The rotation curve and mass models of the Milky Way after the Gaia mission

by Francesco Sylos Labini (Centro ricerche Enrico Fermi)

Monday 18 Dec 2023, 16:00 → 18:00 Europe/Rome

Aula Conversi (Dipartimento di Fisica - Ed. Marconi)

The circular velocity curve of the Milky Way (MW) has been measured over the past few decades using various tracers and methods. Until a few years ago, the prevailing result was that the rotation curve was nearly flat. However, recent measurements of the Milky Way's circular velocity curve, utilizing stellar samples observed by the Gaia mission, have unveiled a significant decline in velocity at larger galactic radii, extending up to approximately 30 kpc. The qualitative improvement of the new stellar samples lies in the availability of all six dimensions for a large sample of stars in the MW, marking a new phase in determining its rotation curve.

In this presentation, after presenting an overview to the Gaia data, I will discuss a novel statistical deconvolution technique aimed at addressing parallax errors, which has been applied to the second and third data releases of Gaia. This method has allowed us to construct maps of the three velocity components up to distances of 20 kpc (with the 2nd release) and 30 kpc (with the 3rd release). By leveraging this extended range of distance measurements, we have successfully determined the rotation curve of the Milky Way. Remarkably, our findings indicate a clear decline in the rotation curve up to 30 kpc, challenging previous assumptions of a flat rotation curve. These measurements have been independently confirmed by several other studies utilizing different datasets.

The implications of these findings are wide-reaching, significantly impacting our understanding of the kinematics and dynamics of the Milky Way, the estimation of dark matter abundance, and the interpretation of alternative gravity theories such as Modified Newtonian Dynamics (MOND). Specifically, we discuss a new model to explain the rotation curve based on the concept that dark matter is confined to a disk rather than distributed in a spherical halo, as in the standard model. In this model the total MW’s mass is only twice that of visible matter, but the local density is higher than in the standard model. We believe this line of research holds promise, and we are currently studying a number of external galaxies where we can apply this model as well.