Evidence of a spectral steepening of the gamma-ray emission from the Galactic center ridge

Very high-energy (VHE) γ-ray emission detected from the central molecular zone (CMZ) hints at diffusive propagation of cosmic rays (CRs) injected continuously near the Galactic center (GC). Using H.E.S.S. VHE γ-ray observations, we aim to construct a multicomponent description of the region in order to derive the spatial and spectral distributions of CRs and discuss their potential origin. We rely on a spectro-morphological analysis, in which spectral and spatial parameters of different components are fit simultaneously, and on a three-dimensional description of the CMZ and CR distributions. The GC ridge emission is well reproduced with the assumed gas distribution model weighted by a 1/r^α CR density profile with We detect no significant spectral variations across the CMZ, which supports a continuous injection from the GC. We report a significant (> 3 σ) curvature in the GC ridge γ-ray spectrum, implying a parent proton break (or cutoff energy) of E_b ∼ 15-50 TeV (E_ α = 1.10 ± 0.05_ stat ± 0.10_ syst . cut ∼ 60-160 TeV). We also derive the best-fit spectral parameters of the other sources in the region and report a significant cutoff at E ∼ 5 TeV in the spectrum of the pulsar wind nebula G0.9+0.1. Finally we constrain the position of the injection site to be close to the GC (ruling out the Arches and Quintuplet clusters as major contributors to the emission). We discuss possible origins for the GC ridge emission, potential mechanisms for causing the observed curvature, and a possible contribution of the gas content to the emission of HESS J1745-290, and derive qualitative constraints on the line-of-sight positions of the clouds shaping the CMZ.