Comparison of star formation rates from Halpha and infrared luminosities
  as seen by Herschel
  We empirically 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 = 0.06 - 0.46 with both Ha detection (from 20k 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 to estimate extinction values for each
galaxy even if the Ha emission line measurement is not reliable. We calculate
the LIR by integrating from 8 up to 1000 {\mu}m the SED that is best fitting
our data. We compare SFR(Ha) with the SFR(LIR). We find a very good agreement
between the two SFR estimates, with a slope of m = 1.01 \pm 0.03 in the
SFR(LIR) vs SFR(Ha) diagram, a normalization constant of a = -0.08 \pm 0.03 and
a dispersion of sigma = 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 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 approx. 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 normalization. For the Sd, irregular
sample (Sd/Irr), the formal best-fit slope becomes much steeper (m = 1.62 \pm
0.43), but it is still consistent with 1 at the 1.5 sigma level, because of the
reduced statistics of this sub-sample.
