Spectrum obtained toward Sgr B2(M) at the frequency of SH⁺ 2₁–1₁ with the APEX telescope in main beam brightness temperature. The red curve is the synthetic spectrum described in Sect. 4.1, i.e., showing no detectable emission, the horizontal dotted line shows the continuum level, and the vertical dotted line marks the systemic velocity of the source (62 km s^-1). The absorption line appearing at  ~− 105 km s^-1 is the J_KaKc = 1₁₁−0₀₀ ground-state line of HDO at 893 638.7 MHz (Comito et al. 2003).