MSc defence by Mariam El-Souri – University of Copenhagen

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MSc defence by Mariam El-Souri

Title: A Spectroscopic Studyof the New Gravitationally Lensed Quasar SDSS J1442+4055

Supervisor: Johan Fynbo

Abstract: The study of the chemical properties of the interstellar medium (ISM) in galaxies and the intergalactic medium and evolution of those over cosmic time is essential for a coherent understanding of galaxy formation and evolution. The absorption lines imprinted on the spectra of quasars are powerful probes of the properties of the gas in and around galaxies. An important part of this study is the detection of tracers of cold, low-ionization gas in absorption. This gas can be efficiently traced by detecting H i damped Lyman-_ absorption lines. Damped Lyman-_ (DLA) systems observed in spectra of quasars have column densities of N(H i) > 2×1020 atoms cm−2. These are known to contain most of the neutral gas in the Universe. The metallicity of DLAs is generally low, and their dust to gas ratio is typically less than what is observed in the Galactic ISM. In addition, there is a correlation between the metallicity and the dust depletion in DLAs. Damped Lyman systems with high metallicity are expected to contain more dust and therefore exhibit a larger fraction of H2. Since dust grains act as an important catalyst for the formation of H2. Nevertheless, even DLAs with the highest metallicities and strong dust signatures, the detection of H2 is rare. However, when detected, H2 is usually coincident with neutral atomic carbon C i. Thus C i is an optimal tracer of the molecular gas in star forming galaxies. The data studied in this thesis are from the Nordic Optical Telescope (NOT) and the W. M. Keck Observatory. Probing the quasar SDSS J 1442+4055 in a gravitational lens at z _ 2.578 and an intervening DLA at zabs _ 1.948. The thesis work has included analysis of spectra to first establish that the system is indeed a lensed quasar. I have then analyses absorption lines in both sightlines to infer how homogeneous the gas is as the two sightlines are separated by 1.19 kpc at the redshift of the DLA. I find that the DLA appear to be surprisingly similar in the two sightlines. In particular, it is surprising that the very cold gas probed by C i is seen in both sightlines.