The Bending Feature of the Fermi Bubbles: A Presumed Horizontal Galactic
  Wind and Its Implication on the Bubbles' Age
  There are two spectacular structures in our Milky Way: the {\it 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 {\it Fermi} bubbles and NPS. In
this model, the contact discontinuity (CD) marks the boundary of the {\it
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 ~ 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, and the HGW is ~ 110-190 km/s. We
argue that, the age of the NPS and the {\it 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 {\it Fermi} bubble/NPS.
