Multi-Tracer Cross-Correlations of the Unresolved $γ$-Ray Sky

TL;DR

This work uses a multi-tracer cross-correlation approach to study the unresolved $\gamma$-ray background (UGRB) by correlating Fermi-LAT maps with DES tracers of large-scale structure. It combines galaxy clustering and weak-lensing measurements to extract a significant cross-correlation signal, interprets it with both phenomenological (PL/LP) and physical (blazar-dominated) halo-model frameworks, and finds a preference for a curved energy spectrum. The introduction of a full multi-tracer analysis yields tighter constraints on the source-population parameters and establishes a robust extragalactic origin for the UGRB with a combined detection significance of $\mathrm{SNR}=8.6$. The results suggest that the faint end of the $\gamma$-ray sky cannot be simply extrapolated from resolved sources and motivate exploring additional populations or mechanisms, including possible dark matter contributions, in future work.

Abstract

Our understanding of the $γ$-ray sky has greatly advanced, yet studying the unresolved $γ$-ray background (UGRB) can unveil the nature of the faintest $γ$-ray source populations in the Universe. Statistical cross-correlations between the UGRB and tracers of large-scale cosmic structure allow us to infer which sources contribute the most to this emission. In this work, we examine the angular correlation between the UGRB and the matter distribution traced by galaxies, using twelve years of Fermi Large Area Telescope (LAT) observations along with three years of Dark Energy Survey (DES) data. We detect a correlation with a signal-to-noise ratio of 7.96, primarily driven by large angular scales. We then perform a multi-tracer analysis that combines this measurement with the cross-correlation between $γ$ rays and DES weak lensing. The two single-tracer results are mutually consistent, and their combination yields a total significance of 8.6, firmly establishing the extragalactic origin of the UGRB. Intriguingly, the properties inferred for the sources contributing to the UGRB show departures from those of the resolved γ-ray population, suggesting that the faint end of the $γ$-ray sky is not a simple extrapolation of currently resolved sources.

Figures (9)

• Figure 1: The redshift distributions for the DES Y3 source and redMaGiC lens galaxies. The data used to generate the plots has been taken from the publicly available DES database (https://des.ncsa.illinois.edu/releases/y3a2/Y3key-catalogues) and has been normalised such that $\int n(z)dz = 1$ for both samples.
• Figure 2: The DES Y3 mass map jeffrey2021dark along with the overlapping region in the Fermi-LAT 12 year $\gamma$-ray background with energies between 9.120 and 17.38 GeV. The plot is in a Mollweide projection with equatorial coordinates, and the $\gamma$-ray map has been downgraded to $\mathrm{N_{side}}=256$ for visualization purposes.
• Figure 3: Top: Angular scale (left) and redshift (right) behaviour for LP (phenomenological) and Physical Models. Bottom left: Energy behaviour for the LP and Physical Models in terms of raw matched filter amplitude, showing a near-flat behaviour compatible with that of the naive model. Bottom right: Energy behaviour for the LP, and Physical Models in terms of the differential energy amplitude with respect to the PL .
• Figure 4: Left: Constraints on the parameters of the power-law (PL) and the log-parabola (LP) phenomenological models. In all panels, the 2D contours refer to the 68% and 95% credible regions, with the shaded areas in the 1-D subplots denoting the 68% credible interval for the associated posterior distributions.
• Figure 5: Constraints on the parameters describing the physical model BLZ, based on the BL Lac model from Ref. korsmeier2022flat, and assuming a single population accounting for the measured cross- correlation. As before, the 2D contours refer to the 68% and 95% credible regions, with the shaded areas in the 1D subplots denoting the 68% credible interval for the 1D posterior distribution.