The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo

TL;DR

This study models the thermal gas structure of the Fermi bubbles using X-ray emission lines, constraining their properties and supporting a formation scenario linked to Sgr A* activity rather than star formation.

Contribution

It provides the first detailed modeling of the bubbles' thermal gas using OVII and OVIII emission lines, estimating their physical parameters and origin.

Findings

Bubble expansion rate of approximately 490 km/s

Bubble age estimated at around 4.3 million years

Energy injection rate consistent with Sgr A* accretion event

Abstract

The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays, extend $\approx$10 kpc vertically from the Galactic center, and formed from either nuclear star formation or accretion activity on Sgr A*. Simulations predict a range of shock strengths as the bubbles expand into the surrounding hot gas halo distribution ($T_{halo} \approx 2 \times 10^6$ K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between $10^6$ and $10^8$ K, with potential X-ray signatures. In this work, we constrain the bubbles' thermal gas structure by modeling the OVII and OVIII emission line strengths from archival XMM-Newton and Suzaku data. Our emission model includes a hot thermal volume-filled bubble component cospatial with the gamma-ray region, and a shell of compressed material. We find that a bubble/shell model with $n \approx 1 \times 10^{-3}$ cm$^{-3}$ and with log($T$) $\approx$ 6.60-6.70 is consistent with the observed line intensities. In the framework of a continuous Galactic outflow, we infer a bubble expansion rate, age, and energy injection rate of $490_{-77}^{+230}$ km s$^{-1}$, $4.3_{-1.4}^{+0.8}$ Myr, and $2.3_{-0.9}^{+5.1} \times 10^{42}$ erg s$^{-1}$. These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.