  The Milky Way galaxy is a typical spiral galaxy which consists of a black
hole in its centre, a barred bulge and a disk which contains spiral arms. The
complex structure of the Galaxy makes it extremely difficult and challenging to
model its mass distribution, particularly for the Galactic disk which plays the
most important role in the dynamics and evolution of the Galaxy. Conventionally
an axisymmetric disk model with an exponential brightness distribution and a
constant mass-to-light ratio is assumed for the Galactic disk. In order to
generate a flat rotation curve, a dark halo has also to be included. Here, by
using the recently released Gaia billion-star map, we propose a Galactic disk
mass distribution model which is based on the star density distribution rather
than the brightness and mass-to-light ratio. The model is characterized by two
parameters, a bulge radius and a characteristic length. Using the mass
distribution model and solving the Poisson equation of the Galaxy, we obtain a
flat rotation curve which reproduces the key observed features with no need for
a dark halo.
