Space Warps– II. New gravitational lens candidates from the CFHTLS discovered through citizen science

Citations Over TimeTop 10% of 2015 papers

Abstract

We report the discovery of 29 promising (and 59 total) new lens candidates from the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS) based on about 11 million classifications performed by citizen scientists as part of the first S<scp>pace</scp> W<scp>arps</scp> lens search. The goal of the blind lens search was to identify lens candidates missed by robots (the <scp>ringfinder</scp> on galaxy scales and <scp>arcfinder</scp> on group/cluster scales) which had been previously used to mine the CFHTLS for lenses. We compare some properties of the samples detected by these algorithms to the S<scp>pace</scp> W<scp>arps</scp> sample and find them to be broadly similar. The image separation distribution calculated from the S<scp>pace</scp> W<scp>arps</scp> sample shows that previous constraints on the average density profile of lens galaxies are robust. S<scp>pace</scp> W<scp>arps</scp> recovers about 65 per cent of known lenses, while the new candidates show a richer variety compared to those found by the two robots. This detection rate could be increased to 80 per cent by only using classifications performed by expert volunteers (albeit at the cost of a lower purity), indicating that the training and performance calibration of the citizen scientists is very important for the success of S<scp>pace</scp> W<scp>arps</scp>. In this work we present the SIMCT pipeline, used for generating <it>in situ</it> a sample of realistic simulated lensed images. This training sample, along with the false positives identified during the search, has a legacy value for testing future lens-finding algorithms. We make the pipeline and the training set publicly available.

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