Contribution of Globular Clusters to Diffuse Gamma-ray Emission from Galactic Plane

TL;DR

This paper assesses the contribution of unresolved globular clusters to the diffuse gamma-ray emission in our galaxy, finding their impact to be negligible at both GeV and TeV energies.

Contribution

It introduces a nonparametric method to estimate globular cluster luminosity and spatial distributions, evaluating their contribution to gamma-ray emission.

Findings

Globular clusters contribute about 2% to TeV gamma-ray emission.

Their contribution is less than 1% in the GeV range.

The impact of unresolved globular clusters on DGE is negligible.

Abstract

The diffuse Galactic $\gamma$-ray emission (DGE) provides a valuable probe for investigating the cosmic ray propagation and interactions within our Galactic environment. Recent observations have demonstrated systematic excesses of DGE compared with the conventional cosmic-ray propagation model predictions. While $\gamma$-ray emissions have been detected in a subset of globular clusters, their undetected populations may significantly contribute to the DGE. Motivated by this possibility, we present a comprehensive assessment of potential contributions from unresolved globular clusters to the DGE. In our analysis, a nonparametric method is employed to estimate the luminosity function and spatial distribution function of globular clusters using the Fermi-LAT fourth source catalog (4FGL) combined with a reference globular cluster catalog. Based on these distributions, we calculate the cumulative contribution of unresolved globular cluster populations to the DGE observed by Fermi-LAT and the Large High Altitude Air Shower Observatory (LHAASO). Our results reveal that globular clusters account for only $\sim$2\% of the DGE at the TeV range, and smaller than $1\%$ in the GeV regime, which is effectively negligible.