Comparison of star formation rates from Ha and infrared luminosity as seen by Herschel

We empirically MD test the relation between the SFR(LIR) derived from the infrared luminosity, LIR, and the SFR(Ha) derived from the Ha emission line luminosity using simple conversion relations. We use a sample of 474 galaxies at z = 0.060.46 with both Ha detection [from 20k redshift Cosmological Evolution (zCOSMOS) survey] and new far-IR Herschel data (100 and 160 mu m). We derive SFR(Ha) from the Ha extinction corrected emission line luminosity. We find a very clear trend between E(B - V) and LIR that allows us to estimate extinction values for each galaxy even if the H beta emission line measurement is not reliable. We calculate the LIR by integrating from 8 up to 1000 mu m the spectral energy distribution (SED) that is best fitting our data. We compare the SFR(Ha) with the SFR(LIR). We find a very good agreement between the two star formation rate (SFR) estimates, with a slope of m = 1.01 +/- 0.03 in the log SFR(LIR) versus log SFR(Ha) diagram, a normalization constant of a = -0.08 +/- 0.03 and a dispersion of s = 0.28 dex. We study the effect of some intrinsic properties of the galaxies in the SFR(LIR)SFR(Ha) relation, such as the redshift, the mass, the specific star formation rate (SSFR) or the metallicity. The metallicity is the parameter that affects most the SFR comparison. The mean ratio of the two SFR estimators log[SFR(LIR)/SFR(Ha)] varies by similar to 0.6 dex from metal-poor to metal-rich galaxies [8.1 < log?(O/H) + 12 < 9.2]. This effect is consistent with the prediction of a theoretical model for the dust evolution in spiral galaxies. Considering different morphological types, we find a very good agreement between the two SFR indicators for the Sa, Sb and Sc morphologically classified galaxies, both in slope and in normalization. For the Sd, irregular sample (Sd/Irr), the formal best-fitting slope becomes much steeper (m = 1.62 +/- 0.43), but it is still consistent with 1 at the 1.5s level, because of the reduced statistics of this sub-sample.