Systematic Surveys for Quasars with the Slitless Spectrum Technique
Objects of High Redshift
Recent results with the slitless spectrum technique are reviewed and the total surface densities from five independent surveys are compared. The observational base for extending the technique to search for quasars with z>3.5 is now large enough that either the apparent cutoff at z = 3.5 will be confirmed in new surveys or quasars with z > 3.5 will be found. I. SCOPE In the previous talk Schmidt has reviewed the distribution in depth of quasars as determined from radio-selected samples and from
... d samples and from optical surveys with the ultraviolet excess technique. Here I discuss recent results from the slitless spectrum technique. This technique, which is best suited for redshifts larger than 1.8, is a natural complement to the ultraviolet excess method, which is best suited for redshifts between 0 and 2.3. The excellent review by Smith (1978) describes in detail the slitless spectrum technique, its application to various problems in quasar research, and the results up to May, 1978. Therefore I shall limit myself to events occurring subsequent to Smith's article. In particular I shall discuss the question of where are the redshift four quasars. The statistical base for investigating the question is now solid enough for a feasible program either to find quasars with z>3.5 or show that their space density is indeed low. Although the recent results have implications for the emission-line and absorption-line problems in quasars, time does not allow them to be discussed here. II. DEFINITION OF THE SLITLESS SPECTRUM TECHNIQUE The problem in carrying out optical surveys for quasars is how to distinguish them from the myriad star images. For example, there may be 100,000 stars on an exposure with the UK Schmidt that contains G.O. Abell and P. J. E. Peebles feds.), Objects of High Redshift, 77-82.