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The Nature of Low-ionization Broad Absorption Line QSOs


The tight correlations between properties of galaxy bulges and their central supermassive black holes have been reproduced successfully in simulations of galaxy collisions if feedback processes are invoked. Mergers of gas-rich galaxies of comparable size have been shown to trigger starbursts, fuel the central black holes, and transform disks into ellipticals. Feedback from the black hole accretion in the form of extreme outflows has need suggested as the mechanism by which the black hole stop its own growth and quenches the star formation in the galaxy by expelling the gas supply. Such winds have been detected in Broad Absorption Line (BAL) QSOs. However, observational evidence that BAL QSOs may be an evolutionary link between mergers and QSO is missing.

In this thesis, we provide the first detailed study of the spectral energy distributions and host galaxy morphologies of a statistically significant volume-limited sample of 22 optically-selected low-ionization Broad Absorption Line QSOs (LoBALs) at 0.5 < z < 0.6. By comparing their mid-IR spectral properties and far-IR SEDs with those of a control sample of 35 non-LoBALs (non-LoBALs) matched in M_i, we investigate the differences between the two populations in terms of their infrared emission and star formation activity. We model the SEDs and decouple the AGN and starburst contributions to the far-infrared luminosity in LoBALs and in non-LoBALs. We estimate star formation rates (SFRs) corrected for the AGN contribution to the FIR flux and find that LoBALs have comparable levels of star formation activity to non-LoBALs when considering the entire samples. Overall, our results show that there is no strong evidence from the mid- and far-IR properties that LoBALs are drawn from a different parent population than non-LoBALs.

We conducted the first high-resolution morphological analysis of LoBALs using observations obtained with the {Hubble Space Telescope} Wide Field Camera 3 in two channels. Signs of recent or ongoing tidal interaction are seen in 59% of the host galaxies, including interacting companions, tidal tails, bridges, asymmetries, plumes, and boxy isophotes. The presence of a second nucleus within ~1" (6.4 kpc) is revealed in seven of the systems. A detailed two-dimensional surface brightness analysis with GALFIT indicates that the majority (73%) have prominent early-type (bulge, n>4) morphology and only four systems have exponential disk profiles (n<2). Two of the disks and one bulge are better described as pseudobulges (n<2.2). The dominance of bulges and unambiguous signs of tidal interaction strongly suggests that the population LoBALs are QSOs that result from major mergers. Nevertheless, most of the merger-induced start formation is already quenched. This sample of LoBALs represents merger systems at various stages of the interaction process, hence, either the outflows which characterize these systems persist for as long as the interaction signs are observable in the galaxy, or very short-lived outflows are triggered and die out during various stages of the merger process.

Overall, our results are consistent with LoBALs representing the last stages of the blowout phase when QSO winds have already quenched the star formation.

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