Quasars from a complete spectroscopic survey

Abstract

With the advent of multi-fibre spectrographs such as the ‘Two-Degree Field’ (2dF) instrument at the Anglo-Australian Telescope, quasar surveys that are free of any pre-selection of candidates and any biases this implies have become possible for the first time. The first of these is that which is being undertaken as part of the Fornax Spectroscopic Survey, a survey of the area around the Fornax Cluster of galaxies, and aims to obtain the spectra of all objects in the magnitude range 16. 5<bj<19. 7. To date, 3679 objects in the central π-deg2 area have been successfully identified from their spectral characteristics. Of these, 71 are found to be quasars, 61 with redshifts 0. 3<z<2. 2 and 10 with redshifts z>2. 2. Using this complete quasar sample, a new determination of quasar number counts is made, enabling an independent check of existing quasars surveys. Cumulative counts per square degree at a magnitude limit of bj<19. 5 are found to be 11. 5±2. 2 for 0. 3<z<2. 2,2. 22±0. 93 for z>2. 2 and 13. 7±3. 1 for z>0. 3. Given the likely detection of extra quasars in the Fornax survey, we make a more detailed examination of existing quasar selection techniques. First, looking at the use of a stellar criterion, four of the 71 quasars are ‘non-stellar’ on the basis of the automated plate measuring facility (APM)bj classification, however inspection shows all are consistent with stellar, but misclassified due to image confusion. Examining the ultraviolet excess and multicolour selection techniques, for the selection criteria investigated, ultraviolet excess would find 69±6 per cent of our 0. 3<z<2. 2 quasars and only 50−18+14 per cent of our z>2. 2 quasars, while the completeness level for multicolour selection is found to be 90−4+3 per cent for 0. 3<z<2. 2 quasars and 80−12+14 per cent for z>2. 2 quasars. The extra quasars detected by our all-object survey thus have unusually red star-like colours, and this appears to be a result of the continuum shape rather than any emission features. An intrinsic dust extinction model may, at least partly, account for the red colours.