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BALQSOs in SDSS: On the absence of Observational Evidence for Ly-α―N V line-locking and a discussion of the physical conditions within BAL outflows

By Christopher Euan Cottis


Broad absorption line quasars show broad blue shifted absorption indicative of high velocity outflowing material. The mechanism responsible for accelerating these outflows is a matter of much debate. In this thesis I investigate the possibility that radiative line driving is responsible for driving the outflows in some objects.\ud Various metrics have been proposed to compile homogeneous samples of BALQSOs. Using a new hybrid algorithm we have compiled a sample of 3552 BALQSOs representing approximately 12.5% of the quasars for which SDSS spectra (from DR5) covering the C IV broad absorption line region are available.\ud From this BALQSO catalogue we have selected the largest sample of objects exhibiting evidence for radiative line driving (the ghost of Ly-α), a feature observed in the BAL troughs of around 1% of BALQSOs.\ud Using this sample we test the criteria purported necessary for ghost-formation in order to confirm whether the identified feature is in fact a ghost. This investigation suggests that the feature observed in the majority of objects arises from the superposition of multiple absorption components, which mimic the appearance of a ghost and unrelated to interactions between N V and Ly-α.\ud Follow up spectroscopic observations of some of the ghost candidate objects taken using the William Herschel Telescope and the Gemini North telescope are presented. The reduction of these data is described in detail. The criteria for ghost formation are tested on the new spectra with similar results.\ud One object whose absorption has undergone significant changes is identified and photo-ionisation models used to provide constraints on the physical conditions within the absorber. These models suggest that the changes are due to an increase in the ionisation parameter and not due to changes in the gas covering fraction. The location and mass outflow rate in this object is consistent with previous estimates in similar systems.\ud The work presented within this thesis does not preclude radiative line driving as a mechanism responsible for high velocity quasar outflows. It does however suggest that the ghost of Ly-α, previously considered the strongest evidence for radiative line driving is rarely observed

Publisher: University of Leicester
Year: 2010
OAI identifier: oai:lra.le.ac.uk:2381/8970

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