Gaia-DR2 extended kinematical maps

II. Dynamics in the Galactic disk explaining radial and vertical velocities

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: fuego.templado@gmail.com
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ South–Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China
⁴ Department of Astronomy, China West Normal University, Nanchong 637009, PR China
⁵ Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi, Compendio del Viminale, 00184 Rome, Italy
⁶ Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, 00185 Roma, Italy
⁷ Istituto Nazionale Fisica Nucleare, Unità Roma 1, Dipartimento di Fisica, Universitá di Roma “Sapienza”, 00185 Roma, Italy
⁸ Faculty of Mathematics, Physics, and Informatics, Comenius University, Mlynska dolina, 842 48 Bratislava, Slovakia
⁹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
¹⁰ Purple Mountain Observatory, the Partner Group of MPI für Astronomie, 2 West Beijing Road, Nanjing 210008, PR China
¹¹ Nordic Institute of Theoretical Physics, Roslagstullsbacken 23, 106 91 Stockholm, Sweden

Received: 16 September 2019
Accepted: 21 December 2019

Abstract

Context. In our Paper I, by using statistical deconvolution methods, extended kinematics maps of Gaia-DR2 data have been produced in a range of heliocentric distances that are a factor of two to three larger than those analyzed previously by the Gaia Collaboration with the same data. It added the range of Galactocentric distances between 13 kpc and 20 kpc to the previous maps.

Aims. Here, we investigate the dynamical effects produced by different mechanisms that can explain the radial and vertical components of these extended kinematic maps, including a decomposition of bending and breathing of the vertical components. This paper as a whole tries to be a compendium of different dynamical mechanisms whose predictions can be compared to the kinematic maps.

Methods. Using analytical methods or simulations, we are able to predict the main dynamical factors and compare them to the predictions of the extended kinematic maps of Gaia-DR2.

Results. The gravitational influence of Galactic components that are different from the disk, such as the long bar or bulge, the spiral arms, or a tidal interaction with Sagittarius dwarf galaxy, may explain some features of the velocity maps, especially in the inner parts of the disk. However, they are not sufficient in explaining the most conspicuous gradients in the outer disk. Vertical motions might be dominated by external perturbations or mergers, although a minor component may be due to a warp whose amplitude evolves with time. Here, we show with two different methods, which analyze the dispersion of velocities, that the mass distribution of the disk is flared. Despite these partial explanations, the main observed features can only be explained in terms of out-of-equilibrium models, which are either due to external perturbers or to the fact that the disk has not had time to reach equilibrium since its formation.