Evolution of Lyman break galaxies beyond z=4

The formation rate of luminous galaxies seems to be roughly constant from z similar to 2 to similar to 4 from the recent observations of Lyman break galaxies (LBGs). The abundance of luminous quasars, on the other band, appears to drop off by a factor of more than 20 from z similar to 2 to z similar to 5. The difference in evolution between the two classes of objects in the overlapping, observed redshift range (z = 2-4) can be explained naturally if we assume that quasar activity is triggered by mergers of luminous LBGs and one quasar lifetime is similar to 10(7)-10(8) yr. If this merger scenario holds at higher redshift, for the evolutions of these two classes of objects to be consistent at z > 4, the formation rate of luminous LBGs is expected to drop off at least as rapidly as exp [-(z - 4)(6/5)] at z > 4.