The Quasar Redshift Limit [chapter]

Patrick S. Osmer
1983 Early Evolution of the Universe and its Present Structure  
The concept of a redshift limit for quasars was first mentioned a decade ago by Schmidt (1970 Schmidt ( , 1972 and Sandage (1972) who independently realized that faint, high redshift quasars were not being found in the numbers expected from the redshift and magnitude distribu tion of bright quasars. Subsequently the limit moved out in redshift to z^3.5 from z^2.5 after the discovery of OQ172 *(z=3. 53, Wampler et al. 1973) and numerous other quasars with z > 3. It should be said at the outset
more » ... aid at the outset that the limit concept is in need of definition. While it would be most interesting if an absolute limit existed, i.e., no^ quasars beyond a certain redshift, in reality the limit value is more likely to be the redshift at which the quasar space density turns down significantly. What is the case for a quasar redshift limit? On purely observational grounds there has been great difficulty in finding quasars with z>3.5. In the nine years since the discovery of OQ172, the number of known quasars has increased by more than a factor of 5, yet only in March of this year was a larger redshift found, PKS 2000-330, with z= 3.78 (Peterson et al. 1982) . In the meantime more than 20 quasars with 3.02.5 because of their loss of a characteristic ultraviolet excess, such concern is no longer warranted. The development and success of the objective-prism technique for finding quasars has shown that a colorindependent approach can be very effective for z > 3 (see review by Smith 1978) . Similarly, the improvement of radio positions means that sources now can be identified only from positional coincidences. Indeed there has probably been a bias in favor of large redshifts in recent work on radio catalogs as a result of researchers looking for neutral or red stellar objects near radio positions. Specific searches for quasars with z>3.5 with the objectiveprism technique by Kron (1980) and Osmer (1982) have failed to find any despite limiting magnitudes that should have been more than faint enough. Osmer addressed the space denity question in particular. Because he did discover 15 emission-line quasars and galaxies with 0.03 <z< 3.36 in his survey, he could make a good estimate of the 35 G. O. Abell and G. Chincarini (eds.), Early Evolution of the Universe and Its Present Structure, 35-38.
doi:10.1007/978-94-009-7220-9_7 fatcat:ydbskkmnyvfwrnnrbpjifn64jy