Stochastic bias and ellipsoidal collapse

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Dr. Sheth will extend his work on the statistics of dark matter halos, to address halos that form in surroundings that are described by an ellipsoidal, rather than a spherical, perturbation of the dark-matter density. The research will be largely analytic, checked by comparison with cosmological N-body simulations. In ellipsoidal collapse, three numbers determine the fate of each halo: the local overdensity of dark matter, and two quantities related to the ellipticity and prolateness of the surrounding distribution of mass. Thus the abundance and the formation history of dark halos depend on the surrounding large-scale structure: the halo population in filaments of the cosmic web should be different from that in sheets and voids. Dr Sheth will calculate how ellipsoidal collapse should affect the relation between 2-point, 3-point and higher-order clustering statistics. The next generation of galaxy surveys will measure these quantities to within a few percent. This work is essential for interpreting those measurements accurately, to estimate the level of fluctuations in the dark-matter density and determine how its distribution differs from that of the luminous galaxies.A graduate student will be trained by participating in this research. Models of the abundance and clustering of dark matter halos are required to interpret measures of galaxy clustering and the baryon acoustic oscillation signal, and hence constrain the equation of state for the dark energy.