  We present grism spectra taken with the Advanced Camera for Surveys to
identify 29 red sources with (i'-z') > 0.9 in the Hubble Ultra Deep Field
(HUDF). Of these 23 are found to be galaxies at redshifts between z=5.4 and
6.7, identified by the break at 1216 angstrom due to IGM absorption; two are
late type dwarf stars with red colors; and four are galaxies with colors and
spectral shape similar to dust reddened or old galaxies at redshifts z~1-2.
This constitutes the largest uniform, flux-limited sample of spectroscopically
confirmed galaxies at such faint fluxes z'<27.5.
  Many are also among the most distant spectroscopically confirmed galaxies (at
redshifts up to z=6.7).
  We find a significant overdensity of galaxies at redshifts z=5.9 +/- 0.2.
Nearly two thirds of the galaxies in our sample (15/23) belong to this peak.
Taking into account the selection function and the redshift sensitivity of the
survey, we get a conservative overdensity of at least a factor of two along the
line-of-sight. The galaxies found in this redshift peak are also localized in
the plane of the sky in a non-random manner, occupying about half of the ACS
chip. Thus the volume overdensity is a factor of four. The star-formation rate
derived from detected sources in this overdense region is sufficient to
reionize the local IGM.
