Table 1
Summary of the signatures predicted in the thick disc formation scenarios.
| Scenario | External/ | Fast/ | Counter-rotating material | Chemical and/or age discontinuity |
| internal | secular | in the thick disc | between the thin and thick discs | |
|
|
||||
| Merger of gas-rich galaxies | External | Fast | Possiblya | Yes |
| Turbulent early disc | Internal | Fast | Nob | Yes |
| Accretion of stars from satellites | External | Secular | Possiblya | Yes |
| Heating by satellites | External | Secular | No | No |
| Internal heating | Internal | Secular | No | No |
Notes.
Those processes would cause some amount of counter-rotating material provided that some of the mergers are retrograde.
In the Bournaud et al. (2014) simulations, some counter-rotating stars are created in this process. The fraction of retrograde stars decreases as a function of the mass of the galaxy. A high-z galaxy with a 1.4 × 1010 ℳ⊙ baryonic mass, similar to the mass of ESO 533-4’s thick disc, would have 2.2% of counter-rotating stars (F. Bournaud, priv. comm.). This small fraction of counter-rotating stars would be undetectable with the techniques used in this paper.
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