tokens: the posteriors estimates of the TNM model is shown by the red line. Right-hand panel: V φ distribution for mono-age stellar particles. Colour code as in Table 2 . Table 4. TNM model parameters: w i is the mixture weight of the i -th component (i.e. young disc, old disc/thick disc, halo), while 〈 V φ 〉 and are the corresponding mean and the standard deviation of the normal distribution. Note that 0 ≤ w i ≤ 1 and ∑ i w i = 1. We also tested alternative models with a different number of components. The results for a double Gaussian distribution such as halo+disc or disc+disc are not statistically significant and do not provide good curve fitting of the data. A four-component model, aimed to represent additional disc populations, produced non-significant improvements to the model. We note that the weights of the two disc components listed in Table 4 correspond to the total mass of the young disc ( Gyr) and of the old disc/thick disc (), respectively (cf. Table 2 considering the different vertical cut applied). The comparison with the mono-age distributions shown in Fig. 7 (right-hand panel) confirms that these two disc components represent the superposition of the stellar generations formed in the age intervals 0–4 and 4–10 Gyr. Note that the old thin disc and the thick disc populations are difficult to distinguish because of their very similar velocity distributions. Moreover, the difference between the mean rotation velocity of the AqC4 discs is This value is smaller than in the MW, where (Han et al. 2020 ). Actually, this difference depends on the fact that in AqC4 the two main disc components represent the young and the old/thick discs instead of the whole thin disc and the thick disc, as in the MW. Moreover, the high rotation velocity of the AqC4 disc is consistent with its more compact structure with respect to the MW disc (Section 3.1 ). Finally, the halo component shows a small prograde rotation as observed in the inner halo of the MW, while its velocity dispersion km s −1 is about 70–100 per cent higher than in the solar neighbourhood (Re Fiorentin et al. 2015 ; Bland-Hawthorn Gerhard 2016 , and references therein). In summary, here we robustly support what is discussed in Section 3 : in the SSR, we can distinguish at least two kinematic disc components which rotate faster than what is measured in the solar neighbourhood (as discussed globally in Section 4.1 ), and with a smaller discrepancy between the mean values. These results extend the spatial characterization of AqC4 stellar disc beyond the determination of the scale lengths and scale heights, allowing us to properly investigate the complete phase-space of mono-age populations, as usually done in Galactic surveys. 4.3 Kinematics of mono-age disc populations Fig. 8 shows the three median velocity components , , and the dispersions (, , ) as a function of R , computed in radial annular bins of Δ R = 0.25 kpc, for the mono-age stellar disc populations younger than 10 Gyr and with | Z |
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