High-redshift Metals. II. Probing Reionization Galaxies with Low-ionization Absorption Lines at Redshift Six

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 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