An overdensity of galaxies at z=5.9 +/-0.2 in the Ultra Deep Field   confirmed using the ACS grism
  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. 
S. Malhotra, J.E. Rhoads, N. Pirzkal, Z. Haiman, C. Xu, E. Daddi, H.   Yan, L.E. Bergeron, J. Wang, H.A. Ferguson, A. Koekem...