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gaia early data release 3 documentation

7.3 Comparison to models

7.3.1 Star density

For testing the star density we compute star counts and compare their absolute values as well as the relative differences (data-model)/data, in different magnitude bins.

Results for magnitude bins 8-9, 12-13 and 17-18 are presented in Figure 7.9. The overall picture of the sky densities are very well comparable in data and model, ensuring realistic star counts in both. There are however significant discrepancies close to the Galactic plane due to the extinction model used in GOG20 which is not optimal. It is also possible that the data show some incompleteness at latitudes near 0 towards the inner Galaxy as seen in the relative difference plot for 17<G<18. At 8<G<9 at high latitudes the small number of stars lead to a noisy map of relative difference which does not show significant discrepancies. At 12<G<13 and 17<G<18, one can see the imprint of the Magellanic clouds, not present in the model. We also start to see some discrepancies in the outer Galaxy, close to the plane, that are most probably due to the warp, the flare, and the disc scale heights, not perfectly modeled in GOG20.

Figure 7.9: Star counts in Gaia EDR3 (left), GOG20 (middle), and relative difference (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 – (EDR3-GOG20)/EDR3 – of -1 (resp. +1) corresponds to a deficit (resp. an excess) of 100% in GOG20 model with regards to Gaia EDR3 data.

The average value of star counts over the whole sky has been computed in each magnitude bin to see a signature of incompleteness. Figure 7.10 shows the results for Gaia EDR3 as well as for Gaia DR2 and GOG20. Gaia EDR3 has more faint stars than Gaia DR2, but still not as many as expected from GOG20.

Figure 7.10: Average of the star counts over the whole sky as a function of magnitude, in Gaia EDR3 and Gaia DR2, compared to GOG20.

Summary of the results:

  • The overall picture of the sky densities are very well comparable in data and model.

  • The data may show some incompleteness towards the inner Galaxy for faint stars.

  • At G>18, Gaia EDR3 has more stars than Gaia DR2, but not as many as expected from GOG20.