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Session 20 - QSOs.
Oral session, Monday, January 13
Frontenac Ballroom,
Approximately 10% of all optically selected radio quiet QSOs exhibit broad absorption lines (BALs) in their spectra due to highly ionized gas. Typical observed outflow velocities of ions such as N^+4, C^+3, O^+5 and Si^+3 are in the range 10,000--30,000 km/s. Approximately 5--15% of broad absorption line QSOs (BALQSOs) exhibit BALs due to low--ionization ions such as Mg^+, Al^+2, and/or C^+1 in addition to the high--ionization BALs. Although BALQSOs have been observed for decades, there is still no consensus as to their relationship to other radio quiet QSOs. BALQSOs must also be considered within the context of the currently popular unified models of active galactic nuclei (AGNs) and Quasars.
The relatively recent capability of observing BALQSOs at multiple wavelengths allows us to place more stringent constraints on models of the ionization structure and chemical composition of this subclass of radio quiet QSOs. In this work we consider recently acquired optical, ultraviolet, and X--ray observations of BALQSOs to accomplish this.
A detailed study of the spectrum of one low--redshift (z\sim0.384) BALQSO (PG0043+039) reveals it to have a number of features typical of low--ionization BALQSOs (a lack of narrow--line O[III] emission, very strong FeII emission, and a strong deficit of flux in the UV shortward of \sim 2200Åbut shows no clear evidence for the low--ionization BAL troughs themselves. Intrinsic dust--extinction is likely in this object, and there are indications that the BAL region covering factor in a subclass of QSOs may be near unity.
Analysis of the composite optical/UV spectrum of a representative moderate--redshift (z \sim 2) BALQSO (Q0226--1024) within the context of a photoionization model provides evidence for BAL region ionization parameter fluctuations on the order of \sim 32-16, nitrogen enhancements relative to oxygen of \sim 4-3 times solar values, as well as overall metal--to--hydrogen abundance enhancements.
Long--exposure pointed--mode observations of several moderate--redshift BALQSOs using the ROSAT PSPC suggest that BALQSOs are X--ray quiet. The deduced range of \alpha_ox (1.8 < \alpha_ox < 2.5, for f_\nu \sim \nu^-\alpha) is well outside the range of 1.3--1.8 quoted in the literature for moderate--redshift radio quiet QSOs. The presence of photoelectric absorption would be consistent with enhanced BAL region elemental abundances.
