What determines large scale galaxy clustering: halo mass or local
  density?
  Using dark matter simulations we show how halo bias is determined by local
density and not by halo mass. This is not totally surprising, as according to
the peak-background split model, local density is the property that constraints
bias at large scales. Massive haloes have a high clustering because they reside
in high density regions. Small haloes can be found in a wide range of
environments which determine their clustering amplitudes differently. This
contradicts the assumption of standard Halo Occupation Distribution (HOD)
models that the bias and occupation of haloes is determined solely by their
mass. We show that the bias of central galaxies from semi-analytic models of
galaxy formation as a function of luminosity and colour is not correctly
predicted by the standard HOD model. Using local density instead of halo mass
the HOD model correctly predicts galaxy bias. These results indicate the need
to include information about local density and not only mass in order to
correctly apply HOD analysis in these galaxy samples. This new model can be
readily applied to observations and has the advantage that the galaxy density
can be directly observed, in contrast with the dark matter halo mass.
