Born this way: thin disc, thick disc, and isotropic spheroid formation in FIRE-2 Milky Way–mass galaxy simulations
Creators
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1.
University of California, Irvine
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2.
Northwestern University
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3.
Tel Aviv University
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4.
The University of Texas at Austin
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5.
University of California, Davis
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6.
California Institute of Technology
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7.
Pomona College
Abstract
We investigate the formation of Milky Way–mass galaxies using FIRE-2 ΛCDM cosmological zoom-in simulations by studying the orbital evolution of stars formed in the main progenitor of the galaxy, from birth to the present day. We classify in situ stars as isotropic spheroid, thick-disc, and thin-disc according to their orbital circularities and show that these components are assembled in a time-ordered sequence from early to late times, respectively. All simulated galaxies experience an early phase of bursty star formation that transitions to a late-time steady phase. This transition coincides with the time that the inner CGM virializes. During the early bursty phase, galaxies have irregular morphologies and new stars are born on radial orbits; these stars evolve into an isotropic spheroidal population today. The bulk of thick-disc stars form at intermediate times, during a clumpy-disc 'spin-up' phase, slightly later than the peak of spheroid formation. At late times, once the CGM virializes and star formation 'cools down,' stars are born on circular orbits within a narrow plane. Those stars mostly inhabit thin discs today. Broadly speaking, stars with disc-like or spheroid-like orbits today were born that way. Mergers on to discs and secular processes do affect kinematics in our simulations, but play only secondary roles in populating thick-disc and in situ spheroid populations at z = 0. The age distributions of spheroid, thick disc, and thin disc populations scale self-similarly with the steady-phase transition time, which suggests that morphological age dating can be linked to the CGM virialization time in galaxies.
Copyright and License
Acknowledgement
We thank an anonymous referee for several suggestions that helped our presentation significantly. We also acknowledge Charlie Conroy, David Weinberg, Andrey Kravtsov, Vasily Belokurov, and Ana Bonaca for inspiring discussions. Patrick Staudt, Stefani Germanotta, and Jeppe Laursen suggested we were on the right track during initial stages of this research.
Funding
SY and JSB were supported by the NSF grants AST-1910346 and AST-1518291. JS was supported by the Israel Science Foundation (grant number 2584/21). MBK acknowledges support from the NSF CAREER award AST-1752913, NSF grants AST-1910346 and AST-2108962, NASA grant 80NSSC22K0827, and HST-AR-15809, HST-GO-15658, HST-GO-15901, HST-GO-15902, HST-AR-16159, and HST-GO-16226 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. CAFG was supported by the NSF through grants AST-1715216, AST-2108230, and CAREER award AST-1652522; by NASA through grants 17-ATP17-0067 and 21-ATP21-0036; by STScI through grants HST-AR-16124.001-A and HST-GO-16730.016-A; by CXO through grant TM2-23005X; and by the Research Corporation for Science Advancement through a Cottrell Scholar Award. AW received support from: NSF via CAREER award AST-2045928 and grant AST-2107772; NASA ATP grant 80NSSC20K0513; HST grants AR-15809, GO-15902, and GO-16273 from STScI. We ran simulations using: XSEDE, supported by the NSF grant ACI-1548562; Blue Waters, supported by the NSF; Pleiades, via the NASA HEC program through the NAS Division at Ames Research Center. Allocations AST21010 and AST20016 were supported by the NSF and TACC.
Data Availability
The data supporting the plots within this article are available on reasonable request to the corresponding author. A public version of the GIZMO code is available at http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html. FIRE-2 simulations are publicly available (Wetzel et al. 2022) at http://flathub.flatironinstitute.org/fire. Additional data including simulation snapshots, initial conditions, and derived data products are available at https://fire.northwestern.edu/data/. Some of the publicly available software packages used to analyse these data are available at: https://bitbucket.org/awetzel/gizmo_analysis, and https://bitbucket.org/awetzel/utilities.
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2210.03845 (arXiv)
Funding
- National Science Foundation
- AST-1910346
- National Science Foundation
- AST-1518291
- Israel Science Foundation
- 2584/21
- National Science Foundation
- AST-1752913
- National Science Foundation
- AST-1910346
- National Science Foundation
- AST-2108962
- National Aeronautics and Space Administration
- 80NSSC22K0827
- National Aeronautics and Space Administration
- HST-AR-15809
- National Aeronautics and Space Administration
- HST-GO-15658
- National Aeronautics and Space Administration
- HST-GO-15901
- National Aeronautics and Space Administration
- HST-GO-15902
- National Aeronautics and Space Administration
- HST-AR-16159
- National Aeronautics and Space Administration
- HST-GO-16226
- National Aeronautics and Space Administration
- NAS5-26555
- National Science Foundation
- AST-1715216
- National Science Foundation
- AST-2108230
- National Science Foundation
- AST-1652522
- National Aeronautics and Space Administration
- 17-ATP17-0067
- National Aeronautics and Space Administration
- 21-ATP21-0036
- Space Telescope Science Institute
- HST-AR-16124.001-A
- Space Telescope Science Institute
- HST-GO-16730.016-A
- National Aeronautics and Space Administration
- TM2-23005X
- Research Corporation for Science Advancement
- National Science Foundation
- AST-2045928
- National Science Foundation
- AST-2107772
- National Aeronautics and Space Administration
- 80NSSC20K0513
- National Aeronautics and Space Administration
- AR-15809
- National Aeronautics and Space Administration
- GO-15902
- National Aeronautics and Space Administration
- GO-16273
- National Science Foundation
- ACI-1548562
- National Science Foundation
- AST21010
- National Science Foundation
- AST20016
Dates
- Accepted
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2023-06-09
- Available
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2023-06-16Published
- Available
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2023-07-03Corrected and typeset