10.3.3 Parallax distributions

Parallax distributions are computed for data and simulations separately, then combined and inter-compared. Absolute difference, between Gaia DR2 and GOG18, of the mean and standard deviation of the parallaxes are computed on the sky in each of the 4800 HEALPix bins and per magnitude bins. The absolute differences are presented Figure 10.26 in four magnitude bins. The mean parallaxes are generally in very good agreement between Gaia DR2 and GOG18. There are however a few places where the difference is significant. It is in the Magellanic Clouds (MC) and close to the Galactic center or the Galactic plane (at bright magnitudes). In the case of the MC, the difference is due to the fact that GOG18 does not generate the clouds, while the mean parallaxes in the MC are significantly different from the parallaxes of the field stars, by about 1 mas. This is expected because the mean parallax of field stars at similar latitudes from the model is expected to be of the order of 1 mas at magnitude 17 (depending on magnitude), while the mean parallax in the MC should be of the order of 0.02 mas.

Near the Galactic center and in regions close to the plane, specially at bright magnitudes, there are significant differences between GOG18 and Gaia DR2. The differences are more significant in regions where the model also disagrees in density. These regions are where the extinction model used in GOG18 is significantly inaccurate. Hence there is no reason to believe that the Gaia DR2 solution is problematic in this case.

Figure 10.26: Parallax distribution: absolute differences in the mean parallaxes in mas between Gaia DR2 data and GOG18 model in 4 magnitude bins.
Figure 10.27: Mean parallaxes in mas as a function of magnitude in three latitude zones. (a) Pole: |b|>70, (b) Intermediate: 20<|b|<70, (c) Plane: |b|<20. Data are blue circles and model green squares (filled: north, open: south).

The mean parallax dependency with magnitude, as seen Figure 10.27, is very similar between the model and the data. However there is a systematic difference which, in absolute value, depends on magnitude, as also seen in Table 10.3 and in Table 10.4. The difference is high at bright magnitudes, but this is where the effect of extinction plays more, as also seen in the density plots. At faint magnitudes, where most of the stars have small parallaxes, the data and the model are in very good agreement, at the level or better than 0.1 mas.

Table 10.3: Mean and standard deviation of the parallax as a function of magnitude in 5 latitude bins, for Gaia DR2 and GOG18. Magnitude bins are 1 mag large. Latitude bins are -90<b<-70,-70<b<-20,-20<b<20,20<b<70 and 70<b<90.
Mag. Latitude DR2 Mean DR2 StDev GOG Mean GOG StDev
6 0 5.98 7.49 4.87 9.81
6 1 7.35 11.30 4.05 8.13
6 2 6.52 8.93 4.13 9.58
6 3 6.64 10.88 3.26 6.06
6 4 5.95 9.08 2.89 6.44
7 0 7.62 6.02 7.18 5.98
7 1 7.65 9.86 5.90 4.34
7 2 5.85 3.57 5.03 4.80
7 3 7.61 6.17 6.07 4.59
7 4 6.55 4.79 6.53 5.40
8 0 6.77 3.43 6.36 3.49
8 1 5.89 2.61 4.98 3.02
8 2 4.38 1.81 3.59 1.36
8 3 6.25 3.10 5.16 2.59
8 4 7.15 4.06 6.84 4.55
9 0 5.44 1.48 6.20 2.12
9 1 4.54 1.27 4.39 1.45
9 2 3.23 1.00 2.70 0.67
9 3 4.75 1.50 4.73 1.81
9 4 6.14 2.06 6.40 3.15
10 0 4.59 1.07 5.18 1.14
10 1 3.60 0.79 3.93 1.03
10 2 2.38 0.58 2.09 0.47
10 3 3.73 0.80 4.08 0.98
10 4 4.62 0.94 5.33 1.63
11 0 3.67 0.98 4.26 0.48
11 1 2.89 0.54 3.38 0.69
11 2 1.79 0.38 1.72 0.44
11 3 3.02 0.55 3.49 0.66
11 4 3.81 0.52 4.33 0.48
12 0 2.88 0.25 3.37 0.28
12 1 2.33 0.40 2.76 0.45
12 2 1.38 0.31 1.45 0.42
12 3 2.43 0.39 2.83 0.41
12 4 2.98 0.31 3.38 0.29
13 0 2.34 0.15 2.66 0.16
13 1 1.88 0.31 2.19 0.32
13 2 1.09 0.26 1.23 0.35
13 3 1.92 0.27 2.23 0.29
13 4 2.35 0.18 2.70 0.18
Table 10.4: (Continue previous Table 10.3) Mean and standard deviation of the parallax as a function of magnitude in 5 latitude bins, for Gaia DR2 and GOG18. Magnitude bins are 1 mag large. Latitude bins are -90<b<-70,-70<b<-20,-20<b<20,20<b<70 and 70<b<90 ∘.
Mag. Latitude DR2 Mean DR2 StDev GOG Mean GOG StDev
14 0 1.92 0.10 2.20 0.12
14 1 1.52 0.26 1.77 0.28
14 2 0.87 0.21 1.02 0.27
14 3 1.53 0.23 1.79 0.26
14 4 1.89 0.13 2.22 0.11
15 0 1.60 0.08 1.92 0.10
15 1 1.24 0.23 1.47 0.27
15 2 0.70 0.17 0.83 0.21
15 3 1.25 0.20 1.50 0.25
15 4 1.58 0.11 1.92 0.10
16 0 1.40 0.08 1.73 0.09
16 1 1.05 0.24 1.29 0.28
16 2 0.57 0.13 0.69 0.17
16 3 1.06 0.20 1.31 0.26
16 4 1.36 0.09 1.71 0.09
17 0 1.25 0.08 1.57 0.09
17 1 0.92 0.24 1.16 0.29
17 2 0.47 0.11 0.58 0.17
17 3 0.93 0.20 1.18 0.27
17 4 1.18 0.08 1.55 0.08
18 0 1.09 0.07 1.36 0.07
18 1 0.82 0.22 1.05 0.26
18 2 0.40 0.10 0.52 0.18
18 3 0.81 0.17 1.06 0.24
18 4 0.99 0.08 1.34 0.06
19 0 0.86 0.06 1.09 0.06
19 1 0.71 0.18 0.91 0.20
19 2 0.37 0.10 0.48 0.18
19 3 0.69 0.12 0.91 0.18
19 4 0.78 0.07 1.06 0.05
20 0 0.69 0.07 0.91 0.06
20 1 0.63 0.14 0.81 0.16
20 2 0.36 0.09 0.46 0.17
20 3 0.61 0.09 0.80 0.14
20 4 0.65 0.06 0.89 0.05