Analysis of galaxy kinematics based on Cepheids from the Gaia DR2 Catalogue

ABSTRACT To construct the rotation curve of the Galaxy, classical Cepheids with proper motions, parallaxes and line-of-sight velocities from the Gaia DR2 Catalogue are used in large part. Our working sample formed from literature data contains about 800 Cepheids with estimates of their age. We determined that the linear rotation velocity of the Galaxy at the solar distance is V0 = 240 ± 3 km s−1. In this case, the distance from the Sun to the axis of rotation of the Galaxy is found to be R0 = 8.27 ± 0.10 kpc. A spectral analysis of radial and residual tangential velocities of Cepheids younger than 120 Myr showed close estimates of the parameters of the spiral density wave obtained from data both at the present time and in the past. Therefore, the value of the wavelength λR, θ is in the range [2.4–3.0] kpc, the pitch angle iR, θ is in the range [−13○, −10○] for a four-arm pattern model, and the amplitudes of the radial and tangential perturbations are fR ∼ 12 km s−1 and fθ ∼ 9 km s−1, respectively. Velocities of Cepheids older than 120 Myr currently give a wavelength λR, θ ∼ 5 kpc. This value differs significantly from the one we obtained from samples of young Cepheids. An analysis of the positions and velocities of old Cepheids, calculated by integrating their orbits backward in time, made it possible to determine significantly more reliable values of the parameters of the spiral density wave: wavelength λR, θ = 2.7 kpc and amplitudes of radial and tangential perturbations fR = 7.9 km s−1 and fθ = 5 km s−1, respectively.

2D-Galactic chemical evolution: the role of the spiral density wave

ABSTRACT We present a 2D chemical evolution code applied to a Milky Way type Galaxy, incorporating the role of spiral arms in shaping azimuthal abundance variations, and confront the predicted behaviour with recent observations taken with integral field units. To the usual radial distribution of mass, we add the surface density of the spiral wave and study its effect on star formation and elemental abundances. We compute five different models: one with azimuthal symmetry which depends only on radius, while the other four are subjected to the effect of a spiral density wave. At early times, the imprint of the spiral density wave is carried by both the stellar and star formation surface densities; conversely, the elemental abundance pattern is less affected. At later epochs, however, differences among the models are diluted, becoming almost indistinguishable given current observational uncertainties. At the present time, the largest differences appear in the star formation rate and/or in the outer disc (R ≥ 18 kpc). The predicted azimuthal oxygen abundance patterns for t ≤ 2 Gyr are in reasonable agreement with recent observations obtained with VLT/MUSE for NGC 6754.