2000A&A...357..414S

Result. of a careful analysis of the absorption systems with z_abs≃z_em seen toward the bright, z_em∼3.91, gravitationally lensed quasar APM 08279+5255 are presented. Two of the narrow-line systems, at z_abs=3.8931 and z_abs=3.9135, show absorptions from singly ionized species with weak or no N V and O VI absorptions at the same redshift. Absorption due to fine structure transitions of C II and Si II (excitation energies corresponding to, respectively, 156µm and 34µm) are detected at z_abs = 3.8931. Excitation by IR radiation is favored as the column density ratios are consistent with the shape of APM 08279+5255 IR spectrum. The low-ionization state of the system favors a picture where the cloud is closer to the IR source than to the UV source, supporting the idea that the extension of the IR source is larger than ∼200pc. The absence of fine structure lines at z_abs=3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are ∼0.01 and 1Z_☉ at z_abs=3.913 and 3.8931 and aluminum could be over-abundant with respect to silicon and carbon by at least a factor of two and five. All this suggests that whereas the z_abs=3.8931 system is probably located within 200 pc from the QSO and ejected at a velocity larger than 1000km/s, the z_abs=3.9135 system is farther away and part of the host-galaxy. Several narrow-line systems have strong absorption lines due to C IV, O VI and N V and very low neutral hydrogen optical depths. This probably implies metallicities Z≥Z_☉ although firm conclusion cannot be drawn as the exact value depends strongly on the shape of the ionizing spectrum. The C IV broad absorption has a complex structure with mini-BALs (width ≤1000km/s) and narrow components superposed on a continuous absorption of smaller optical depth. The continuous absorption is much stronger in O VI indicating that the corresponding gas-component is of higher ionization than the other components in the flow and that absorption structures in the BAL-flow are mainly due to density inhomogeneities. There is a tendency for mini-BALs to have different covering factors for different species. It is shown that a few of the absorbing clouds do not cover all the three QSO images, especially we conclude that the z_abs=3.712 system covers only image C. Finally we identify narrow components within the BAL-flow with velocity separations within 5km/s of the O VI, N V and Si IV doublet splittings suggesting that line driven radiative acceleration is an important process to explain the out-flow.