While a non-negligible fraction of stars have disc-like kinematics, the majority of the UIP stars, as well as the VMP and EMP stars, have halo-like kinematics. This does not necessarily mean that they are all accreted, since a fraction of the halo can be made of stars formerly in the disc, but later kinematically heated to halo kinematics by one or several satellite accretions. This has indeed proven to be the dominant in-situ mode of formation of the Galactic halo for stars at higher metallicities, and at few kpc from the Sun (see Di Matteo et al. 2019), as we discuss more extensively in the next section. It is, however, interesting to note that, compared to stars at higher metallicities, such as stars in the Nissen & Schuster sample and stars in the Gaia DR2-APOGEE sample (middle and right columns in Fig. 5), in the Toomre diagram, the UIP and LP samples seem to lack stars with null angular momentum, meaning along the Lz = 0 line. At values of 





(



V
R


2

+



V
Z


2

)


≳
200


$ \sqrt{({V_R}^2+{V_Z}^2)} \gtrsim 200 $


 km s−1, stars with [Fe/H]   −4 have rather prograde or retrograde motions, but none seem to lie along the sequence of accreted halo stars discovered by Nissen & Schuster (2010), and later confirmed in Gaia DR1 and DR2 data by Belokurov et al. (2018) (Gaia Sausage), Haywood et al. (2018), Helmi et al. (2018) (Gaia Enceladus). The Gaia Sausage, which is this group of halo stars with very radial orbits, and null VΦ, seems to indeed disappear at [Fe/H]   −2 in this plane. We emphasise that this apparent difference between the kinematics of VMP and EMP stars, on the one side, and stars with [Fe/H] >  −2, on the other side, is simply the consequence of these stars probing different regions and distances from the Galactic centre. Indeed, when one compares the kinematics of these different samples of stars in the quasi-integral-of-motion space Lz − Lperp plane (see Fig. 5, second row), rather than in the Toomre diagram, the kinematic properties of these samples are the same, over the whole [Fe/H] interval. Before moving further, we need, however, to emphasise two points of the comparison with samples at higher metallicity. Firstly, when compared to the Nissen & Schuster (2010) sample, which, we remind the reader, is a kinematically selected sample of thick disc and halo stars, the VMP and EMP stars in the LP First stars sample show an excess of stars at retrograde motions (positive Lz, i.e. Lz >  5) and high values of Lperp (Lperp >  15). None of the Nissen & Schuster (2010) stars occupy this region of the Lz − Lperp diagram, and we suggest this is a consequence of the “local” character of stars in the Nissen & Schuster (2010) study, which are all limited to a few hundred parsecs from the Sun. Indeed, when VMP and EMP stars are compared to stars in the Gaia DR2-APOGEE sample, one can see that stars with Lz and Lperp as extreme as Lz >  5 and Lperp >  15 are found also in the latter. Secondly, because in the comparison shown in this figure we used all stars in the Gaia DR2-APOGEE sample, not restricting ourself to stars from the kinematically defined thick disc and halo, the reader will not be surprised to find that the majority of stars in Gaia DR2-APOGEE sample are stars with cold (i.e. thin) disc-like kinematics, their distribution peaking at Lz ∼ 20 and Lperp ≤ 5. At this stage, what is important to retain is that the region occupied by all these samples, independently of their [Fe/H] ratio, is the same: the relative fraction of stars in one or in another region of the space under analysis can change from one sample to another, but not their overall distribution.
Citation: Nissen & Schuster (2010)
Citation function: Compare/Contrast; Compare/Contrast
Citation: Nissen & Schuster (2010)
Citation function: Compare/Contrast; Compare/Contrast
Citation: Nissen & Schuster (2010)
Citation function: Compare/Contrast; Compare/Contrast
Citation: Nissen & Schuster (2010)
Citation function: Compare/Contrast; Compare/Contrast