IC 342: estimated from Nguyen et al. (1992)(Note that a low HCO⁺/H¹³CO⁺ 1–0 line ratio of 7 suggests that HCO⁺ columns may be higher. NGC 253: from Martín et al. (2006) (6.5 × 10¹³ for T_ex = 12 K and for 35″ source size. Rescaled N(HCO⁺) for the 10″ by 14″ H₃O⁺ source size of Knudsen et al. 2007). NGC 1068: we used the data provided in Pérez-Beaupuits et al. (2009) and made a rotational diagram (assuming a 2″ source size) finding T_ex(HCO⁺) = 36 K. NGC 4418: column density for HCO⁺ (based on data in Aalto et al. (2007) and unpublished JCMT HCO⁺ 4–3 data) for a 0''̣5 source size. NGC 6240: We estimate the HCO⁺ column density from the excitation information given in Iono et al. (2007) and for a source size of 1″.