Mg Ⅱ ABSORPTION LINES IN z = 2.974 DAMPED Lyα SYSTEM TOWARD GRAVITATIONALLY LENSED QSO APM 08279+5255: DETECTION OF SMALL-SCALE STRUCTURE IN Mg Ⅱ ABSORBING CLOUDS

摘要

1.02-1.16 μm spectra (λ/Δλ ～7000) of the gravitationally lensed quasi-stellar object (QSO) APM 08279+5255 at z_(em) = 3.911 were obtained during the commissioning run of IRCS, the 1-5 μm near-infrared camera and spectrograph for the Subaru 8.2 m Telescope. Strong Mg n doublet λλ2976, 2800 and Fe Ⅱ λ2600, Fe Ⅱ λ2587 absorption lines at z_(abs) = 2.974 were clearly detected in the rest-frame U.V spectra, confirming the presence of a damped Lya system at the redshift as suggested by Petitjean et al. In addition, the Mg Ⅰ λ2853 absorption line is probably detected. An analysis of the absorption lines including velocity decomposition was performed. This is a first detailed study of the Mg Ⅱ absorption system at high redshift (z > 2.5), where the Mg Ⅱ doublet shifts into the near-infrared in the observer's frame. The spectra of the lensed QSO pair A and B with 0.38″ separation were resolved in some exposure frames under excellent seeing condition. We extracted the Mg Ⅱ doublet spectra of A and B separately. Although three velocity components (v ～ -28, +5, and +45 km s~(-1)) are known to exist in this Mg n system (Petitjean et al.), the v ～ +45 km s~(-1) absorption line was not detected toward source B, showing that the +45 km s~(-1) Mg Ⅱ cloud lies only in the line of sight to the image A. Our results suggest that the size of the Mg Ⅱ absorbing clouds is as small as 200 pc, which corresponds to the separation of A and B at the redshift of the absorber. This is the first direct detection of the small-scale structure of Mg n clouds at high redshift, confirming the estimated cloud sizes from the photoionization model by Churchill and Charlton.