Projekt A

The Ionizing Radiation Field in the Vicinity of Quasars Constrained by High-Resolution Absorption Spectroscopy

Prof. Dr. Philipp Richter (advisor) and PD Dr. Volker Müller (co-advisor)


Background: The majority of baryonic matter in the Universe resides in the form of diffuse intergalactic gas, spatially confined in cosmological filaments (»cosmic web«). This intergalactic medium (IGM) represents a major matter reservoir to fuel star formation in galaxies. At high redshifts the IGM is almost fully ionized by photons from the metagalactic UV background. In the vicinity of quasars the UV radiation field is locally enhanced due to the additional contribution by the quasar, a phenomenon referred to as »proximity effect« [1][2].

Aims: We want to constrain the intensity and in particular the spectral shape of the radiation field close to and far from individual sources of ionizing radiation by spectroscopy of metal-absorption systems seen in the spectra of distant quasars [3][4]. This will allow us to infer the relative (and absolute) abundances of metal ions in the absorbers, which reflect the ionization state of the gas in its local environment [5] and thereby the shape of the ionizing spectrum [6].

Methods: We will use more than 450 high-resolution (R > 45,000) optical spectra obtained with the UVES instrument at the Very Large Telescope (VLT). These data are publicly available in the ESO archive. After the identification of suitable absorption systems we will analyze the absorption lines by various intermediate and high ions, such as C II, C III, C IV, Si II, Si III, and Si IV. Using line-profile fitting techniques, we will derive column densities and Doppler parameters. Ion ratios such as C II / C III / C IV will be compared to the predictions of photoionization models to constrain both intensity and spectral shape of the ionizing radiation near the background source. The final goal of the project is to better understand physical conditions in the IGM in the proximity zone of quasars in the young Universe and the role of the proximity effect for our understanding of the IGM based on a statistically relevant (N > 50) number of metal-absorption systems.