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

Context. The two hot molecular cores Sgr？B2(M) and Sgr？B2(N), which are located at the center of the giant molecular cloud complex Sagittarius？B2, have been the targets of numerous spectral line surveys, revealing a rich and complex chemistry.Aims. We seek to characterize the physical and chemical structure of the two high-mass star-forming sites Sgr？B2(M) and Sgr？B2(N) using high-angular resolution observations at millimeter wavelengths, reaching spatial scales of about 4000 au.Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to perform an unbiased spectral line survey of both regions in the ALMA band 6 with a frequency coverage from 211 GHz to 275 GHz. The achieved angular resolution is 0.？4, which probes spatial scales of about 4000 au, i.e., able to resolve different cores and fragments. In order to determine the continuum emission in these line-rich sources, we used a new statistical method, STATCONT, which has been applied successfully to this and other ALMA datasets and to synthetic observations.Results. We detect 27 continuum sources in Sgr？B2(M) and 20 sources in Sgr？B2(N). We study the continuum emission variation across the ALMA band 6 (i.e., spectral index) and compare the ALMA 1.3 mm continuum emission with previous SMA 345 GHz and VLA 40 GHz observations to study the nature of the sources detected. The brightest sources are dominated by (partially optically thick) dust emission, while there is an important degree of contamination from ionized gas free-free emission in weaker sources. While the total mass in Sgr？B2(M) is distributed in many fragments, most of the mass in Sgr？B2(N) arises from a single object, with filamentary-like structures converging toward the center. There seems to be a lack of low-mass dense cores in both regions. We determine H_{2 volume densities for the cores of about 10^{7–109 cm-3 (or 105–107 M⊙ pc-3), i.e., one to two orders of magnitude higher than the stellar densities of super star clusters. We perform a statistical study of the chemical content of the identified sources. In general, Sgr？B2(N) is chemically richer than Sgr？B2(M). The chemically richest sources have about 100 lines per GHz and the fraction of luminosity contained in spectral lines at millimeter wavelengths with respect to the total luminosity is about 20%–40%. There seems to be a correlation between the chemical richness and the mass of the fragments, where more massive clumps are more chemically rich.}}