2006ApJ...643..667K

An NIR (1.18-1.35 µm) high-resolution spectrum of the gravitationally lensed QSO APM 08279+5255 was obtained with the Subaru Telescope IRCS using the AO system. We have detected strong Na I D λλ5891, 5897 doublet absorption in high-redshift DLAs at z_abs=1.062 and 1.181, confirming the presence of Na I, which was first reported for the rest-frame UV Na I λλ3303.3, 3303.9 doublet by Petitjean et al. This is the first detection of Na I D absorption in a high-redshift (z>1) DLA. We have also detected a new Na I component in the z_abs=1.062 DLA and four new components in the z_abs=1.181 DLA. Using an empirical relationship between Na I and H I column density, we found that all components have large H I column density (logN_Hi[cm^–2]≳20.3), so each component is classified as DLA absorption. We also detected strong Na I D absorption associated with a Mg II system at z_abs=1.173. Because no other metal absorption lines were detected in this system at the velocity of the Na I absorption in previously reported optical spectra, we infer that this Na I absorption cloud probably appeared in the line of sight toward the QSO after the optical observation. This newly found cloud is likely to be a DLA based on its large estimated H I column density. We found that the N_NaI_/NCaII_ratios in these DLAs are systematically smaller than those observed in the Galaxy; they are more consistent with the ratios seen in the LMC. This is consistent with dust depletion generally being smaller in lower metallicity environments. However, all five clouds of the z_abs=1.181 system have a high N_NaI_/NCaII_ratio, which is characteristic of cold dense gas. We tentatively suggest that the host galaxy of this system may be the most significant contributor to the gravitational lens toward APM 08279+5255.