Intrinsic galaxy shapes and alignments - I. Measuring and modelling COSMOS intrinsic galaxy ellipticities

The statistical properties of the ellipticities of galaxy images depend on how galaxies form and evolve, and therefore constrain models of galaxy morphology, which are key to the removal of the intrinsic alignment contamination of cosmological weak lensing surveys, as well as to the calibration of weak lensing shape measurements. We construct such models based on the halo properties of the Millennium Simulation and confront them with a sample of 90 000 galaxies from the COSMOS Survey, covering three decades in luminosity and redshifts out to z = 2. The ellipticity measurements are corrected for effects of point spread function smearing, spurious image distortions and measurement noise. Dividing galaxies into early, late and irregular types, we find that early-type galaxies have up to a factor of 2 lower intrinsic ellipticity dispersion than late-type galaxies. None of the samples shows evidence for redshift evolution, while the ellipticity dispersion for late-type galaxies scales strongly with absolute magnitude at the bright end. The simulation-based models reproduce the main characteristics of the intrinsic ellipticity distributions although which model fares best depends on the selection criteria of the galaxy sample. We observe fewer close-to-circular late-type galaxy images in COSMOS than expected for a sample of randomly oriented circular thick discs and discuss possible explanations for this deficit.