A WideField Multicolor Survey for HighRedshift Quasars, Z >= 2.2. III. The Luminosity Function
Abstract
In two previous papers we presented the aims, methods, and spectroscopic results, and computed the completeness of a sixband multicolor survey for highredshift quasars, covering an effective area of 43.0 deg^2^, to m_or_ = 20.0. In this paper the complete sample of 86 quasars 2.2 <= z < 4.5 is combined with other published samples to determine the continuum and line luminosity functions, {PHI}_C_, {PHI}_L_ over the redshift range 2.0 <= z < 4.5. The estimate of the completeness of our multicolor sample has been revised following the development of a more sophisticated treatment of the line and continuum absorption from intervening matter, and the incorporation of the effects of quasar variability. Appendices describe the new model procedures. The luminosity function calculation also accounts for the range of line strength and continuum spectral index, as well as the photometric errors. The results are presented in terms of two magnitudes, M_C(1216)_ and M_L(1216)_, which are logarithmic measures of the absolute continuum flux under the Lyα/N V emission line, and of the Lyα/N V line luminosity. Additionally the intrinsic distribution of continuum spectral indices α is derived. This completes the parameterization of the restframe UV spectralenergy distributions of the quasar population, in the fourdimensional space with axes M_C(1216)_, M_L(1216)_, α, z. The nature of the evolution of the continuum luminosity function changes near the midpoint of the redshift interval, and no simple model provides a satisfactory fit over the entire redshift range 2.0 <= z < 4.5. For redshifts z < 3.5, noevolution luminosity functions, and luminosity functions of evolving single powerlaw form are inadequate. The best fit is obtained with functions of the Schechter or double power law type, in which only the bright end evolves, to brighter magnitudes at higher redshift. The evolution ceases near z = 3.3, with a marked decline in space density beyond. Comparison of the number of survey quasars in the redshift range 3.5 <= z < 4.5 against the number expected if there is no decline beyond z = 3.3 shows a shortfall by a factor 6.1(6.5), for q_0_ = 0.1(0.5). At the 95% confidence level there is a decline in the space density of quasars M_C_ <  25.6(<  24.5) between redshifts z = 3.3 and 4.0 by a factor greater than 3.1(> 3.3). These limits include an allowance for the uncertainty in the estimate of the luminosity function at z = 3.3. Our results are very similar to those obtained by Osmer a decade ago. A similar comparison against the preliminary results of the brighter survey of Irwin et al. indicates that the amount of the decline is approximately independent of absolute magnitude, brighter than the above limits. The observed decline in the space density of quasars beyond z = 3.3 is compatible with a constant true space density and obscuration by dust in intervening damped Lyα systems only if the parameters of the obscuration (dust to gas ratio, etc.) are at the upper end of current observational limits. The line luminosity function at z = 3 and 4 is computed, to compare against the forthcoming results of the widefield grism survey of Schmidt et al.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 1994
 DOI:
 10.1086/173660
 Bibcode:
 1994ApJ...421..412W
 Keywords:

 Cosmology;
 Quasars;
 Red Shift;
 Sky Surveys (Astronomy);
 Stellar Color;
 Stellar Luminosity;
 Stellar Mass;
 Stellar Models;
 Galactic Evolution;
 Schmidt Telescopes;
 Stellar Evolution;
 Astronomy;
 GALAXIES: LUMINOSITY FUNCTION;
 MASS FUNCTION;
 GALAXIES: QUASARS: GENERAL;
 SURVEYS