Accepted for publication in MNRAS.

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Daniel Rosa-González 1, Elena Terlevich 1* and Roberto Terlevich, 2†

1 INAOE, Luis Enrique Erro 1. Tonantzintla, Puebla 72840. México.
2 Institute of Astronomy, Madingley Road, CB3 OHA Cambridge, U.K.

Abstract. The observational determination of the behaviour of the star formation rate (SFR) with look-back time or redshift has two main weaknesses: 1 - the large uncertainty of the dust/extinction corrections, and 2 - that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. Most frequently, the luminosity of the Halpha emission line, that of the forbidden line [OII]lambda3727 and that of the far ultraviolet continuum (UV) are used with low, intermediate and high redshift galaxies respectively.

To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using Halpha, SFR(Halpha), [OII]lambda3727Å, SFR(OII), UV, SFR(UV) and FIR, SFR(FIR) luminosities of a sample comprising the 31 nearby star forming galaxies having high quality photometric data in the UV, optical and FIR.

We review the different "standard" methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(Halpha) and SFR(FIR), both SFR(OII) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law.

We show that the excess in the SFR(OII) and SFR(UV) is mainly due to an overestimate of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(OII) and SFR(UV) in line with the SFR(FIR) and simultaneously reduces the internal scatter of the SFR estimations.

Based on these results we have derived "unbiased" SFR expressions for the SFR(UV), SFR(OII) and SFR(Halpha). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys. The main results are that the use of the unbiased SFR estimators brings into agreement the results of all surveys. Particularly important is the agreement achieved for the SFR derived from the FIR/mm and optical/UV surveys. The "unbiased" star formation history of the Universe shows a steep rise in the SFR from z = 0 to z = 1 with SFR propto (1 + z)4.5 followed by a decline for z > 2 where SFR propto (1 + z)-1.5. Galaxy formation models tend to have a much flatter slope from z = 0 to z = 1.

Key words: Stars: formation, Galaxies: star forming, Galaxies: HII, Galaxies: evolution

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* Visiting Fellow at IoA, UK

Visiting Professor at INAOE, Mexico