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

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 II doublet λλ2976, 2800 and Fe II λ2600, Fe II λ2587 absorption lines at z_abs=2.974 were clearly detected in the rest-frame UV spectra, confirming the presence of a damped Lyα 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 II 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 II clouds at high redshift, confirming the estimated cloud sizes from the photoionization model by Churchill and Charlton. Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.