We use the infrared excess (IRX) FIR/UV luminosity ratio to study the relation between the effective UV attenuation (A IRX) and the UV spectral slope (β) in a sample of 450 1 z 2.5 galaxies. The FIR data are from very deep Herschel observations in the GOODS fields that allow us to detect galaxies with star formation rates (SFRs) typical of galaxies with log(M *) 9.3. Thus, we are able to study galaxies on and even below the main SFR-stellar mass relation (main sequence). We find that main-sequence galaxies form a tight sequence in the IRX-β plane, which has a flatter slope than commonly used relations. This slope favors a Small-Magellanic-Cloud-like UV extinction curve, though the interpretation is model dependent. The scatter in the A IRX-β plane correlates with the position of the galaxies in the SFR-M * plane. Using a smaller sample of galaxies with CO gas masses, we study the relation between the UV attenuation and the molecular gas content. We find a very tight relation between the scatter in the IRX-β plane and the specific attenuation SA , a quantity that represents the attenuation contributed by the molecular gas mass per young star. SA is sensitive to both the geometrical arrangement of stars and dust and to the compactness of the star-forming regions. We use this empirical relation to derive a method for estimating molecular gas masses using only widely available integrated rest-frame UV and FIR photometry. The method produces gas masses with an accuracy between 0.12 and 0.16 dex in samples of normal galaxies between z ~ 0 and z ~ 1.5. Major mergers and submillimeter galaxies follow a different SA relation.
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