A slitless spectroscopic survey for quasars near quasars

We present the results of the “Quasars near Quasars” (QNQ) survey, a CCD-based slitless spectroscopic survey for faint V <∼ 22 quasars at 1.7 <∼ z <∼ 3.6 on 18 26.2 × 33.5 fields centred on bright quasars at 2.76 < z < 4.69. In total 169 quasar candidates with emission lines were selected from the extracted flux-calibrated spectra on the basis of well-defined automatic selection criteria followed by visual inspection and verification. With follow-up spectroscopy of 81 candidates that were likely to reside at z > 1.7 we were able to confirm 80 new quasars at 0.580 ≤ z ≤ 3.586 on 16 of our fields. 64 of the newly discovered quasars are located at z > 1.7. The overall high success rate implies that most of the remaining 88 candidates are quasars as well, although the majority of them likely resides at z < 1.7 on the basis of the observed line shapes and strengths. Due to the insufficient depth of the input source catalogues needed for extraction of the slitless spectra our survey is not well defined in terms of limiting magnitude for faint 2.5 <∼ z <∼ 3.6 quasars whose Lyα emission is detectable well beyond V = 22, albeit at a continuum S/N <∼ 1. While not useful for characterising the evolving space density of quasars, our sample provides many new closely spaced quasar sightlines around intensely studied quasars for further investigations on the three-dimensional distribution of the intergalactic medium.