Ultraviolet to infrared emission of z>1 galaxies: Can we derive reliable
  star formation rates and stellar masses?
  We seek to derive star formation rates (SFR) and stellar masses (M_star) in
distant galaxies and to quantify the main uncertainties affecting their
measurement. We explore the impact of the assumptions made in their derivation
with standard calibrations or through a fitting process, as well as the impact
of the available data, focusing on the role of IR emission originating from
dust. We build a sample of galaxies with z>1, all observed from the UV to the
IR (rest frame). The data are fitted with the code CIGALE, which is also used
to build and analyse a catalogue of mock galaxies. Models with different SFHs
are introduced. We define different set of data, with or without a good
sampling of the UV range, NIR, and thermal IR data. The impact of these
different cases on the determination of M_star and SFR are analysed.
Exponentially decreasing models with a redshift formation of the stellar
population z ~8 cannot fit the data correctly. The other models fit the data
correctly at the price of unrealistically young ages when the age of the single
stellar population is taken to be a free parameter. The best fits are obtained
with two stellar populations. As long as one measurement of the dust emission
continuum is available, SFR are robustly estimated whatever the chosen model
is, including standard recipes. M_star measurement is more subject to
uncertainty, depending on the chosen model and the presence of NIR data, with
an impact on the SFR-M_star scatter plot. Conversely, when thermal IR data from
dust emission are missing, the uncertainty on SFR measurements largely exceeds
that of stellar mass. Among all physical properties investigated here, the
stellar ages are found to be the most difficult to constrain and this
uncertainty acts as a second parameter in SFR measurements and as the most
important parameter for M_star measurements.
