:{\rm Gyr}$}$ ( z = 0.34) significantly contaminate the bulge, at a level higher than is observed at high latitudes in the Milky Way, implying that its bar cannot have formed as late as inm12m . We also study the model's vertex deviation of the velocity ellipsoid as a function of stellar metallicity and age in the equivalent of Baade's Window. The formation of the bar leads to a non-zero vertex deviation. We find that metal-rich stars have a large vertex deviation (∼40°), which becomes negligible for metal-poor stars, a trend also found in the Milky Way, despite not matching in detail. We demonstrate that the vertex deviation also varies with stellar age and is large for stars as old as$9 \mbox{$\:{\rm Gyr}$}$, while$13\mbox{$\:{\rm Gyr}$}$ old stars have negligible vertex deviation. When we exclude stars that have been accreted, the vertex deviation is not significantly changed, demonstrating that the observed variation of vertex deviation with metallicity is not necessarily due to an accreted population.