Testing anthropic reasoning for the cosmological constant with a
  realistic galaxy formation model
  The anthropic principle is one of the possible explanations for the
cosmological constant ($\Lambda$) problem. In previous studies, a dark halo
mass threshold comparable with our Galaxy must be assumed in galaxy formation
to get a reasonably large probability of finding the observed small value,
$P(<$$\Lambda_{\rm obs})$, though stars are found in much smaller galaxies as
well. Here we examine the anthropic argument by using a semi-analytic model of
cosmological galaxy formation, which can reproduce many observations such as
galaxy luminosity functions. We calculate the probability distribution of
$\Lambda$ by running the model code for a wide range of $\Lambda$, while other
cosmological parameters and model parameters for baryonic processes of galaxy
formation are kept constant. Assuming that the prior probability distribution
is flat per unit $\Lambda$, and that the number of observers is proportional to
stellar mass, we find $P(<$$\Lambda_{\rm obs}) = 6.7 \%$ without introducing
any galaxy mass threshold. We also investigate the effect of metallicity; we
find $P(<$$\Lambda_{\rm obs}) = 9.0 \%$ if observers exist only in galaxies
whose metallicity is higher than the solar abundance. If the number of
observers is proportional to metallicity, we find $P(<$$\Lambda_{\rm obs}) =
9.7 \%$. Since these probabilities are not extremely small, we conclude that
the anthropic argument is a viable explanation, if the value of $\Lambda$
observed in our universe is determined by a probability distribution.
