Pixel-to-pixel correlation between H¹³CO⁺ (1–0) and C¹⁸O (1–0) integrated intensities in K km s⁻¹ and Herschel H₂ column density in mag (assuming , Bohlin et al. 1978). The blue and red points indicate the correlation between C¹⁸O (1–0) integrated intensity and Herschel H₂ column density and between H¹³CO⁺ (1–0) integrated intensity and Herschel H₂ column density, respectively. The dashed and solid lines indicate the best-fit result for the C¹⁸O - Herschel H₂ column density correlation and for the H¹³CO⁺ - Herschel H₂ column density correlation.