Abstract
We examine the absorption line spectra of a sample of 31 luminous (M_UV~-23)
Lyman break galaxies at redshift z~6 using data taken with the FOCAS and OSIRIS
spectrographs on the Subaru and GTC telescopes. For two of these sources we
present longer exposure data taken at higher spectral resolution from ESO's
X-shooter spectrograph. Using these data, we demonstrate the practicality of
stacking our lower resolution data to measure the depth of various interstellar
and stellar absorption lines to probe the covering fraction of low ionization
gas and the gas-phase and stellar metallicities near the end of the era of
cosmic reionization. From maximum absorption line depths of SiII1260 and
CII1334, we infer a mean covering fraction of >0.85 +/-0.16 for our sample.
This is larger than that determined using similar methods for lower luminosity
galaxies at slightly lower redshifts, suggesting luminous galaxies do not play
a prominent role in concluding reionization. Using various interstellar
absorption lines we deduce gas-phase metallicities close to solar indicative of
substantial early enrichment. Using selected stellar absorption lines, we model
our spectra with a range of metallicities using techniques successfully
employed at lower redshift and deduce a stellar metallicity of 0.4 +0.3/-0.1
solar, consistent with the stellar mass - stellar metallicity relation recently
found at z~3-5. We discuss the implications of these metallicity estimates for
the typical ages of our luminous galaxies and conclude our results imply
initial star formation at redshifts z~10, consistent with independent analyses
of earlier objects.
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