Measuring the slope of the intrinsic alignment amplitude as a function of mass using simulation and real data

Tidal gravitational forces can modify the shape of galaxies and clusters of galaxies, thus correlating their orientation with the surrounding density field. We study the dependence of this phenomenon, known as intrinsic alignment (IA), on the mass of the dark matter haloes that host these bright structures, analysing the Millennium and Millennium-XXL N-body simulations. We closely follow the observational approach, measuring the halo position-halo shape alignment and subsequently dividing out the dependence on halo bias. We derive a theoretical scaling of the IA amplitude with mass in a dark matter universe, and predict a power-law with slope ß_M in the range 1/3 to 1/2, depending on mass scale. We find that the simulation data agree with each other and with the theoretical prediction, with the joint analysis yielding an estimate of ß_M = 0.36+0.01-0.01, and that this result does not depend on redshift or on the details of the halo shape measurement. We repeat the analysis using observational data, obtaining a significantly higher value, ß_M = 0.56+0.05-0.05. We discuss possible reasons for this discrepancy, which could be investigated with large hydrodynamical simulations