The physical and chemical structure of Sagittarius B2 VI. UCHII regions in Sgr B2

Context. The giant molecular cloud Sagittarius B2 (hereafter Sgr B2) is the most massive region with ongoing high-mass star formation in the Galaxy. Two ultra-compact HII (UCHIT ) regions were identified in Sgr B2's central hot cores, Sgr B2(M) and Sgr B2(N). Aims. Our aim is to characterize the properties of the HII regions in the entire Sgr B2 cloud. Comparing the Hii regions and the dust cores, we aim to depict the evolutionary stages of different parts of Sgr B2. Methods. We use the Very Large Array in its A, CnB, and D configurations, and in the frequency band C (similar to 6 GHz) to observe the whole Sgr B2 complex. Using ancillary VLA data at 22.4 GHz and ALMA data at 96 GHz, we calculated the physical parameters of the UCHIT regions and their dense gas environment. Results. We identify 54 UCHIT regions in the 6 GHz image, 39 of which are also detected at 22.4 GHz. Eight of the 54 UCHII regions are newly discovered. The UCHIT regions have radii between 0.006 pc and 0.04 pc, and have emission measure between 10(6) pc cm(-6) and 10 9 pc cm(-6). The UCHIT regions are ionized by stars of types from B0.5 to O6. We found a typical gas density of similar to 10(6)-10(9) cm(-3) around the UCHIT regions. The pressure of the UCHIT regions and the dense gas surrounding them are comparable. The expansion timescale of these UCHIT regions is determined to be similar to 10(4)-10(5) yr. The percentage of the dust cores that are associated with HII regions are 33%, 73%, 4%, and 1% for Sgr B2(N), Sgr B2(M), Sgr B2(S), and Sgr B2(DS), respectively. Two-thirds of the dust cores in Sgr B2(DS) are associated with outflows. Conclusions. The electron densities of the UCHIT regions we identified are in agreement with that of typical UCHIT regions, while the radii are smaller than those of the typical UCHIT regions. The dust cores in Sgr B2(M) are more evolved than in Sgr B2(N). The dust cores in Sgr B2(DS) are younger than in Sgr B2(M) or Sgr B2(N).