The rest-frame UV-to-optical spectroscopy of APM 08279+5255 - BAL classification and black hole mass estimates

Abstract

We present the analysis of the rest-frame optical-to-UV spectrum of APM 08279+5255, a well-known lensed broad absorption line (BAL) quasar at $z=3.911$. The spectroscopic data are taken with the optical DOLoRes and near-IR NICS instruments at TNG, and include the previously unexplored range between C III] $\hat{I}»$1910 and [O III] $\hat{I}»\hat{I}»$4959,5007. We investigate the possible presence of multiple BALs by computing "balnicity" and absorption indexes (i.e. BI, BI${}_0$ and AI) for the transitions Si IV $\hat{I}»$1400, C IV $\hat{I}»$1549, Al III $\hat{I}»$1860 and Mg II $\hat{I}»$2800. No clear evidence for the presence of absorption features is found in addition to the already known, prominent BAL associated to C IV, which supports a high-ionization BAL classification for APM 08279+5255. We also study the properties of the [O III], H${\hat{I}}^2$ and Mg II emission lines. We find that [O III] is intrinsically weak ($F_{[\mathrm{OIII}]}/F_{\mathrm{H}{\hat{\mathrm{I}}}^2}\lesssim 0.04$), as it is typically found in luminous quasars with a strongly blueshifted C IV emission line ($\sim$2500 km s${}^{-1}$ for APM 08279+5255). We compute the single-epoch black hole mass based on Mg II and H${\hat{I}}^2$ broad emission lines, finding $M_{\mathrm{BH}}=(2\tilde{A}\cdot 3)\times 10^{10}\hat{I}{¼}^{-1}$ M${}_{\odot }$, with the magnification factor $\hat{I}¼$ that can vary between 4 and 100 according to CO and rest-frame UV-to-mid-IR imaging respectively. Using a Mg II equivalent width (EW)-to-Eddington ratio relation, the EW${}_{\mathrm{MgII}}\sim 27$ Ã measured for APM 08279+5255 translates into an Eddington ratio of $\sim$0.4, which is more consistent with $\hat{I}¼=4$. This magnification factor also provides a value of $M_{\mathrm{BH}}$ that is consistent with recent reverberation-mapping measurements derived from C IV and Si IV.

10 pages, 4 figures, 4 tables, accepted for publication in A&A