High-redshift metals. II. Probing reionization galaxies with low-ionization absorption lines at redshift six

Abstract

We present a survey for low-ionization metal absorption line systems toward 17 QSOs at redshifts z em = 5.8-6.4. Nine of our objects were observed at high resolution with either Keck/HIRES or Magellan/MIKE, and the remainder at moderate resolution with Keck/ESI. The survey spans 5.3 < z abs < 6.4 and has a path length interval ΔX = 39.5 or Δz = 8.0. In total we detect ten systems, five of which are new discoveries. The line-of-sight number density, ℓ(X) = 0.25+0.21- 0.13 (95% confidence), is consistent with the combined number density at z 3 of damped Lyα systems (DLAs) and sub-DLAs, which comprise the main population of low-ionization systems at lower redshifts. This apparent lack of evolution may occur because low-ionization systems are hosted by lower-mass halos at higher redshifts, or because the mean cross section of low-ionization gas at a given halo mass increases with redshift due to the higher densities and lower ionizing background. The roughly constant number density notably contrasts with the sharp decline at z > 5.3 in the number density of highly ionized systems traced by C IV. The low-ionization systems at z 6 span a similar range of velocity widths as lower-redshift sub-DLAs but have significantly weaker lines at a given width. This may imply that the mass-metallicity relation of the host galaxies evolves toward lower metallicities at higher redshifts. These systems lack strong Si IV and C IV, which are common among lower-redshift DLAs and sub-DLAs. This is consistent, however, with a similar decrease in the metallicity of the low- and high-ionization phases, and does not necessarily indicate a lack of nearby, highly ionized gas. The high number density of low-ionization systems at z 6 suggests that we may be detecting galaxies below the current limits of i-dropout and Lyα emission galaxy surveys. These systems may therefore be the first direct probes of the "typical" galaxies responsible for hydrogen reionization. © 2011. The American Astronomical Society. All rights reserved..