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We analyse 14 LBGs at z~2.8-3.8 constituting the only sample where both aspectroscopic measurement of their metallicity and deep IR observations(CANDELS+HUGS survey) are available. Fixing the metallicity of populationsynthesis models to the observed values, we determine best-fit physicalparameters under different assumptions about the star-formation history andalso consider the effect of nebular emission. For comparison we determine theUV slope of the objects, and use it to estimate their SFR_UV99 by correctingthe UV luminosity following Meurer et al. (1999). A comparison between SFRobtained through SED-fitting (SFR_fit) and the SFR_UV99 shows that the latterare underestimated by a factor 2-10, regardless of the assumed SFH. Other SFRindicators (radio, far-IR, X-ray, recombination lines) coherently indicate SFRsa factor of 2-4 larger than SFR_UV99 and in closer agreement with SFR_fit. Thisdiscrepancy is due to the solar metallicity implied by the usual beta-A1600conversion factor. We propose a refined relation, appropriate for sub-solarmetallicity LBGs: A1600 = 5.32+1.99beta. This relation reconciles thedust-corrected UV with the SED-fitting and the other SFR indicators. We showthat the fact that z~3 galaxies have sub-solar metallicity implies an upwardrevision by a factor of ~1.5-2 of the global SFRD, depending on the assumptionsabout the age of the stellar populations. We find very young best-fit ages(10-500 Myrs) for all our objects. From a careful examination of theuncertainties in the fit and the amplitude of the Balmer break we conclude thatthere is little evidence of the presence of old stellar population in at leasthalf of the LBGs in our sample, suggesting that these objects are probablycaught during a huge star-formation burst, rather than being the result of asmooth evolution.

Constraints on the star-formation rate of z~3 LBGs with measured metallicity in the CANDELS GOODS-South field