Chemical abundances and ionization in sub-Damped Lyman-alpha absorbers at z < 1.5.

摘要

The chemical composition of galaxies provide important clues into galaxy formation and evolution. Quasar (QSO) absorption line systems offer a unique window into the high redshift Universe and the properties of normal galaxies at high redshift. QSO absorbers have long been used to study distant galaxies and the intergalactic medium (IGM). The Damped Lyman-alpha systems (DLAs), with neutral Hydrogen column densities of log NH I > 20.3, and sub-Damped Lyman-alpha systems (sub-DLAs) with 19.0 < log NH I < 20.3 contain the majority of the neutral gas in the Universe at high redshift, probe metallicities over ∼90% of the cosmic history, and are believed to be the progenitors of modern day galaxies. Models of the chemical evolution of galaxies predict that the mean metallicity of galaxies should reach a solar value by z ∼ 0 due to the ongoing cycles of star formation which enrich the galaxy with heavy elements. The DLA systems which have been the preferred class of absorbers for these investigations however appear to be metal poor at all redshifts, and show little evolution in their metallicity, contradicting the models of chemical evolution, the "missing metals problem". We have amassed a sample of 32 sub-DLAs and 3 DLAS at zabs < 1.5 using the 6.5m Magellan II telescope with the MIKE spectrograph, and the 8.2m VLT-Kueyen telescope with the UVES spectrograph to study the properties of these systems and determine their metal content. We have measured the absorption lines of multiple lines in these systems and determined column densities and abundances. We have also created grids of photoionization models using CLOUDY to determine the effects of ionization in these systems. Although the gas is largely ionized, the abundances appear not to require significant ionization corrections. We find that the sub-DLAs, especially at low z are more metal rich than the DLA systems, with [Zn/H]subDLA = --0.30 +/- 0.15 and [Zn/H]DLA = --0.94 +/- 0.11. These systems appear to contain ∼ 40--75% of the comoving mass density of metals that is seen in DLAs, WsubDLAmet ∼ (4--11) x 10-7, dependent on the ionization. Kinematically, the sub-DLAs from this sample have larger velocity widths than the DLAs, perhaps implying that they arise in more massive galaxies with deeper potential wells. We also investigate the relative abundances of [Cr/Zn], [Fe/Zn], [Mn/Fe], and [Si/Fe] in these systems to study dust depletion and nucleosynthetic effects.