Simulating the LOcal Web (SLOW) - VI: $γ$-ray Emission in the Local Universe

TL;DR

This study uses the first constrained cosmological MHD simulation with an on-the-fly spectral CR solver to predict diffuse γ-ray emission from CR protons in the local Universe. CR protons accelerate at structure-formation and accretion shocks, advect through cluster and filament volumes, and produce π0-decay γ-rays in the Fermi-LAT band; the resulting emission is several orders of magnitude below current Fermi-LAT limits, with Coma needing roughly $ ext{F}_ ext{γ} < 10^{-11} ext{ s}^{-1} ext{ cm}^{-2}$ for detection. The work highlights that shock finder resolution and shock obliquity strongly influence CR acceleration, and it discusses how CR transport and injection parameters shape the predicted γ-ray outputs. The results are consistent with non-detections and set the stage for higher-resolution zoom-ins and expanded CR transport treatments to tighten predictions and guide future observations.

Abstract

Context: Diffuse $γ$-ray emission from cosmic ray (CR) protons scattering off the gas in the intracluster and intergalactic medium (ICM and IGM) remains out of reach for current observations. Detecting this emission would provide constraints on intergalactic magnetic fields as well as dark matter interaction models. Aims: We aim to provide estimates for diffuse $γ$-ray emission in the Fermi-LAT band from galaxy clusters and the cosmic web in the local Universe. Methods: In this work we show results from the first cosmological MHD simulation with an on-the-fly spectral CR model. We model CR injection at shocks, account for adiabatic energy changes and advection of CR protons, and obtain their $γ$-ray emissivity directly from the simulated CR energy density and spectra. For this we use constrained initial conditions, which evolve in a field closely resembling that of the local Universe, allowing direct comparison to Fermi-LAT data on massive clusters. Results: We find CR proton acceleration at all structure formation and accretion shocks in galaxy clusters and cosmic web filaments. These protons provide the basis for diffuse $γ$-ray emission in these regimes. The absolute value of the diffuse $γ$-ray emission in our simulation lies a few orders of magnitude below the current upper limits found by Fermi-LAT. Under the assumption of our model, a sensitivity of $F_γ< 10^{-11} \: γ~ \text{s}^{-1}~\text{cm}^{-2}$ is required for a detection of diffuse emission in Coma.

Figures (8)

• Figure 1: Full-sky projection in galactic coordinates of simulation box. Top: CR proton pressure component as the mean value along the line of sight between $r = 10 - 300$ Mpc. This shows predominantly the total injected proton component with adiabatic compression as it settles into the higher-density regions of clusters and filaments. Circles indicate the projected $r_\mathrm{vir}$ of each of the labeled cluster matches. Bottom: Integrated $\gamma$-ray intensity obtained by integrating over the CR proton spectra according to Eq. \ref{['eq:gammaweb_gamma_luminosity']} in the Fermi-LAT band $E_\gamma \in [0.5-200]$ GeV.
• Figure 2: Results for clusters and groups in the simulation box with a virial mass $M_\mathrm{vir} > 5 \times 10^{13} M_\odot$. Top panel: CR proton to thermal pressure ratio obtained in the momentum range $\hat{p} \in [0.1, 10^5]$. Middle panel: $\gamma$-ray flux obtained at the position of our point of view in Fig. \ref{['fig:gammaweb_allsky']} in the energy band $\gamma \in [0.5-200]$ GeV. Bottom panel: $\gamma$-ray luminosity in the same energy band. In all panels, dots represent the respective quantity $\mathcal{Q}$ as obtained from the simulation. Crosses indicate the same quantity up-scaled by the ratio between $X_\mathrm{CR}$ in the whole cluster volume and $X_\mathrm{CR}$ only in particles that contain a CR population.
• Figure 3: Mean distribution function in energy space, within our Coma cluster replica. We bin the spectra in 20 radial bins indicated by the different colors. The vertical dashed line indicates the threshold energy beyond which protons can be scattered into $\pi^0$-ons.
• Figure 4: We show the integrated $\gamma$-ray intensity for three prominent clusters. Each image has a width of $3r_\mathrm{vir}$ of the respective cluster. The top panels show the simulation output. The bottom panels show the same images as above but smoothed with a Gaussian beam with the single photon angular resolution at 1 GeV of $\theta = 0.6^\circ$ of the Fermi-LAT instrument Atwood2009. The white circle in the lower-left corner indicates the size of $\theta = 0.6^\circ$ at the observed redshift of the cluster.
• Figure 5: Integrated $\gamma$-ray spectra for our three selected clusters: Coma, Virgo and Perseus. Solid lines show our results, while arrows indicate the upper limits from observations, where available. Different dashed lines show the results from applying the model by Pfrommer2004 for a fixed $X_\mathrm{cr} = 0.01$ under varying proton energy slopes $\alpha_p$.