Galaxies come in a variety of shapes. Nevertheless, in a simplified view of galaxy formation, one can classify them either as spirals or ellipticals. However, the orientation and ellipticity of these spirals and ellipticals is not random; it depends on their formation history and their environment. In particular, tidal forces from the large-scale structure of the Universe (see figure below) play a role in determining correlations between ellipticity, orientation and the underlying (dark) matter density field. Such correlations are referred to as ‘intrinsic alignments’.
On the other hand, future imaging surveys from the Euclid satellite and the Large Synoptic Survey Telescope, aim to measure per cent level perturbations on galaxy ellipticities caused by a different effect: the deviation of photons from their original path due to the gravity of these same large-scale structures. This ‘gravitational lensing’ effect constitutes a powerful probe of the nature of the mysterious dark energy that drives the accelerated expansion of our Universe.
Therefore, the quest for improving our understanding of intrinsic alignments to avoid them contaminating dark energy observables is paramount. Thanks to the DiRAC facility, our group (Horizon-UK) lead an analysis of unprecedented statistical scope and detail of galaxy shape and spin alignments, mining the huge dataset from the state-of-the-art hydrodynamic cosmological simulation Horizon-AGN. These pioneering results are reported in Chisari et al, 2015, MNRAS, 454, 2736.