Author

Suraj Poudel

Date of Award

Summer 2020

Document Type

Open Access Dissertation

Department

Physics and Astronomy

First Advisor

Varsha P. Kulkarni

Abstract

Metal abundance measurements throughout the cosmic ages track the history of galaxy formation and evolution. Measuring abundances during the first ∼1 billion years is especially important, as they are influenced by the nucleosynthetic signatures from the early stars. Evolution of metallicity of Damped/sub-Damped Lyman-alpha Absorbers (DLAs/sub-DLAs) detected in the spectra of quasars is a powerful tracer of the cosmic star formation history. A sudden drop in DLA metallicity at z>4.7 was reported in some recent studies. However, these studies were primarily based on refractory elements such as Fe and Si. We present ten new abundance measurements of the elements O, C and Si at z > 4.5, increasing the existing sample of weakly depleted elements in gas-rich galaxies significantly at z∼5. These measurements are based on spectra of quasars with DLAs/sub-DLAs obtained with the MIKE and MagE spectrographs on Magellan Telescope, the X-Shooter spectrograph on the Very Large Telescope, and the ESI on the Keck Telescope. We combine these new measurements with those drawn from the literature to estimate the NH I -weighted binned mean metallicity of −1.51 ± 0.18 dex at hzi =4.8. This metallicity value is in excellent agreement with the prediction from lower redshift DLAs, suggesting that the metallicity evolution is smooth at z ∼ 5, rather than showing a sudden decline at z > 4.5. Furthermore, the metallicity evolution trends for the DLAs and sub-DLAs are similar within our uncertainties. We also determine the extent of dust depletion using a combination of both the volatile element O and the refractory elements Si and/or Fe. Some of the absorbers show evidence of depletion of elements on dust grains. The relative abundances of these absorbers along with other z∼5 absorbers

from the literature show some peculiarities, e.g. low [C/O] in several absorbers and high [Si/O] in some absorbers. Additionally, using [C/O] and [Si/O] to constrain the nucleosynthesis models, we estimate that the probability distributions of the progenitor star masses for three relatively metal-poor DLAs are centered around 12 M⊙ to 17 M⊙. Furthermore, the z ∼ 5 absorbers show a different metallicity-velocity dispersion relation than lower redshift DLAs, suggesting that they may be tracing a different population of galaxies. Finally, we comment on the implications of our studies for galaxy evolution, the need for further expanding the samples of z & 5 absorbers, and the promise offered by the next generation of large telescopes.

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