We present a measurement of the Type I quasar luminosity function at z = 5 using a large sample of spectroscopically confirmed quasars selected from optical imaging data .
We measure the bright end ( M _ { 1450 } < -26 ) with Sloan Digital Sky Survey ( SDSS ) data covering \sim 6000 ~ { } { deg } ^ { 2 } , then extend to lower luminosities ( M _ { 1450 } < -24 ) with newly discovered , faint z \sim 5 quasars selected from 235 deg ^ { 2 } of deep , coadded imaging in the SDSS Stripe 82 region ( the celestial equator in the Southern Galactic Cap ) .
The faint sample includes 14 quasars with spectra obtained as ancillary science targets in the SDSS-III Baryon Oscillation Spectroscopic Survey ( BOSS ) , and 59 quasars observed at the MMT and Magellan telescopes .
We construct a well-defined sample of 4.7 < z < 5.1 quasars that is highly complete , with 73 spectroscopic identifications out of 92 candidates .
Our color selection method is also highly efficient : of the 73 spectra obtained , 71 are high redshift quasars .
These observations reach below the break in the luminosity function ( M _ { 1450 } ^ { * } \approx - 27 ) .
The bright end slope is steep ( \beta \lesssim - 4 ) , with a constraint of \beta < -3.1 at 95 % confidence .
The break luminosity appears to evolve strongly at high redshift , providing an explanation for the flattening of the bright end slope reported previously .
We find a factor of \sim 2 greater decrease in the number density of luminous quasars ( M _ { 1450 } < -26 ) from z = 5 to z = 6 than from z = 4 to z = 5 , suggesting a more rapid decline in quasar activity at high redshift than found in previous surveys .
Our model for the quasar luminosity function predicts that quasars generate \sim 30 % of the ionizing photons required to keep hydrogen in the universe ionized at z = 5 .