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Context. The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. Two ultra-compact H ii (UCHii ) regions were identified in SgrB2’s central hot cores, SgrB2(M) and SgrB2(N). Aims. Our aim is to characterize the properties of the H ii regions in the entire SgrB2 cloud. Comparing the H ii regions and the dust cores, we aim to depict the evolutionary stages of different parts of SgrB2. Methods. We use the Very Large Array in its A, CnB, and D configurations, and in the frequency band C (~6GHz) to observe the whole SgrB2 complex. Using ancillary VLA data at 22.4 GHz and ALMA data at 96 GHz, we calculated the physical parameters of the UCH ii regions and their dense gas environment. Results. We identify 54 UCHii 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 UCHii 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 UCHii regions are ionized by stars of types from B0.5 to O6. We found a typical gas density of ~10 6 –10 9 cm 3 around the UCH ii regions. The pressure of the UCH ii regions and the dense gas surrounding them are comparable. The expansion timescale of these UCHii regions is determined to be ~10 4 –10 5 yr. The percentage of the dust cores that are associated with H ii regions are 33%, 73%, 4%, and 1% for SgrB2(N), SgrB2(M), SgrB2(S), and SgrB2(DS), respectively. Two-thirds of the dust cores in SgrB2(DS) are associated with outflows. Conclusions. The electron densities of the UCHii regions we identified are in agreement with that of typical UCHii regions, while the radii are smaller than those of the typical UCHii regions. The dust cores in SgrB2(M) are more evolved than in SgrB2(N). The dust cores in SgrB2(DS) are younger than in SgrB2(M) or SgrB2(N). 

The morphology of the Milky Way is still a matter of debate. In order to shed light on uncertainties surrounding the structure of the Galaxy, in this paper, we study the imprint of spiral arms on the distribution and properties of its molecular gas. To do so, we take full advantage of the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic Interstellar Medium) survey that observed a large area of the inner Galaxy in the 13 CO (2–1) line at an angular resolution of 28′′. We analyse the influences of the spiral arms by considering the features of the molecular gas emission as a whole across the longitude–velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to the properties of their counterparts in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ significantly from the emission between the arms. On average, spiral arms show masses per unit length of ~10 5 –10 6 M ⊙ kpc −1 . This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one.