An Overdensity of Galaxies atz= 5.9 + 0.2 in the Hubble Ultra Deep Field Confirmed Using the ACS Grism

We present grism spectra taken with the Advanced Camera for Surveys toidentify 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 and6.7, identified by the break at 1216 angstrom due to IGM absorption; two arelate type dwarf stars with red colors; and four are galaxies with colors andspectral shape similar to dust reddened or old galaxies at redshifts z~1-2.This constitutes the largest uniform, flux-limited sample of spectroscopicallyconfirmed galaxies at such faint fluxes z'<27.5. Many are also among the most distant spectroscopically confirmed galaxies (atredshifts 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 thesurvey, we get a conservative overdensity of at least a factor of two along theline-of-sight. The galaxies found in this redshift peak are also localized inthe plane of the sky in a non-random manner, occupying about half of the ACSchip. Thus the volume overdensity is a factor of four. The star-formation ratederived from detected sources in this overdense region is sufficient toreionize the local IGM.Comment: Accepted for publication in ApJ. Minor changes to text and figures 1 and