Arcfinder: an algorithm for the automatic detection of gravitational arcs

Zentrum für Astronomie, Universität Heidelberg, ITA, Albert-Ueberle-Straße 2, 69117 Heidelberg, Germany e-mail: gseidel@ita.uni-heidelberg.de

Received: 24 July 2006
Accepted: 8 June 2007

Abstract

We present an efficient algorithm designed for and capable of detecting elongated, thin features such as lines and curves in astronomical images, and its application to the automatic detection of gravitational arcs. The algorithm is sufficiently robust to detect such features even if their surface brightness is near the pixel noise in the image, yet the amount of spurious detections is low. The algorithm subdivides the image into a grid of overlapping cells which are iteratively shifted towards a local centre of brightness in their immediate neighbourhood. It then computes the ellipticity for each cell, and combines cells with correlated ellipticities into objects. These are combined to graphs in a next step, which are then further processed to determine properties of the detected objects. We demonstrate the operation and the efficiency of the algorithm applying it to HST images of galaxy clusters known to contain gravitational arcs. The algorithm completes the analysis of an image with 3000  3000 pixels in about 4 s on an ordinary desktop PC. We discuss further applications, the method's remaining problems and possible approaches to their solution.