The SOAR Gravitational Arc Survey – I. Survey overview and photometric catalogues

Abstract

We present the first results of the SOAR (Southern Astrophysical Research) Gravitational Arc Survey (SOGRAS). The survey imaged 47 clusters in two redshift intervals centred at z = 0.27 and z = 0.55, targeting the richest clusters in each interval. Images were obtained in the g¹ , r¹ and i¹ bands using the SOAR Optical Imager (SOI), with a median seeing of 0.83, 0.76 and 0.71 arcsec, respectively, in these filters. Most of the survey clusters are located within the Sloan Digital Sky Survey (SDSS) Stripe 82 region and all of them are in the SDSS footprint. Photometric calibration was therefore performed using SDSS stars located in our SOI fields. We reached for galaxies in all fields the detection limits of g ∼ 23.5, r ∼ 23 and i ∼ 22.5 for a signal-to-noise ratio (S/N) = 3. As a byproduct of the image processing, we generated a source catalogue with 19 760 entries, the vast majority of which are galaxies,wherewe list their positions, magnitudes and shape parameters. We compared our galaxy shape measurements to those of local galaxies and concluded that they were not strongly affected by seeing. From the catalogue data, we are able to identify a red sequence of galaxies in most clusters in the lower z range. We found 16 gravitational arc candidates around eight clusters in our sample. They tend to be bluer than the central galaxies in the lensing cluster. A preliminary analysis indicates that ∼10 per cent of the clusters have arcs around them, with a possible indication of a larger efficiency associated with the high-z systems when compared to the low-z ones. Deeper follow-up images with Gemini strengthen the case for the strong lensing nature of the candidates found in this survey.