Detection and characterisation of dark-matter halos by gravitational shear and flexion; constraints on the non-linear cosmic structure growth

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Gravitational lensing is among the most powerful tools to study dark structures in the Universe and their growth over cosmological epochs. Over the past years, the precise measurement of cosmic shear has developed impressively quickly into one of the pillars supporting the cosmological standard model. The shear traces the gravitational tidal field, i.e. second derivatives of the gravitational potential. Important information must also be contained in its third derivatives, which can conveniently be combined to form the two-component flexion. Despite its higher order, flexion has comparable or superior signal-to-noise properties compared to the shear. Its sensitivity to smaller scales in the matter distribution makes it a highly interesting and welcome tool to detect dark-matter halos and to study their internal composition, and thus to test generic predictions of the CDM cosmogony which are still controversial. We have continued our study of the shapelet technique for measuring flexion, removed several of its limitations and problems, developed a new, reliable implementation and demonstrated that it has now reached similar reliability as alternative shear measurements. We here propose to continue this programme towards a systematic calibration of shapelet measurements of the flexion, applications of flexion measurements to the detailed reconstruction of galaxy clusters, the detection of dark-matter halos and their substructures, and the study of nonlinear cosmic structure growth.