The Bispectrum of IRAS Redshift Catalogs

We compute the bispectrum for the galaxy distribution in the IRAS QDOT, 2 Jy, and 1.2 Jy redshift catalogs for wavenumbers 0.05<=k<=0.2 h Mpc^-1 and compare the results with predictions from gravitational instability in perturbation theory. Taking into account redshift-space distortions, nonlinear evolution, the survey selection function, and discreteness and finite-volume effects, all three catalogs show evidence for the dependence of the bispectrum on the configuration shape predicted by gravitational instability. Assuming Gaussian initial conditions and local biasing parameterized by linear and nonlinear bias parameters b₁ and b₂, a likelihood analysis yields 1/b₁=1.32^+0.36_-0.58, 1.15^+0.39_-0.39 and b₂/b²₁=-0.57^+0.45_{- 0.30}, -0.50^+0.31_-0.51 for the 2 and 1.2 Jy samples, respectively. This implies that IRAS galaxies trace dark matter increasingly weakly as the density contrast increases, consistent with their being underrepresented in clusters. In a model with χ² non-Gaussian initial conditions, the bispectrum displays an amplitude and scale dependence different from that found in the Gaussian case; if IRAS galaxies do not have bias b₁>1 at large scales, χ² non-Gaussian initial conditions are ruled out at the 95% confidence level. The IRAS data do not distinguish between Lagrangian and Eulerian local bias.