Restframe UV-to-optical spectroscopy of APM 08279+5255

BAL classification and black hole mass estimates^★

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
e-mail: francescogabriele.saturni@oa-roma.inaf.it
² Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
³ Dip. di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁶ Scuola Normale Superiore, P.zza dei Cavalieri 7, 56126 Pisa, Italy
⁷ SRON – Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁸ Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching b. München, Germany
⁹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
¹⁰ Dip. di Fisica e Astronomia, Alma Mater Studiorum, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
¹¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3. 40129 Bologna, Italy

Received: 8 February 2018
Accepted: 24 April 2018

Abstract

We present the analysis of the restframe optical-to-UV spectrum of APM 08279+5255, a well-known lensed broad absorption line (BAL) quasar at z = 3.911. The spectroscopic data were taken with the optical DOLoRes and near-IR NICS instruments at TNG, and include the previously unexplored range between C III] λ1910 and [O III] λλ4959,5007. We have investigated the possible presence of multiple BALs by computing “balnicity” and absorption indexes (i.e., BI, BI₀, and AI) for the transitions Si IV λ1400, C IV λ1549, Al III λ1860, and Mg II λ2800. 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 studied the properties of the [O III], Hβ, and Mg II emission lines. We find that [O III] is intrinsically weak (F_[OIII]∕F_Hβ ≲ 0.04), as it is typically found in luminous quasars with a strongly blueshifted C IV emission line (~2500 km s⁻¹ for APM 08279+5255). We computed the single-epoch black hole mass based on Mg II and Hβ broad emission lines, finding M_BH = (2 ÷ 3) × 10¹⁰μ⁻¹ M_⊙, with the magnification factor μ that can vary between 4 and 100 according to CO and restframe UV-to-mid-IR imaging respectively. Using a Mg II equivalent width (EW)-to-Eddington ratio relation, the EW_MgII ~ 27 Å measured for APM 08279+5255 translates into an Eddington ratio of ~0.4, which is more consistent with μ = 4. This magnification factor also provides a value of M_BH that is consistent with recent reverberation-mapping measurements derived from C IV and Si IV.