First Detection of NaiD Lines in High‐Redshift Damped Lyα Systems

Abstract

An NIR (1.18–1.35 � m) 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 kk5891, 5897 doublet absorption in high-redshift DLAs at zabs ¼ 1:062 and 1.181, confirming the presence of Na i ,w hich was firstreportedfortherest-frameUVNaikk3303.3,3303.9doubletbyPetitjeanetal.ThisisthefirstdetectionofNa i D absorption in a high-redshift (z > 1)DLA. We have also detected a new Nai component in thezabs ¼ 1:062DLA and four new components in the zabs ¼ 1:181 DLA. Using an empirical relationship between Na i and H i column density,wefoundthatallcomponentshavelargeHicolumndensity( logNH i ½cm � 2 � k20:3),soeachcomponentis classified as DLA absorption. We also detected strong Na i D absorption associated with a Mg ii system at zabs ¼ 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 NNa i/NCa ii 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 zabs ¼ 1:181 system have a high NNa i/NCa ii 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. Subject headings: galaxies: formation — gravitational lensing — intergalactic medium — quasars: absorption lines — quasars: individual (APM 08279+5255)