Revealing the outer galactic disc with gaia dr2

Resumen

The outer stellar disk of the Milky Way at Galactocentric distances larger than 15 kpc is one of the Galaxy's most unknown regions. As stellar populations in the external disk have low densities and their measured distance has high errors, it is difficult to constrain their distribution. This thesis aims to explore the outer part of the Galaxy in three consisting topics. First, we investigate the density distribution of the outer disk and explore its structural features. Next, we study the outer disk's rotation curve and deviations from equilibrium. The last goal is to analyse the warp precession and its implication on the warp origin. To this end, we develop a statistical deconvolution method that enables us to recover star counts up to Galactocentric distance 20 kpc. Moreover, we apply the Jeans equation to derive rotation curve from kinematic data obtained with the same method. Throughout the thesis we use the second Gaia data release, which is the most advanced astrometric information of the Milky Way up to date. The obtained density distribution reveals that the disk is asymmetrically warped, with northern Galactic hemisphere amplitude being larger than the southern one by sim 25%. Moreover, the warp amplitude is lower than previously thought and it exhibits a strong dependence on the age of the studied stellar population. When we compare our result with that of Cepheids, we find a 2-3 times lower warp amplitude. The analysis of rotation curves of the outer disk reveals that it has little dependence on both Galactic radius and height. Upon fitting the curves with models including either a dark matter halo or Modified Newtonian dynamics (MOND), we find that both approaches fit them satisfyingly. By performing N-body simulations of mock galactic systems out of equilibrium, we discover that the Jeans equation, used to calculate the rotation curve, notably overestimates rotational velocity at distances larger than 20 kpc, therefore the non-equilibrium does not significantly affect our curves. The final result is analysis of warp precession. We apply the warp model that we derived using the whole stellar population and find that the data are compatible with a model with slow precession of several Gyr or a nonprecessing model as well, contrary to other works that measured a faster precession with a period of of 600 Myr, not taking into account warp dependence on the age of the population. These results support the theory that warp is a long-lived feature triggered by a non-gravitational mechanism. We also conclude that due to non-equilibrium, the Jeans equation may produce unreliable results at high Galactocentric distances. Thus, this thesis contributes to the exploration of the outer Galactic disk and laid the foundation for future work to be carried out with the upcoming Gaia data releases.