Gravity’s Smoking Gun?

We present a new constraint on the biased galaxy formation picture. Gravitational instability theory predicts that the two-point mass density correlation function, ξ(r), has an inflection point at the separation r = r0, corresponding to the boundary between the linear and nonlinear regime of clustering, ξ ≃ 1. We show how this feature can be used to constrain the biasing parameter b2 ≡ ξg(r)/ξ(r) on scales r ≃ r0, where ξg is the galaxy-galaxy correlation function, which is allowed to differ from ξ. We apply our method to real data: the ξg(r), estimated from the Automatic Plate Measuring (APM) galaxy survey. Our results suggest that the APM galaxies trace the mass at separations r ≳ 5 h-1 Mpc, where h is the Hubble constant in units of 100 km s-1 Mpc-1. The present results agree with earlier studies, based on comparing higher order correlations in the APM with weakly nonlinear perturbation theory. Both approaches constrain the b factor to be within 20% of unity. If the existence of the feature that we identified in the APM ξg(r)—the inflection point near ξg = 1—is confirmed by more accurate surveys, we may have discovered gravity's smoking gun: the long-awaited "shoulder" in ξ, predicted by Gott and Rees 25 years ago.