title: A comprehensive analysis of optical and near-infrared spectroscopy of z~6 quasars
creator: De Rosa, Gisella
subject: 530
subject: 530 Physics
description: High redshift (z~6) quasars (QSOs) are unique probes of the early growth of accreting supermassive black holes (BHs) and are fundamental tools to study the ionization state of the intergalactic medium and its chemical enrichment. Until now, only the most luminous QSOs have been studied, often one object at the time. In this thesis we present the most extended consistent analysis to date of optical and near-infrared spectra of z~6 QSOs. Our NIR sample is composed of 22 spectra of QSOs at 4.5<z<6.45, covering the MgII and FeII emission features, and includes new spectroscopic VLT-ISAAC observations of three faint z~6 SDSS QSOs. The optical sample consists of 26 high quality spectra of QSOs at 5.70<z<6.45, covering the Ly-alpha complex, and the OI+SiII, CII and SiIV+OIV] broad lines. Based on the NIR spectra we estimate the BH mass (MBH), the Eddington ratio and the FeII/MgII line ratio, a proxy for the chemical enrichment of the BLR. The QSOs in our sample host BHs with masses of ~10^9 Msun that are accreting significantly faster than a luminosity–matched comparison sample at lower redshifts. The measured FeII/MgII line ratios show no evolution with cosmic time in the redshift range 4.5<z<6.45, implying that the QSOs in our sample have undergone a major episode of Fe enrichment in the few 100s Myr preceding the cosmic age at which they are observed. An analysis of the wavelength shifts of the optical lines shows that the redshifts inferred from the OI+SiII complex, visible in most of the optical spectra, are consistent with those obtained from the MgII line. This implies that zOI is a reliable proxy for the QSO systemic redshift. We create a composite template of the z~6 QSOs that, red-ward the Ly-alpha line, remarkably resembles the one obtained for low-redshift QSOs. Investigating the existence of possible correlations amongst optical emission properties, we do not detect the expected anti-correlation between the EW and the QSO continuum luminosity (Baldwin effect) for the high ionization lines (NV & SiIV+OIV]). We show that a strong correlation holds between the line FWHM and its luminosity. Given the narrow distribution of Eddington ratios that characterizes the z~6 QSOs population, this implies that QSOs with brighter lines are the ones that host more massive BHs. We further confirm this statement by testing the relation between the line luminosity and MBH. Finally we use our optical spectra to study the ionization state of the IGM at high redshift, by measuring the evolution of the near zone (NZ) sizes and that of the Gunn Peterson (GP) effective optical depth. Our comprehensive analysis shows that the correlation between the NZ radii and the QSO systemic redshifts is significantly reduced with respect to previous results. This implies that changes of the IGM ionization state are difficult to constrain through such measurements. We also outline a new method to detect the evolution of the GP effective optical depth through the analysis of the Ly-alpha and Ly-beta flux ratios. For each QSO we build a model based on the SDSS QSO composite spectrum and on the optical depth parametrization of lower-redshift absorbers (z<5.7). Comparing the QSOs flux ratios with the ones obtained from the models we detect a steep increase in the evolution of the effective optical depth.
date: 2011
type: Dissertation
type: info:eu-repo/semantics/doctoralThesis
type: NonPeerReviewed
format: application/pdf
identifier: https://archiv.ub.uni-heidelberg.de/volltextserverhttps://archiv.ub.uni-heidelberg.de/volltextserver/12859/1/DeRosa_Thesis_biblio.pdf
identifier: DOI:10.11588/heidok.00012859
identifier: urn:nbn:de:bsz:16-opus-128592
identifier:   De Rosa, Gisella  (2011) A comprehensive analysis of optical and near-infrared spectroscopy of z~6 quasars.  [Dissertation]     
relation: https://archiv.ub.uni-heidelberg.de/volltextserver/12859/
rights: info:eu-repo/semantics/openAccess
rights: http://archiv.ub.uni-heidelberg.de/volltextserver/help/license_urhg.html
language: eng