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The statistical properties of the ellipticities of galaxy images depend onhow galaxies form and evolve, and therefore constrain models of galaxymorphology, which are key to the removal of the intrinsic alignmentcontamination of cosmological weak lensing surveys, as well as to thecalibration of weak lensing shape measurements. We construct such models basedon the halo properties of the Millennium Simulation and confront them with asample of 90,000 galaxies from the COSMOS Survey, covering three decades inluminosity and redshifts out to z=2. The ellipticity measurements are correctedfor effects of point spread function smearing, spurious image distortions, andmeasurement noise. Dividing galaxies into early, late, and irregular types, wefind that early-type galaxies have up to a factor of two lower intrinsicellipticity dispersion than late-type galaxies. None of the samples showsevidence for redshift evolution, while the ellipticity dispersion for late-typegalaxies scales strongly with absolute magnitude at the bright end. Thesimulation-based models reproduce the main characteristics of the intrinsicellipticity distributions although which model fares best depends on theselection criteria of the galaxy sample. We observe fewer close-to-circularlate-type galaxy images in COSMOS than expected for a sample of randomlyoriented circular thick disks and discuss possible explanations for thisdeficit.

Intrinsic galaxy shapes and alignments I: Measuring and modelling COSMOS intrinsic galaxy ellipticities