Galaxy formation on the largest scales: The impact of astrophysics on
  the BAO peak
  We investigate the effects of galaxy formation on the baryonic acoustic
oscillations (BAO) peak by applying semi-analytic modelling techniques to the
Millennium-XXL, a $3 \times 10^{11}$ particle N-body simulation of similar
volume to the future EUCLID survey. Our approach explicitly incorporates the
effects of tidal fields and stochasticity on halo formation, as well as the
presence of velocity bias, spatially correlated merger histories, and the
connection of all these with the observable and physical properties of
galaxies. We measure significant deviations in the shape of the BAO peak from
the expectations of a linear bias model built on top of the nonlinear dark
matter distribution. We find that the galaxy correlation function shows an
excess close to the maximum of the BAO peak ($r\sim110 Mpc/h$) and a deficit at
$r\sim90 Mpc/h$. Depending on the redshift, selection criteria and number
density of the galaxy samples, these bias distortions can be up to 5% in
amplitude. They are, however, largely absorbed by marginalization over nuisance
parameters in current analytical modelling of the BAO peak in configuration
space, in particular into the parameter that controls the broadening due to
nonlinear evolution. As a result, the galaxy formation effects detected here
are unlikely to bias the high-precision measurements planned by the upcoming
generation of wide-field galaxy surveys.
