Day 1254

Tuesday.

1705.02420
Reconstructing the gravitational field of the local universe
Desmond, Ferreira, Lavaux, Jasche

Tests of gravity at the galaxy scale are in their infancy.  As a first step to systematically uncovering the gravitational significance of galaxies, map 3 fundamental gravitational variables -- the Newtonian potential, acceleration and curvature -- over the galaxy environments of the local universe to a distance of approximately 200 Mpc.  The method combines the contributions from galaxies in an all-sky redshift survey, haloes from an N-body simulation hosting low-luminosity objects, and linear and quasi-linear modes of the density field.  Use the ranges of these variables to determine the extent to which galaxies expand the scope of generic tests of gravity and are capable of constraining specific classes of model for which they have special significance.  Finally, investigate the improvements afforded by upcoming galaxy surveys.


1705.02629
Testing the accuracy of clustering redshift with simulations
Scottez, Benoit-Lévy, Coupon, Ilbert, Mellier

Explore the accuracy of the clustering-based redshift inference within the MICE2 simulation.  This method uses the spatial clustering of galaxies between a spectroscopic reference sample and an unknown sample.  The goal of this study is to give a preview of the redshift accuracy one can reach with this method.  To do so, first highlight the requirements of this technique in terms of number of objects in both the reference and unknown samples.  Also confirm that this method does not require a representative spectroscopic sample for calibration.  Estimate that a density of spectroscopic objects of 1e-5 arcmin^-2 per redshift bin of width delta_z=0.01 over 9000 deg^2 allows to reach 0.1% accuracy in the man redshift for a galaxy density compatible with next generation of cosmological surveys.  This number is compatible with the density of QSOs in BOSS.  Second, demonstrate the ability to measure individual redshifts for galaxies independently from the photo-z procedure.  The resulting individual clustering redshifts have a bias =-0.001, an outlier fraction of eta=2.57% and a scatter of sigma=0.027 to i<25.  The advantage of this procedure is 3-fold: i) it allows the use of clustering redshifts for any field in astronomy, ii) it allows the possibility to combine photometric and clustering based z to get an improved redshift estimation, iii) it allows the use of cluster-z to define tomographic bins for weak lensing.  Finally, explore this last option and build 5 clustering z selected tomographic bins from redshift 0.2 to1.  Found a bias on the man redshift estimate of 0.002 per bin.