Context. The structure of outer disc of our Galaxy is still not well described, and many features need to be better understood. The second Gaia data release (DR2) provides data in unprecedented quality that can be analysed to shed some light on the outermost parts of the Milky Way.
Aims: We calculate the stellar density using star counts obtained from Gaia DR2 up to a Galactocentric distance R = 20 kpc with a deconvolution technique for the parallax errors. Then we analyse the density in order to study the structure of the outer Galactic disc, mainly the warp.
Methods: In order to carry out the deconvolution, we used the Lucy inversion technique for recovering the corrected star counts. We also used the Gaia luminosity function of stars with MG < 10 to extract the stellar density from the star counts.
Results: The stellar density maps can be fitted by an exponential disc in the radial direction hr = 2.07 ± 0.07 kpc, with a weak dependence on the azimuth, extended up to 20 kpc without any cut-off. The flare and warp are clearly visible. The best fit of a symmetrical S-shaped warp gives zw ≈ z☉ + (37 ± 4.2(stat.) - 0.91(syst.))pc ⋅ (R/R☉)2.42 ± 0.76(stat.) + 0.129(syst.)sin(ϕ + 9.3° ±7.37° (stat.) + 4.48° (syst.)) for the whole population. When we analyse the northern and southern warps separately, we obtain an asymmetry of an ∼25% larger amplitude in the north. This result may be influenced by extinction because the GaiaG band is quite prone to extinction biases. However, we tested the accuracy of the extinction map we used, which shows that the extinction is determined very well in the outer disc. Nevertheless, we recall that we do not know the full extinction error, and neither do we know the systematic error of the map, which may influence the final result. The analysis was also carried out for very luminous stars alone (MG < -2), which on average represents a younger population. We obtain similar scale-length values, while the maximum amplitude of the warp is 20 - 30% larger than with the whole population. The north-south asymmetry is maintained.