Clues to the origin of Fermi bubbles from O viii/O vii line ratio

Abstract

We constrain the origin of Fermi bubbles using 2D hydrodynamical simulations of both star formation-driven and black hole accretion-driven wind models. We compare our results with recent observations of O viii to O vii line ratio within and near Fermi bubbles. Our results suggest that independent of the driving mechanisms, a low-luminosity ( L∼0.7--1×1041 erg s−1) energy injection best reproduces the observed line ratio for which the shock temperature is ≈3 × 106 K. Assuming the Galactic halo temperature to be 2 × 106 K, we estimate the shock velocity to be ∼300 km s−1 for a weak shock. The corresponding estimated age of the Fermi bubbles is ∼15–25 Myr. Such an event can be produced either by a star formation rate of ∼0.5 M⊙ yr−1 at the Galactic Centre or a very low-luminosity jet/accretion wind arising from the central black hole. Our analysis rules out any activity that generates an average mechanical luminosity ≳1041 erg s−1 as a possible origin of the Fermi bubbles.