B. Joachimi, E. Semboloni, P. E. Bett, J. Hartlap, S. Hilbert, H. Hoekstra, P. Schneider, T. Schrabback
The statistical properties of the ellipticities of galaxy images test the paradigm of structure formation and galaxy evolution, and constrain models of galaxy morphology, which are key to the removal of the intrinsic alignment contamination of cosmological weak lensing surveys. We construct such models based on the halo properties of the Millennium Simulation and confront them with a sample of 150,000 galaxies from the COSMOS Survey, covering 4 decades in luminosity and redshifts out to z=2. The ellipticity measurements are corrected for effects of PSF smearing, spurious image distortions, and measurement noise. We find that early-type galaxies have a 25% lower intrinsic ellipticity dispersion than late-type galaxies, which is quantitatively reproduced by our best models. 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 low ellipticity dispersions predicted by models based on reduced inertia tensors of simulated haloes are generally disfavoured by the observations. The fraction of close to circular late-type galaxy images in COSMOS is much lower than expected for a sample of circular inclined thick disks, indicating a substantial fraction of galaxies with irregular morphology.
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http://arxiv.org/abs/1203.6833
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