  We study the sources of biases and systematics in the derivation of galaxy
properties of observational studies, focusing on stellar masses, star formation
rates, gas/stellar metallicities, stellar ages and magnitudes/colors. We use
hydrodynamical cosmological simulations of galaxy formation, for which the real
quantities are known, and apply observational techniques to derive the
observables. We also make an analysis of biases that are relevant for a proper
comparison between simulations and observations. For our study, we post-process
the simulation outputs to calculate the galaxies' spectral energy distributions
(SEDs) using Stellar Population Synthesis models and also generating the
fully-consistent far UV-submillimeter wavelength SEDs with the radiative
transfer code SUNRISE. We compared the direct results of simulations with the
observationally-derived quantities obtained in various ways, and found that
systematic differences in all studied galaxy properties appear, which are
caused by: (1) purely observational biases, (2) the use of
mass-weighted/luminosity-weighted quantities, with preferential sampling of
more massive/luminous regions, (3) the different ways to construct the template
of models when a fit to the spectra is performed, and (4) variations due to the
use of different calibrations, most notably in the cases of the gas
metallicities and star formation rates. Our results show that large differences
can appear depending on the technique used to derive galaxy properties.
Understanding these differences is of primary importance both for simulators,
to allow a better judgement on similarities/differences with observations, and
for observers, to allow a proper interpretation of the data.
