  We present the results of narrow-band and broad-band imaging with the Very
Large Telescope of the field surrounding the radio galaxy TN J0924-2201 at z =
5.2. Fourteen candidate Ly alpha emitters with a rest-frame equivalent width of
> 20 A were detected. Spectroscopy of 8 of these objects showed that 6 have
redshifts similar to that of the radio galaxy. The density of emitters at the
redshift of the radio galaxy is estimated to be a factor 1.5-6.2 higher than in
the field, and comparable to the density of Ly alpha emitters in radio galaxy
protoclusters at z = 4.1, 3.1 and 2.2. The Ly alpha emitters near TN J0924-2201
could therefore be part of a structure that will evolve into a massive cluster.
These observations confirm that substantial clustering of Ly alpha emitters
occurs at z > 5 and support the idea that radio galaxies pinpoint high density
regions in the early Universe.
