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 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. These structures demonstrate a 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 the NPS. In this model, the contact discontinuity 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 ∼1.9–2.3, and the velocity of the HGW is ∼0.7–0.9cs. Depending on the temperature of the pre-shock ISM, the velocity of the expanding NPS in Galactic coordinates is around 180–290 $\mathrm{km}\,{{\rm{s}}}^{-1}$, and the HGW is ∼110–190 $\mathrm{km}\,{{\rm{s}}}^{-1}$. 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.