Clues to the origin of Fermi Bubbles from OVIII/OVII line ratio

TL;DR

This study uses hydrodynamical simulations and OVIII/OVII line ratios to constrain the origin of Fermi Bubbles, suggesting a low-luminosity event about 15-25 million years ago from either star formation or black hole activity.

Contribution

It provides the first detailed comparison of hydrodynamical models with observed X-ray line ratios to determine the Fermi Bubbles' origin and age.

Findings

Low luminosity energy injection best matches observed line ratios.

Estimated shock velocity is around 300 km/s.

Fermi Bubbles are approximately 15-25 million years old.

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 OVIII to OVII line ratio within and near Fermi Bubbles. Our results suggest that independent of the driving mechanisms, a low luminosity ($\mathcal{L} \sim 0.7-1\times 10^{41}$ erg s$^{-1}$) energy injection best reproduces the observed line ratio for which the shock temperature is $\approx 3\times 10^6$ K. Assuming the Galactic halo temperature to be $2\times 10^6$K, we estimate the shock velocity to be $\sim 300$ km s$^{-1}$ for a weak shock. The corresponding estimated age of the Fermi bubbles is $\sim 15-25$ Myr. Such an event can be produced either by a star formation rate of $\sim 0.5$ M$_\odot$ 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 $\gtrsim 10^{41}$ \ergps as a possible origin of the Fermi Bubbles.