Comparison of effective screen extinction, as inferred from SED analysis of the four-band HST images, and the CO molecular gas mass surface density Σ _{mol gas}. Left panel: comparisons of the two-dimensional distributions (CO in white contours, A( V) in color) at the same FWHM resolution of ~0 75, and sampled on a 0 25 grid. Right panel: pixel-by-pixel correlation between molecular column density (horizontal scale) and effective screen optical depth at Hα (τ(Hα) = 0.73 × A( V); Calzetti et al. 2000), with a typical uncertainty marked as a black cross. The black and dotted red curves are models for a homogenous mixture of gas, dust, and stars, where τ _{effective screen}(Hα) = ln ((Σ _mol gas/30 b)/{1 − exp (− (Σ _mol gas/30 b))}), where b is the ratio of total molecular gas column to Hα optical depth in units of the diffuse Galactic ISM value (Σ/τ(Hα)) _MW ISM = 30  M _☉ pc ⁻², or ( N(H)/ A( V)) _MW ISM = 2 × 10 ²¹ cm ⁻² mag ⁻¹ (Bohlin et al. 1978; see also Wuyts et al. 2011a; Nordon et al. 2013). The best-fitting curve has b = 4.7 ± 0.3, in good agreement with the findings of Nordon et al. (2013) for z ~ 1–3 main-sequence galaxies observed in the UV, Herschel PACS, and CO.