Context : The giant molecular cloud Sagittarius B2 ( hereafter Sgr B2 ) is the most massive region with ongoing high-mass star formation in the Galaxy .
In the southern region of the 40-pc large envelope of Sgr B2 , we encounter the Sgr B2 ( DS ) region , which hosts more than 60 high-mass protostellar cores distributed in an arc shape around an extended H ii region .
Hints of non-thermal emission have been found in the H ii region associated with Sgr B2 ( DS ) .

Aims : We seek to characterize the spatial structure and the spectral energy distribution of the radio continuum emission in Sgr B2 ( DS ) .
We aim to disentangle the contribution from the thermal and non-thermal radiation , as well as to study the origin of the non-thermal radiation .

Methods : We used the Very Large Array in its CnB and D configurations , and in the frequency bands C ( 4–8 GHz ) and X ( 8–12 GHz ) to observe the whole Sgr B2 complex .
Continuum and radio recombination line maps are obtained .

Results : We detect radio continuum emission in Sgr B2 ( DS ) in a bubble-shaped structure .
From 4 to 12 GHz , we derive a spectral index between -1.2 and -0.4 , indicating the presence of non-thermal emission .
We decomposed the contribution from thermal and non-thermal emission , and find that the thermal component is clumpy and more concentrated , while the non-thermal component is more extended and diffuse .
The radio recombination lines in the region are found to be not in local thermodynamic equilibrium but stimulated by the non-thermal emission .

Conclusions : Sgr B2 ( DS ) shows a mixture of thermal and non-thermal emission at radio wavelengths .
The thermal free-free emission is likely tracing an H ii region ionized by an O7 star , while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration .
We have developed a simple model of the Sgr B2 ( DS ) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index .