Small-scale physical and chemical structure of diffuse and translucent
  molecular clouds along the line of sight to Sgr B2
  The diffuse and translucent molecular clouds traced in absorption along the
line of sight to strong background sources have so far been investigated mainly
in the spectral domain because of limited angular resolution or small sizes of
the background sources. We aim to resolve and investigate the spatial structure
of molecular clouds traced by several molecules detected in absorption along
the line of sight to SgrB2(N). We have used spectral line data from the EMoCA
survey performed with ALMA, taking advantage of the high sensitivity and
angular resolution. We identify, on the basis of the spectral analysis of
c-C3H2 across the field of view, 15 main velocity components along the line of
sight to SgrB2(N) and several components in the envelope of SgrB2. The c-C3H2
column densities reveal two categories of clouds. Clouds in Category I (3 kpc
arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities,
smaller linewidths, and smaller widths of their column density PDFs than clouds
in Category II (Scutum arm, Sgr arm, and other GC clouds). To investigate the
spatial structure we derive opacity maps for the following molecules: c-C3H2,
H13CO+, 13CO, HNC, HN13C, HC15N, CS, C34S, 13CS, SiO, SO, and CH3OH. These maps
reveal that most molecules trace relatively homogeneous structures that are
more extended than the field of view defined by the background continuum
emission (about 15", that is 0.08-0.6pc depending on the distance). SO and SiO
show more complex structures with smaller clumps of size ~5-8". Our analysis
suggests that the driving of the turbulence is mainly solenoidal in the
investigated clouds. On the basis of HCO+, we conclude that most line-of-sight
clouds towards SgrB2 are translucent, including all clouds where complex
organic molecules were recently detected. We also conclude that CCH and CH are
good probes of H2 in both diffuse and translucent clouds.
