14.3.3 Proper motions
The projections of the proper motion along the Galactic coordinates, and , are computed in order to see systematics due to the Galactic rotation. The median of the proper motions is computed in each HEALPix pixel and shown in Figure 14.42.
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 14.43 gives the proper motion averaged over the whole sky per magnitude bin, for Gaia DR3, Gaia DR2, and GOG20. At bright magnitudes, the difference between Gaia DR3 and GOG20 in is at the level of 0.05, and increases at fainter magnitudes (up to 0.4). The systematics are less than 0.05 in .
Summary of the results:
The Gaia DR3 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.