Physical properties of Sgr B2 (N1) derived from dust continuum emission and C¹⁸O 1–0 emission. Panel a: optical depth t as a function of distance from Sgr B2 (N1) in south (S) and west (W) directions and for the 1.3 mm and 3 mm continua, respectively. The 3 mm data have been corrected for free-free emission. Panel b: emissivity spectral index β as a function of distance from Sgr B2 (N1) derived from the optical depth ratio of the 1.3 mm and 3 mm data shown in (a). Panel c: H₂ column densities derived from the 3 mm continuum emission, using β = 1.3 (adopted from Bonfand et al. 2019, see text), and from C¹⁸O 1–0 emission using conversion factors of C¹⁶O/C¹⁸O = 250 ± 30 (Henkel et al. 1994) and H₂/C¹⁶O = 10⁴ (Rodríguez-Fernández et al. 2001). The break in the (dashed) westbound profile indicates the observed velocity shift of the line from ~64 to 69 km s⁻¹. In all panels upper and lower limits are indicated with arrows.