GALAPAGOS: From Pixels to Parameters

Context. To automate source detection, two-dimensional light-profile S´ ersic modelling and catalogue compilation in large survey applications, we introduce a new code Galapagos, Galaxy Analysis over Large Areas: Parameter Assessment by GALFITting Objects from SExtractor. Aims. Based on a single setup, Galapagos can process a complete set of survey images. It detects sources in the data, estimates a local sky background, cuts postage stamp images for all sources, prepares object masks, performs Sfitting including neighbours and compiles all objects in a final output catalogue. Methods. For the initial source detection Galapagos applies SExtractor, while Galfit is incorporated for modelling Sprofiles. It measures the background sky involved in the S´ ersic fitting by means of a flux growth curve. Galapagos determines postage stamp sizes based on SExtractor shape parameters. In order to obtain precise model parameters Galapagos incorporates a complex sorting mechanism and makes use of modern CPU's multiplexing capabilities. It combines SExtractor and Galfit data in a single out- put table. When incorporating information from overlapping tiles, Galapagos automatically removes multiple entries from identical sources. Galapagos is programmed in the Interactive Data Language, IDL. Results. We test the stability and the ability to properly recover structural parameters extensively with artificial image simulations. Moreover, we apply Galapagos successfully to the STAGES data set. For one-orbit HST data, a single 2.2 GHz CPU processes about 1000 primary sources per 24 hours. Note that Galapagos results depend critically on the user-defined parameter setup. This paper provides useful guidelines to help the user make sensible choices.