DDA 2015 – The Relative Influence of Dynamical Nature and Nurture on the Formation of Disk Galaxies

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Jonathan Bird (Vanderbilt) (invited)




  • Are disk galaxies formed by nature or nurture?
  • e.g. NGC 891
  • Thick disk and thin disk (Gilmore & Reid 1983)
    • Extragalactic thick disks are ubiquitous (Dalcanton & Bernstein 2002, Yoachim & Dalcanton 2006)
  • Nature:
    • Stellar kinematics dominated by those of gas from which stars formed
    • Subsequent dynamics are second-order
    • Planetary disk: core accretion; static
    • $\alpha$ abundance is a tracer for stellar age
      • plot: [$\alpha$/Fe] vs [Fe/H]
      • Thick disk is old, $\alpha$-rich, kinematically hot
      • Thin disk is young, (relatively to Fe) $\alpha$-poor, dynamically cold
    • Smooth correlation between chemistry and kinematics
    • APOGEE survey: velocity dispersion increases with stellar age
      • power law
      • $\rightarrow$ disk grows over time
  • Nurture:
    • Stellar kinematics dominated by dynamical interactions after birth
    • Most stars born in dynamical cold gas (level playing field)
    • Resonances play huge role; pebble accretion
    • Scattering processes heat stellar velocity distributions
    • Radial migration
      • Sellwood & Binney 2002: can redistribute stars without globally heating the disk
      • Can outwardly migrating stars create the thick disk?
      • No: vertical action is conserved. (Tolfree et al. 2014)

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