EDP Sciences
Free access
Volume 406, Number 1, July IV 2003
Page(s) 65 - 73
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030793

A&A 406, 65-73 (2003)
DOI: 10.1051/0004-6361:20030793

Outflowing material in the $z_{\mathsf{em}}$ = 4.92 BAL QSO SDSS J160501.21-011220.0

N. Gupta1, R. Srianand2, P. Petitjean3, 4 and C. Ledoux5

1  NCRA, Post Bag 3, Ganeshkhind, Pune 411 007, India  
    e-mail: neeraj@ncra.tifr.res.in
2  IUCAA, Post Bag 4, Ganeshkhind, Pune 411 007, India  
    e-mail: anand@iucaa.ernet.in
3  Institut d'Astrophysique de Paris - CNRS, 98bis Boulevard Arago, 75014 Paris, France
4  LERMA, Observatoire de Paris, 61 rue de l'Observatoire, 75014 Paris, France  
    e-mail: petitjean@iap.fr
5  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile  
    e-mail: cledoux@eso.org

(Received 17 January 2003 / Accepted 6 May 2003)

We present the analysis of broad absorption lines (BALs) seen in the spectrum of the $z_{\rm em}$  $\simeq$ 4.92 QSO SDSS J160501.21-011220.0. Our high spectral resolution UVES spectrum shows two well-detached absorption line systems at $z_{\rm abs}$ = 4.685 and 4.855. The system at $z_{\rm abs}$ = 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 $z_{\rm abs}$ = 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 $z_{\rm abs}$ = 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 $z_{\rm abs}$ = 4.685 as well is not Compton thick. Using simple Eddington arguments we show that the mass of the central black hole is $\sim $ $7.1\times 10^8~M_\odot$. This suggests that the accretion onto a seed black hole must have started as early as $z \sim$ 11.

Key words: quasars: absorption lines -- quasars: individual: SDSS J160501.21-011220.0

Offprint request: R. Srianand

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