Outflowing material in the z_em = 4.92 BAL QSO SDSS J160501.21-011220.0

Abstract

We present the analysis of broad absorption lines (BALs) seen in the spectrum of the Z_em ⋍ 4.92 QSO SDSS J160501.21-011220.0. Our high spectral resolution UVES spectrum shows two well-detached absorption line systems at Z_abs = 4.685 and 4.855. The system at Z_abs = 4.855 covers the background source completely, suggesting that the gas is located outside the broad emission line region. On the contrary, the system at Z_abs = 4.685, which occults only the continuum source, has a covering factor of the order of 0.9. Physical conditions are investigated in the BAL system at Z_abs = 4.855 using detailed photoionization models. The observed H I absorption line together with the limits on C II and Si II absorptions suggest that 16 < log N(H I) (cm ^-2) < 17 in this system. Comparison with models show that the observed column densities of N V , Si IV and C IV in this system require that nitrogen is underabundant by more than a factor of 3 compared to silicon if the ionizing radiation is similar to a typical QSO spectrum. This is contrary to what is usually derived for the emission line gas in QSOs. We show that the relative suppression in the N V column density can be explained for Solar abundance ratios or abundance ratios typical of Starburst abundances if an ionizing spectrum devoid of X-rays is used instead. Thus, if the composition of BAL is like that of the emission line regions it is most likely that the cloud sees a spectrum devoid of X-rays similar to what we observe from this QSO. This is consistent with the fact that none of our models have high Compton optical depths to remove X-rays from the QSO. Similar arguments lead to the conclusion that the system at Z_abs = 4.685 as well is not Compton thick. Using simple Eddington arguments we show that the mass of the central black hole is ∼ 7.1 × MΘ. This suggests that the accretion onto a seed black hole must have started as early as z ∼ 11.