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One aspect that has generated many discussions regarding the SF history diagram in Cosmology, is the lack of confidence on the reddening corrections. This is compounded with the low level of agreement found until now between the optical/UV and FIR determinations of the SFR.

In the first part of this paper, we have investigated the possible systematic differences between SFR estimators by applying them to a sample of nearby star forming galaxies with good photometric data from the UV to the FIR.

We found that the main source of systematic differences among the SFR rate estimators is related to the presence of stellar Balmer absorption in the spectrum of emission line galaxies. The main effect of the Balmer absorptions is to produce an overestimate of the reddennnig when their effect is not included. We showed that taking into account the underlying Balmer absorptions effect in the estimates of reddening, removes most of the systematic differences between the SFR estimators in the optical/UV and FIR. Furthermore the scatter of the SFR estimations is considerably reduced by the application of the corrections. We also found that the escape of photons plays a minor role compared to that of the Balmer absorptions. These results give renewed confidence to the estimates of SFR for star forming galaxies in general and for samples similar to the one presented here in particular.

Thus, our central result is that the extinction correction including the effects of an underlying stellar Balmer absorption brings into agreement all four SFR estimators, and that the photon escape correction seems to play a minor role.

In the second part of the paper we used the average results for our sample to construct a set of "unbiased" SFR estimators. These "unbiased" SFR estimators expressions include statistically the underlying Balmer absorption and photon escape corrections to the extinction estimates and bring the four SFR estimators studied here into the same system. We thus obtained consistent results between the SFR estimators in the optical/UV and FIR. The application of these "unbiased" SFR estimators to a compilation of surveys has produced a SFR history of the universe where all surveys results agree whitin the errors. Particularly important is the level of agreement achieved between the FIR/mm and optical/UV SFR results.

Our "new" and unbiased SFR history of the universe shows a steep rise in the SFR rate from z = 0 to z = 1 with SFR propto (1 + z)4.5 followed by a mild decline for z > 2 where SFR propto (1 + z)-1.5. The steep increase to z = 1 seems in line with recent determinations of the SFR using only the Halpha estimator. Most galaxy formation models tend to have a much flatter slope from z = 0 to z = 1.


We acknowledge fruitful conversations with Miguel Mas-Hesse, Daniela Calzetti, Piero Madau, Max Pettini, Divakara Mayya, Enrique Pérez, Rosa González-Delgado, David Hughes and Enrique Gaztañaga and useful suggestions from an anonymous referee. Daniel Rosa González gratefully acknowledges a research grant from the INAOE Astrophysics Department, a studentship from CONACYT, the Mexican Research Council, as part of ET research grant # 32186-E, and an EC Marie Curie studentship at the IoA Cambridge.

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