Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255

Abstract

Results of a careful analysis of the absorption systems with zabs≃zem seen toward the bright, zem ∼3.91, gravitationally lensed quasar APM 08279+5255 are pre- sented. Two of the narrow-line systems, at zabs = 3.8931 and zabs = 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 cor- responding to, respectively, 156µm and 34µm) are de- tected at zabs = 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 ∼200 pc. The absence of fine structure lines at zabs = 3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are ∼0.01 and 1 Z⊙ at zabs = 3.913 and 3.8931 and alu- minum could be over-abundant with respect to silicon and carbon by at least a factor of two and five. All this sug- gests that whereas the zabs= 3.8931 system is probably located within 200 pc from the QSO and ejected at a ve- locity larger than 1000 km s−1, the zabs= 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 hy- drogen 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 ≤ 1000 km s−1) and narrow compo-nents 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 ten- dency 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 zabs = 3.712 system covers only im- age C. Finally we identify narrow components within the BAL- flow with velocity separations within 5 km s−1 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.