A five square arcminute region around the luminous radio-loud quasar SDSS J0836+0054 ( z = 5.8 ) hosts a wealth of associated galaxies , characterized by very red ( 1.3 < i _ { 775 } - z _ { 850 } < 2.0 ) color .
The surface density of these z \sim 5.8 candidates is approximately six times higher than the number expected from deep ACS fields .
This is one of the highest galaxy overdensities at high redshifts , which may develop into a group or cluster .
We also find evidence for a substructure associated with one of the candidates .
It has two very faint companion objects within two arcseconds , which are likely to merge .
The finding supports the results of a recent simulation that luminous quasars at high redshifts lie on the most prominent dark-matter filaments and are surrounded by many fainter galaxies .
The quasar activity from these regions may signal the buildup of a massive system .