An Overdensity of Galaxies at z = 5.9 +/- 0.2 in the Hubble Ultra Deep Field Confirmed Using the ACS Grism
Daddi, E.; Panagia, N.; Ferguson, H. C.; Stiavelli, M.; di Serego Alighieri, S.; Moustakas, L. A.; Koekemoer, A.; Malhotra, S.; Rhoads, J. E.; Yan, H.; Wang, J.; Xu, C.; Bergeron, L. E.; Walsh, J.; Pasquali, A.; Pirzkal, N.; Gronwall, C.; Windhorst, R. A.; Haiman, Z.; Kuemmel, M.
United States, Germany, China, Switzerland, Italy
Abstract
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.