* Astronomy

Title: Space Warps: I. Crowd-sourcing the Discovery of Gravitational Lenses
Author: Phil Marshall, Aprajita Verma, Anupreeta More, Chris Davis, Surhud More, Amit Kapadia, Michael Parrish, Chris Snyder, Julianne Wilcox, Elisabeth Baeten, Christine Macmillan, Claude Cornen, Michael Baumer, Edwin Simpson, Chris Lintott, David Miller, Edward Paget, Robert Simpson, Arfon Smith, Rafael Kueng, Prasenjit Saha, Tom Collett, Matthias Tecza

We describe Space Warps, a novel gravitational lens discovery service that yields samples of high purity and completeness through crowd-sourced visual inspection. Carefully produced colour composite images are displayed to volunteers via a classi- fication interface which records their estimates of the positions of candidate lensed features. Simulated lenses, and expert-classified images which lack lenses, are inserted into the image stream at random intervals; this training set is used to give the vol- unteers feedback on their performance, as well as to calibrate it in order to allow dynamical updates to the probability of any image they classify to contain a lens. Low probability systems are retired from the site periodically, concentrating the sample towards a set of candidates. Having divided 160 square degrees of Canada-France- Hawaii Telescope Legacy Survey (CFHTLS) imaging into some 430,000 overlapping 84 by 84 arcsecond tiles and displaying them on the site, we were joined by around 37,000 volunteers who contributed 11 million image classifications over the course of 8 months. The sample was reduced to 3368 Stage I candidates; these were then refined to yield a sample that we expect to be over 90% complete and 30% pure. We comment on the scalability of the Space Warps system to the wide field survey era, based on our finding that searches of 10⁵ images can be performed by a crowd of 10⁵ volunteers in 6 days.

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