AN EXPONENTIAL DECLINE AT THE BRIGHT END OF THE z=6 GALAXY LUMINOSITY FUNCTION

We present the results of a search for the most luminous star-forming galaxies at redshifts z approximate to 6 based on Canada-France-Hawaii Telescope Legacy Survey data. We identify a sample of 40 Lyman break galaxies (LBGs) brighter than magnitude z' = 25.3 across an area of almost 4 deg(2). Sensitive spectroscopic observations of seven galaxies provide redshifts for four, of which only two have moderate to strong Ly alpha emission lines. All four have clear continuum breaks in their spectra. Approximately half of the LBGs are spatially resolved in 0.7 arcsec seeing images, indicating larger sizes than lower luminosity galaxies discovered with the Hubble Space Telescope, possibly due to ongoing mergers. The stacked optical and infrared photometry is consistent with a galaxy model with stellar mass similar to 10(10) M-circle dot. There is strong evidence for substantial dust reddening with a best-fit A(V) = 0.75 and A(V) > 0.48 at 2 sigma confidence, in contrast to the typical dust-free galaxies of lower luminosity at this epoch. The spatial extent and spectral energy distribution suggest that the most luminous z approximate to 6 galaxies are undergoing merger-induced starbursts. The luminosity function of z = 5.9 star-forming galaxies is derived. This agrees well with previous work and shows strong evidence for an exponential decline at the bright end, indicating that the feedback processes that govern the shape of the bright end are occurring effectively at this epoch.