SH⁺ column density as a function of the CH⁺ column density per broad velocity interval (see Table 3). The open squares and open circles are from the present analysis and from Menten et al. (2011), respectively. The blue and purple points are from the absorption lines observed along the Galactic centre sight lines (SgrA*+50, SgrB2(N), and SgrB2(M)) and along the other sight lines (DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51), respectively. The black dashed, dotted, and dashed-dotted lines indicate N(SH⁺)/N(CH⁺) ratios of 0.01, 0.1, and 1, respectively.