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The high-mass star forming sites SgrB2(M) and SgrB2(N) have been the targetof numerous studies, revealing e.g. a rich chemistry. We want to characterizetheir physical and chemical structure using ALMA high-angular resolutionobservations at mm wavelengths, reaching spatial scales of about 4000 au, andcovering the whole band 6 (from 211 to 275 GHz). In order to determine thecontinuum emission in line-rich sources, we use a new statistical method:STATCONT. We detect 27 continuum sources in SgrB2(M) and 20 in SgrB2(N). Westudy the continuum emission across the ALMA band 6, and compare it withprevious SMA 345 GHz and VLA 40 GHz observations, to study the nature of thesources detected. The brightest sources are dominated by (partially opticallythick) dust emission, while there is an important degree of contamination fromionized gas free-free emission in weaker sources. While the total mass inSgrB2(M) is distributed in many fragments, most of the mass in SgrB2(N) arisesfrom a single object, with filamentary-like structures converging towards thecenter. There seems to be a lack of low-mass dense cores in both regions. Wedetermine H2 volume densities for the cores of about 10^5-10^7 Msun pc^-3, oneto two orders of magnitude higher than the stellar densities of super starclusters. In general, SgrB2(N) is chemically richer than SgrB2(M). There seemsto be a correlation between the chemical richness and the mass of thefragments, with more massive clumps being more chemically rich. Both SgrB2(N)and SgrB2(M) harbour a cluster of hot molecular cores. We compare the continuumimages with predictions from a detailed 3D radiative transfer model thatreproduces the structure of SgrB2 from 45 pc down to 100 au. This dataset,together with ongoing projects in the range 5 to 200 GHz, better constrain the3D structure of SgrB2, and allow us to understand its physical and chemicalstructure.

The physical and chemical structure of Sagittarius B2. II. Continuum millimeter emission of SgrB2(M) and SgrB2(N) with ALMA