The propagation of uncertainties in stellar population synthesis
  modeling II: The challenge of comparing galaxy evolution models to
  observations
  Models for the formation and evolution of galaxies readily predict physical
properties such as the star formation rates, metal enrichment histories, and,
increasingly, gas and dust content of synthetic galaxies. Such predictions are
frequently compared to the spectral energy distributions of observed galaxies
via the stellar population synthesis (SPS) technique. Substantial uncertainties
in SPS exist, and yet their relevance to the task of comparing galaxy evolution
models to observations has received little attention. In the present work we
begin to address this issue by investigating the importance of uncertainties in
stellar evolution, the initial stellar mass function (IMF), and dust and
interstellar medium (ISM) properties on the translation from models to
observations. We demonstrate that these uncertainties translate into
substantial uncertainties in the ultraviolet, optical, and near-infrared colors
of synthetic galaxies. Aspects that carry significant uncertainties include the
logarithmic slope of the IMF above 1 Msun, dust attenuation law, molecular
cloud disruption timescale, clumpiness of the ISM, fraction of unobscured
starlight, and treatment of advanced stages of stellar evolution including blue
stragglers, the horizontal branch, and the thermally-pulsating asymptotic giant
branch. The interpretation of the resulting uncertainties in the derived colors
is highly non-trivial because many of the uncertainties are likely systematic,
and possibly correlated with the physical properties of galaxies. We therefore
urge caution when comparing models to observations.
