An overdensity of galaxies at z= 5.9 ± 0.2 in the Hubble Ultra Deep Field confirmed using the ACS grism Malhotra, S. Rhoads, J. E. Pirzkal, N. Haiman, Zoltán Xu, C. Daddi, E. Yan, H. Bergeron, L. E. Wang, J. Ferguson, H. C. Gronwall, C. Koekemoer, A. Kuemmel, M. Moustakas, L. A. Panagia, N. Pasquali, A. Stiavelli, M. Walsh, J. Windhorst, R. A. di Serego Alighieri, S. We present grism spectra taken with the Advanced Camera for Surveys (ACS) to identify 29 red sources with (i775 - z850) ≥ 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 Å due to intergalactic medium (IGM) absorption; two are late-type dwarf stars with red colors; and four are galaxies with colors and spectral shapes 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 (z850 ≤ 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 2 along the line of sight. The galaxies found in this redshift peak are also localized in the plane of the sky in a nonrandom manner, occupying about half of the ACS chip. Thus the volume overdensity is a factor of 4. The star formation rate derived from detected sources in this overdense region is sufficient to reionize the local IGM. American Astronomical Society 2005 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/17774 Malhotra S, Rhoads JE, Pirzkal N, et al. An overdensity of galaxies at z= 5.9 ± 0.2 in the Hubble Ultra Deep Field confirmed using the ACS grism. <i>The Astrophysical Journal</i>. 2005;626(2):666-679. doi:<a href="https://doi.org/10.1086/430047">10.1086/430047</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1086/430047 info:eu-repo/semantics/altIdentifier/issn/0004-637X info:eu-repo/semantics/altIdentifier/issn/1538-4357 info:eu-repo/semantics/openAccess