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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 thegravitationally lensed quasi-stellar object (QSO) APM 08279+5255 atzem=3.911 were obtained during the commissioning run of IRCS,the 1-5 μm near-infrared camera and spectrograph for the Subaru 8.2 mTelescope. Strong Mg II doublet λλ2976, 2800 and Fe IIλ2600, Fe II λ2587 absorption lines atzabs=2.974 were clearly detected in the rest-frame UVspectra, confirming the presence of a damped Lyα system at theredshift as suggested by Petitjean et al. In addition, the Mg Iλ2853 absorption line is probably detected. An analysis of theabsorption lines including velocity decomposition was performed. This isa 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 theobserver's frame. The spectra of the lensed QSO pair A and B with 0.38"separation were resolved in some exposure frames under excellent seeingcondition. We extracted the Mg II doublet spectra of A and B separately.Although three velocity components (v~-28, +5, and +45 kms-1) are known to exist in this Mg II system (Petitjean etal.), the v~+45 km s-1 absorption line was not detectedtoward source B, showing that the +45 km s-1 Mg II cloud liesonly in the line of sight to the image A. Our results suggest that thesize of the Mg II absorbing clouds is as small as 200 pc, whichcorresponds to the separation of A and B at the redshift of theabsorber. This is the first direct detection of the small-scalestructure of Mg II clouds at high redshift, confirming the estimatedcloud sizes from the photoionization model by Churchill and Charlton.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
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