Finding new high-redshift quasars by asking the neighbours

Abstract

Quasarswith a high redshift (z) are important to understand the evolution processes of galaxies in the early Universe. However, only a few of these distant objects are known to this date. The costs of building and operating a 10-m class telescope limit the number of facilities and, thus, the available observation time. Therefore, an efficient selection of candidates is mandatory. This paper presents a new approach to select quasar candidates with high redshift (z > 4.8) based on photometric catalogues. We have chosen to use the z > 4.8 limit for our approach because the dominant Lyman a emission line of a quasar can only be found in the Sloan i- and z-band filters. As part of the candidate selection approach, a photometric redshift estimator is presented, too. Three of the 120 000 generated candidates have been spectroscopically analysed in follow-up observations and a new z = 5.0 quasar was found. This result is consistent with the estimated detection ratio of about 50 per cent and we expect 60 000 high-redshift quasars to be part of our candidate sample. The created candidates are available for download at MNRAS or at http://www.astro.rub.de/polsterer/quasar-candidates.csv. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.