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Finding New High-Redshift Quasars by Asking the Neighbours

Quasars with 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-metre 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 alpha 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.