ACADEMIA

The mass of the Milky Way is a fundamental quantity in modern astrophysics and cosmology that has a direct impact on many astrophysical problems.

With the combination of high-precision data from Gaia EDR3 and a new-generation dynamical modeling method, an international research team has found that the total mass of the Milky Way ranges from 500–800 billion solar mass, which indicates a lighter Milky Way when compared to previous measurements.

The study was led by Chinese astronomer WANG Jianling from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), in collaboration with Francois Hammer and YANG Yanbin from the Paris Observatory under the framework of the Sino-French collaborative "Tianguan" project.

Previous studies of galactic dynamics were affected by two factors: Either they were based on a too small data set that introduced large uncertainties, or the tracers they used lacked information. The latter could be a serious matter since simple hypotheses on the equilibrium of some distant tracers have been used, thus introducing unknown systematic problems.

Now we are entering a golden era of galactic archeology with progress on large-scale spectroscopic surveys and high-precision proper motion measurements from the Gaia satellite, which provides a huge amount of high-quality data. These data overcome many difficulties mentioned above, especially by providing full, six-dimensional information for high-precision tracers acquired by Gaia.

Using these unprecedented data to study how our Milky Way and its halo are structured and how they assembled together is the central task facing astronomers, and dynamic modeling with supercomputers is the central tool for accomplishing this task.

The astronomers used a new-generation, analytical dynamic modeling technique, i.e., action-based, distribution function dynamical modeling. They derived the Milky Way baryon mass and dark matter mass distribution function, which in turn provided the accurate total mass of the Milky Way.

Thanks to the precise proper motions of Gaia, they derived the most precise kinematic information for around 150 galactic globular clusters. They combined this information with the accurate rotation curve information from the disk region also based on Gaia data. The flexible action-based distribution function overcomes many simplistic assumptions adopted by previous studies, thus leading to a more realistic distribution function for the tracers, and to a Milky Way mass distribution function.

N-body simulation and realistic cosmological hydrodynamic simulations have been used in this work to quantify any systematics introduced by the Large Magellanic Cloud passing by as well as by unrelaxed substructures.

This study has significant implications for cosmological problems and the origin of Milky Way satellites.