Automated detection of gravitational arcs

This paper presents a method to identify gravitational arcs or more generally elongated structures in a given image. The method is based on the computation of a local estimator of the elongation. The estimation of the local elongation proceed in two steps: first the local orientation of the structure is computed, then in the next step, a rotation is performed, and the marginal distributions are used to compute the elongation. This procedure allows the computation of the local elongation at each point in the image. Then, using a threshold on the elongation map the elongated structures are identified and re-constructed using connectivity criteria. Finally a catalogue of elongated structures is produced, and the properties of each object are computed, allowing the selection of potential arc candidates. The final selection of the arc candidates is performed by visual inspection of multi-color images of a small number of objects. This method is a general tool that may be applied not only to gravitational arcs, but to all problems related to the mapping and measurement of elongated structures, in an image, or a volume.