10.3.1 Star counts

Absolute differences of star counts (data-model) and relative differences (data-model)/data have been computed in the 4800 HEALPix bins. Results for magnitude bins 8-9, 12-13 and 17-18 are presented in Figures 10.21. There are significant discrepancies close to the Galactic plane due to the rough extinction model used in GOG18 and at 8<G<9 at high latitudes due to the small number of stars. At 12<G<13 and 17<G<18, one can see the effect of Magellanic clouds not present in GOG. We also notice some discrepancies that can be due to the warp and the flare, not well modelled in GOG18. At faint magnitudes the effect of the scanning law start to be visible. Figure 10.22 shows relative differences for magnitude ranges 18<G<19 19<G<20, where these effects are clearly visible. We conclude that at bright magnitudes there are no specific problems that can be identified in the data. However at G>17 the imprint of the scanning law is noticeable in the star counts, probably because more sources are missing in regions which have lower number of reliable observations.

Figure 10.21: Star counts in Gaia DR2 (left), GOG18 (middle), and relative difference in star counts (right) per magnitude bin in the magnitude ranges 8<G<9 (top row) and 12<G<13 (middle row) and 17<G<18 (bottom row). A relative difference of -1 (resp. +1) corresponds to a deficit (resp. an excess) of 100% in GOG model with regards to Gaia DR2 data.
Figure 10.22: Star counts relative difference at faint magnitudes. Left: 18<G<19. Right: 19<G<20. A relative difference of -1 (resp. +1) corresponds to a deficit (respectively an excess) of 100% in GOG model with regards to Gaia DR2 data. The imprint of the scanning law is clearly visible at these faint magnitudes.