How to suppress the shot noise in galaxy surveys
  Galaxy surveys are one of the most powerful means to extract the cosmological
information and for a given volume the attainable precision is determined by
the galaxy shot noise sigma_n^2 relative to the power spectrum P. It is
generally assumed that shot noise is white and given by the inverse of the
number density n. In this paper we argue one may be able to considerably
improve upon this: in the halo picture of cosmological structure all of the
dark matter is in halos of varying mass and galaxies are formed inside these
halos, but for the dark matter mass and momentum conservation guarantee that
nonlinear effects cannot develop a white noise in the dark matter power
spectrum on large scales. This suggests that with a suitable weighting a
similar effect may be achieved for galaxies, suppressing their shot noise. We
explore this idea with N-body simulations by weighting central halo galaxies by
halo mass and find that the resulting shot noise can be reduced dramatically
relative to expectations, with a 10-30 suppression at the highest number
density of n=4*10^-3 (Mpc/h)^3 resolved in our simulations. For specific
applications other weighting schemes may achieve even better results and for
n=3*10^-4(Mpc/h)^3 we can reduce sigma_n^2/P by up to a factor of 10 relative
to uniform weighting. These results open up new opportunities to extract
cosmological information in galaxy surveys, such as the recently proposed
multi-tracer approach to cancel sampling variance, and may have important
consequences for the planning of future redshift surveys. Taking full advantage
of these findings may require better understanding of galaxy formation process
to develop accurate tracers of the halo mass.
