Galaxy Lookback Evolution Models -- a Comparison with Magneticum
  Cosmological Simulations and Observations
  We construct empirical models of star-forming galaxy evolution assuming that
individual galaxies evolve along well-known scaling relations between stellar
mass, gas mass and star formation rate following a simple description of
chemical evolution. We test these models by a comparison with observations and
with detailed Magneticum high resolution hydrodynamic cosmological simulations.
Galaxy star formation rates, stellar masses, gas masses, ages, interstellar
medium and stellar metallicities are compared. It is found that these simple
lookback models capture many of the crucial aspects of galaxy evolution
reasonably well. Their key assumption of a redshift dependent power law
relationship between galaxy interstellar medium gas mass and stellar mass is in
agreement with the outcome of the complex Magneticum simulations. Star
formation rates decline towards lower redshift not because galaxies are running
out of gas, but because the fraction of the cold ISM gas, which is capable of
producing stars, becomes significantly smaller. Gas accretion rates in both
model approaches are of the same order of magnitude. Metallicity in the
Magneticum simulations increases with the ratio of stellar mass to gas mass as
predicted by the lookback models. The mass metallicity relationships agree and
the star formation rate dependence of these relationships is also reproduced.
We conclude that these simple models provide a powerful tool for constraining
and interpreting more complex models based on cosmological simulations and for
population synthesis studies analyzing integrated spectra of stellar
populations.
