A survey for quasars with redshifts greater than four.

Results from a survey to detect quasars at redshifts greater than four utilizing CCD imaging on a Schmidt telescope are presented. Gunn gri filters were used to isolate quasars in the redshift range $4<z<5$ based on the location of redshifted emission lines, especially Lyman alpha, and the strong drop in the continuum blueward of the redshifted Lyman alpha emission line. The $g-r$ and $r-i$ colors of some quasars in this range stand out sufficiently on color-color diagrams so that they can be isolated from the stellar loci. Preliminary observations were obtained with a 1024 x 1024 Thomson CCD, with a 0.5$\sp\circ$ field of view, on the Cerro-Tololo Curtis Schmidt Telescope. The technique was successfully tested by the recovery of two known high redshift quasars. Then 2.5 deg$\sp2$ of survey area were obtained in Fall 1994, after which the Thomson CCD was replaced with a 2048 x 2048 STIS CCD, increasing the field of view to 1.1 deg$\sp2.$ Observations were made with the new larger CCD during Spring 1995, Fall 1995, and Fall 1996, making the total area surveyed 22 deg$\sp2.$ The exposures for 17.5 deg$\sp2$ of the survey area were three hours each in gri, with magnitude limits near 22.5, and the exposures for the remaining 4.5 deg$\sp2$ were 30 minutes each in duration. Follow-up spectroscopic confirmation was performed with the Wisconsin-Indiana-Yale-N.O.A.O. (WIYN) 3.5 meter telescope, using its multifiber positioner Hydra. This allowed up to 98 spectra to be taken at once. Candidates from approximately 10 deg$\sp2$ were observed spectroscopically, and yielded two confirmed quasars, one with $z=4.05$ and one with $z=2.8,$ the latter quasar having been discovered by its extreme variability. There were also three other possible quasars and seven carbon stars. Most of the remainder of the candidates proved to be K and M stars. The results, indicating one quasar with $4<z