The physical and chemical structure of Sagittarius B2. II. Continuum
  millimeter emission of SgrB2(M) and SgrB2(N) with ALMA
  The high-mass star forming sites SgrB2(M) and SgrB2(N) have been the target
of numerous studies, revealing e.g. a rich chemistry. We want to characterize
their physical and chemical structure using ALMA high-angular resolution
observations at mm wavelengths, reaching spatial scales of about 4000 au, and
covering the whole band 6 (from 211 to 275 GHz). In order to determine the
continuum 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). We
study the continuum emission across the ALMA band 6, and compare it 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
SgrB2(M) is distributed in many fragments, most of the mass in SgrB2(N) arises
from a single object, with filamentary-like structures converging towards the
center. There seems to be a lack of low-mass dense cores in both regions. We
determine H2 volume densities for the cores of about 10^5-10^7 Msun pc^-3, one
to two orders of magnitude higher than the stellar densities of super star
clusters. In general, SgrB2(N) is chemically richer than SgrB2(M). There seems
to be a correlation between the chemical richness and the mass of the
fragments, with more massive clumps being more chemically rich. Both SgrB2(N)
and SgrB2(M) harbour a cluster of hot molecular cores. We compare the continuum
images with predictions from a detailed 3D radiative transfer model that
reproduces 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 the
3D structure of SgrB2, and allow us to understand its physical and chemical
structure.
