7.3.3 Proper motion distributions

$\mu_{l}$ and $\mu_{b}$, the projections of the proper motion along the Galactic coordinates, are computed in order to see systematics due to the Galactic rotation. The median of the proper motions is computed in each healpix bin and shown in Figure 7.13 ($\mu_{l}$) and Figure 7.14 ($\mu_{b}$). At bright magnitudes, the dispersion is high, due to the small number of stars, and differences are not significant.

The model and data present approximately similar patterns in all magnitude ranges. However, there are systematics, as was already noted in the validation of Gaia DR2. It is most probable that these systematic differences are due to inaccuracy in modelling (e.g. the Galactic rotation).

Figure 7.15 gives the proper motion averaged over the whole sky per magnitude bin, for Gaia EDR3, Gaia DR2, and GOG20. At bright magnitudes, the difference between Gaia EDR3 and GOG20 in $\mu_{l}$ is at the level of 0.05 mas/yr, and increases at fainter magnitudes (up to $\sim$0.4 mas/yr). The systematics are less than 0.05 mas/yr in $\mu_{b}$.

Summary of the results:

• Overall, it seems that the Gaia EDR3 data are as expected from our knowledge of the Galactic kinematics up to very faint magnitudes and it is probably the model which suffers from systematics, or does not account for asymmetries. Indeed we note that the change of kinematics from GOG18 to GOG20 globally allows for a better agreement with the data.