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Title: Selection effects in optical quasar surveys
Author: Keable, Crispin Julian
ISNI:       0000 0001 3595 980X
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1988
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An investigation into the selection effects of optical quasar surveys is described. This is accomplished via the accumlation and comparison of optical quasar surveys o f d ifferen t types, which are then used to generate a su rvey free from selection effects. A visual search and an automatic computerised search for quasars using U.K. Schmidt plates were already available in a single high galactic latitude field of 23 degrees. An automatic search of a deep C.F.H.T. grens plate over 1 degree^ in the centre of the field was undertaken. Slit spectroscopic observations were made of 26 candidates from this survey. These data allow conclusions to be drawn regardin g the grens as a survey instrument. An automatic photometric su rvey was also undertaken, using direct plates in the five wavebands U, J, V, R, and I from the U.K. Schmidt telescope. CCD frames were obtained in each of the wavebands from various sources, to allow calibration o f data from the Schmidt plates. This has allowed the construction o f a conventional ultraviolet excess sample, as well as a less conventional colour space density search generated sample. Locating the various low dispersion spectroscopic su rvey candidates amongst the photometric su rvey data has brought to light various aspects o f these surveys: in particular, the eye discovers a smaller surface density of candidates at most magnitudes. Secondly, the automatic prism plate search has significant progressive object losses b righ ter than the plate limit, which are subject to complex selection effects. Large numbers of galactic stars are mistaken for quasars because of peculiarities in their spectra seen at low dispersion. Also prism dispersion variations force a su rvey limit brighter than the plate limit. Colour synthesis from the prism plate works well at brigh ter magnitudes, unless spectra are saturated. The method is also subject to limitations due to poor spectrum signal to noise at fainter magnitudes. Spectroscopy indicates that the automatic grens survey is complete, except for overlapped spectra. This is in part due to the linear dispersion of the transmission grating. The present survey is however subject to considerable contamination:- the reasons for this are examined and discussed. Conclusions drawn from examination of the grens sample colours are limited because the grens su rvey goes to a much fainter magnitude, over a limited area. The photometric survey readily identifies objects which have peculiar colours, but a detailed examination of the properties of these objects must await further work. Finally, a synthetic survey has been constructed, using all of the information available for each object. Redshift estimates were possible for about 30% of the 130 candidates - the higher redshift objects. Examination of the survey indicates that it is largely free from selection effects below a redshift of z = 3.4.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Astrophysics