  The measurement of baryon acoustic oscillations (BAO) from a galaxy redshift
survey provides one of the most promising methods for probing dark energy. In
this paper, we clarify the assumptions that go into the forecasts of dark
energy constraints from BAO. We show that assuming a constant nP_{0.2}/G^2(z)
(where P_{0.2} is the real space galaxy power spectrum at k=0.2h/Mpc and
redshift z) gives a good approximation of the observed galaxy number density
expected from a realistic flux-limited galaxy redshift survey. We find that
assuming nP_{0.2}/G^2(z)=10 gives very similar dark energy constraints to
assuming nP_{0.2}=3, but the latter corresponds to a galaxy number density
larger by ~70% at z=2. We show how the Figure-of-Merit (FoM) for constraining
dark energy depends on the assumed galaxy number density, redshift accuracy,
redshift range, survey area, and the systematic errors due to calibration and
uncertainties in the theory of nonlinear evolution and galaxy biasing. We find
that an additive systematic noise of up to 0.4-0.5% per dz=0.1 redshift slice
does not lead to significant decrease in the BAO FoM.
