SFR surface density versus the molecular hydrogen surface density. The points with the 3-σ error bars represent pointings in the Arp 158 system with the SFR derived from the FUV + 24 μm relation, and the red circles adopt the SFR derived from the fit of the SED assuming the burst has the extinction calculated in Table 5. The solid black line represents the Schmidt-Kennicutt law for molecular hydrogen derived by Bigiel et al. (2008) for spiral galaxies: log   Σ_SFR = −3.02 + 0.96log   Σ_H2, with Σ_SFR in M_⊙ yr^-1 kpc^-2 and Σ_H2 in M_⊙ pc^-2. The dashed lines represent the typical 0.2 dex scatter. The molecular mass does not take into account the helium contribution in any case.