First Detection of Na I D Lines in High-Redshift Damped Lyα Systems

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/N_CaII 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/N_CaII 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.

Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.