Fermi Bubbles Without AGN: Gamma-Ray Bubbles in MHD Galaxy Formation Simulations with Full Cosmic Ray Spectra

Abstract

For the first time, we show in MHD simulations with cosmological initial conditions that bi-lobed gamma-ray outflows similar to the Fermi bubbles can form from star formation and supernova feedback, without involvement from active galactic nuclei (AGN). We use simulations run with full MHD and dynamical, on-the-fly multi-species cosmic ray transport in MeV-TeV energy bins to model gamma-ray emission in Milky Way-mass spiral galaxies from neutral pion decay, relativistic non-thermal Bremsstrahlung, and inverse Compton scattering. We find that these gamma-ray outflows are present in all three Milky-Way mass simulated galaxies. The amplitude, shape, and the composition of the gamma-ray spectrum of these bubbles fluctuates over time, with lepton-dominated and hadron-dominated phases. Spectra in which there is O(1) more gamma-ray flux from inverse Compton scattering than neutral pion decay are a good fit to the measured Fermi-LAT spectrum. Additionally, these simulations predict multi-wavelength features in soft x-rays and synchrotron radio, potentially providing new observational signatures that can connect the circumgalactic medium to cosmic ray physics and activity in the galactic center.

Figures (5)

• Figure 1: Top: total flux of 5-50 GeV $\gamma$-rays for the three simulated MW analogs in our sample. All galaxies show a distinct bi-lobed feature above and below the plane of the galactic disk; this feature is less pronounced for m12i, which has a warped disk and a strong magnetic field at its center, which leads to higher CR lepton loss rates than in m12f or m12m. Bottom: $\gamma$-ray emission in galaxy m12mfrom the three relevant CR interactions: neutral pion decay, relativistic Bremsstrahlung, and inverse Compton scattering. ICS is responsible for most of the bubble structure, with a slight hadronic contribution at lower latitudes.
• Figure 2: Top: the spread of $\gamma$-ray spectra from all CR interactions across all snapshots in the simulation, spaced between 460 Myr and 0 Myr in lookback time. A global renormalization to the spectrum amplitude has been applied to compare the spectra from simulations to each other and to the Fermi-LAT measurement. While the spectra for galaxies m12f and m12m is consistent with Fermi-LAT, m12i has a steeper tail at all times, due to high loss rates for CR leptons in the galactic center. Bottom: the components of the $\gamma$-ray spectra for the bubbles in galaxy m12m. $\gamma$-ray emission is produced by $\Pi_0$ decay, relativistic Bremsstrahlung, and inverse Compton scattering. The flux from inverse Compton is significantly less variable over time than the flux from $\Pi_0$ decay and relativistic Bremsstrahlung, both of which are proportional to gas density.
• Figure 3: Left: The $\gamma$-ray spectrum for the bubbles in galaxy m12m at select snapshots between 340 and 240 Myr in lookback time. At 340 Myr, the shape of the spectrum is in good agreement with the measured spectrum of the Fermi bubbles. At 320 Myr, an inflow of gas causes a sharp increase in the spectrum, as well as a more peaked shape characteristic of hadronic spectra. After the gas settles, the spectra become flatter, until a cluster of supernovae go off at 240 Myr. Right: The ratio of $\gamma$-ray flux from $\Pi_0$ decay to flux from ICS for the same snapshots. The snapshot where the $\gamma$-ray spectrum is most similar in shape to the measured Fermi-LAT spectrum is indicated by the thick magenta line. Spectra corresponding to gas outflows/ inflows are more hadronic, while those more consistent with the Fermi-LAT measurement are lepton-dominated. However, there are times at which the $\gamma$-ray spectrum for the bubbles are more leptonic than the measured Fermi-LAT spectrum.
• Figure 4: $\gamma$-ray emission (top) and gas density (bottom) for galaxy m12m over a 45 Myr span. At 354 Myr, a clear bi-lobed $\gamma$-ray bubble is visible. At 331 Myr, the bubbles become brighter and less ordered as gas previously ejected by supernova feedback falls back into the disk. By 309 Myr, the gas has re-settled in the disk, and the distinct bi-lobed shape of the bubbles is visible once more.
• Figure 5: Observable features in other wavelengths related to the $\gamma$-ray bubbles in galaxy m12m. Top: Synchrotron radio emission at 20 GHz from CR leptons in galaxy m12m. The flux is roughly comparable to the observed flux of the WMAP haze, and there is structure similar to observed radio loops and spurs in the MW 2004ApJ...614..186FBottom: soft x-ray (0.1-2.4 keV) surface brightness from inverse Compton scattering of CR leptons in galaxy m12m. Contours for $\gamma$-ray flux are shown in the black lines, with each contour separated by $10^{0.5} \, \rm{GeV \, cm^{-2} \, s^{-1} \, sr^{-1}}$. The edges of the $\gamma$-ray bubbles and x-ray halo are roughly coincident. Both of these observable features arise from CR lepton interactions (for synchrotron, with magnetic fields, and for ICS, with radiation), and are bright enough to potentially be detected by current and future surveys.