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Astron. Astrophys. 357, 37-50 (2000)

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5. The BAL systems in HS 1216+5032 B: Some qualitative inferences

Three BAL systems are observed in the UV spectrum of HS 1216+5032 B (see Fig. 2, where the system redshifts have been arbitrarily numbered 1, 2, and 3). Absorption by HI , CIII , NIII , and possibly SIV is observed in at least two of the systems, while OVI and NV are present in all three systems. CII is observed only in system 2, and NIII in systems 2 and 3.

Many features distinguish these systems from the most commonly observed BAL systems (e.g., Turnshek et al. 1996): (1) the lines are particularly weak and several line profiles are not distorted by/or blended with other BALs; (2) the maximum outflow velocity [FORMULA] km s-1 is small compared with typical BALQSOs, which exhibit terminal velocities of several [FORMULA] km s-1 (Turnshek 1984); (3) CII is present (see Wampler et al. 1995for other BAL QSO spectrum with singly-ionized species; Arav et al. 1999b); (4) the strength of HI in systems 2 and 3 decreases more slowly at the red edge of the troughs (Turnshek 1984).

Also remarkable is the undulating shape of the absorbed continuum for [FORMULA] Å (see Fig. 1). The position of the flux depressions coincides quite well with the blue wing of expected emission lines by CIII , NIII , Ly[FORMULA], and OVI . This coincidence seems to suggest that the absorption is indeed dominated by very broad line components, which determine the continuum shape, superimposed to the narrower components, here labeled as systems 1, 2 and 3. The same effect, although less remarkable, is observed for SIV and SVI , thus giving evidence for absorption by these ions associated with the BAL phenomenon (also reported for another QSO by Arav et al. 1999a).

It is customary to define BALs as a continuous absorption with outflow velocities larger than [FORMULA] km s-1 from the emission redshift (Weymann et al. 1991) to make a distinction between BAL systems and "associated systems". However, as pointed out by Arav et al. (1999b), such definition does not hold any physical meaning. In our case, systems 1 and 2 should be then classified as associated systems but, due to our poor resolution, it is not possible to establish whether the line profiles are produced by continuous absorption or whether they are made of several narrower velocity components (as observed in associated systems). The metal lines in these systems (e.g. the NV and OVI doublet lines) are relatively narrow and their widths could be dominated by the instrumental profile (for HI in system 2, however, the situation is less clear). Therefore, the classification of systems 1 and 2 as BAL systems must be considered solely instrumental.

High-resolution spectra of BAL QSOs (Hamann et al. 1997) show that BAL profiles do not necessarily result from an ensemble of discrete narrow, unresolved lines, but as part of a mixture of a continuously accelerated outflow and overlapping narrow components with different non-thermal velocity dispersions. For the systems in HS 1216+5032, it can be assumed that a similar mixture of line widths is present, with the broad components dominating over the narrow ones. In that case, these line profiles should not look so different at higher resolution (with exception, maybe, of systems 1 and 2).

Ionization models have shown that BAL clouds span a range of densities and/or distances from the ionizing source (Turnshek et al. 1996; Hamann et al. 1995). Fig. 2 shows that, while the high-ionization species NV and OVI are present in all three systems in HS 1216+5032, HI , CII and doubly-ionized species are absent in system 1. A possible explanation is that the ionization conditions could change with outflow velocity, with higher ionization level for redshifts closer to [FORMULA]. However, a quantitative study of the ionization conditions in these BAL clouds with the present data is made difficult by our inability to reliably establish the continuum level at the BAL troughs and thus determine column densities (besides the fact that nonblack saturation of the line profiles might be present; see Arav et al. 1999a).

Nevertheless, a more quantitative study of the BAL phenomenon in HS 1216+5032 B should be possible using higher resolution HST spectra to better estimate the QSO continuum and to identify spurious lines at the BAL troughs. In particular, BAL system 2 can be studied in more detail because it shows more ions and less contamination by narrow absorption lines at the lines lying in the Ly[FORMULA] forest. On the other hand, alone medium-resolution optical spectra should considerably improve our knowledge of the BAL systems in HS 1216+5032 through the line profiles of CIV , CIII , and possibly MgII , which we suspect to be present given the presence of CII . The width of the trough in system 2 is sufficiently small to study nonblack saturation effects via doublet ratios provided the emission line profiles can be determined (e.g., at the position of the CIV BAL). Thus, corrected column densities for system 2 appear as very suitable to reliably examine photoionization models because the radiation fields can be constrained by the presence of low-ionization species.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000