There are two spectacular structures in our Milky Way : the Fermi bubbles in gamma-ray observations and the North Polar Spur ( NPS ) structure in X-ray observations .
Because of their morphological similarities , they may share the same origin , i.e. , related to the past activity of Galactic center ( GC ) .
Besides , those structures show significant bending feature toward the west in Galactic coordinates .
This inspires us to consider the possibility that the bending may be caused by a presumed global horizontal galactic wind ( HGW ) blowing from the east to the west .
Under this assumption , we adopt a toy shock expansion model to understand two observational features : ( 1 ) the relative thickness of the NPS ; ( 2 ) the bending of the Fermi bubbles and NPS .
In this model , the contact discontinuity ( CD ) marks the boundary of the Fermi bubbles , and the shocked interstellar medium ( ISM ) marks the NPS X-ray structure .
We find that the Mach number of the forward shock in the east is \sim 1.9 - 2.3 , and the velocity of the HGW is \sim 0.7 - 0.9 c _ { s } .
Depending on the temperature of the pre-shock ISM , the velocity of the expanding NPS in Galactic coordinates is around 180 - 290 ~ { } km~ { } s ^ { -1 } , and the HGW is \sim 110 - 190 ~ { } km~ { } s ^ { -1 } .
We argue that , the age of the NPS and the Fermi bubbles is about 18–34 Myr .
This is a novel method , independent of injection theories and radiative mechanisms , for the estimation on the age of the Fermi bubble/NPS .