MgII Absorption Lines in z=2.974 Damped Lyman-alpha System toward Gravitationally Lensed QSO APM 08279+5255: Detection of Small-scale Structure in MgII Absorbing Clouds

1.02-1.16 micron spectra (R ~ 7,000) of the gravitationally lensed QSO APM 08279+5255 at z_em=3.911 were obtained during the commissioning run of IRCS, the 1-5 micron near-infrared camera and spectrograph for the Subaru 8.2 m Telescope. Strong MgII doublet at 2976,2800 angstrom and FeII (2600 angstrom), FeII (2587 angstrom) absorption lines at z_abs=2.974 were clearly detected in the rest-frame UV spectra, confirming the presence of a damped Lyman-alpha system at the redshift as suggested by Petitjean et al. Also MgI (2853 angstrom) absorption line is probably detected. An analysis of the absorption lines including velocity decomposition was performed. This is a first detailed study of MgII absorption system at high redshift (z > 2.5) where the MgII doublet shifts into the near-infrared in the observer's frame. The spectra of the lensed QSO pair A and B with 0.38 arcsec separation were resolved in some exposure frames under excellent seeing condition. We extracted the MgII doublet spectra of A and B separately. Although three velocity components (v ~ -28, +5, +45 km/s) are known to exist in this MgII system (Petitjean et al.), the v ~ +45 km/s absorption line was not detected toward source B, showing that the +45 km/s MgII cloud lies only in the line of sight to the source A. Our results suggests that the size of the MgII 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 MgII clouds at high-redshift, confirming the estimated cloud sizes from photoionization model by Churchill and Charlton.Comment: ApJ in press (Vol.569, 20 April 2002 issue