Reddening-independent quasar selection from a wide-field optical and near-infrared imaging survey

Abstract

We combine deep, wide-field near-infrared (near-IR) and optical imaging to demonstrate a reddening-independent quasar selection technique based on identifying outliers in the (g -z)/(z - H) colour diagram. In three fields covering a total of ≈0.7 deg2 to a depth of mH ∼ 18, we identified 68 quasar candidates. Follow-up spectroscopy for 32 objects from this candidate list confirmed 22 quasars (0.86 < z < 2.66), five with significant IR excesses. Two of eight quasars from a subsample with U-band observations do not exhibit UVX colours. From these preliminary results, we suggest that this combined optical and near-IR selection technique has a high selection efficiency (>65 per cent success rate), a high surface density of candidates and is relatively independent of reddening. We discuss the implications for star-galaxy separation for IR base surveys for quasars. We provide the coordinate list and follow-up spectroscopy for the sample of 22 confirmed quasars.