Metallicity Evolution of Damped Lyman Alpha Systems In Lambda CDM Cosmology
Abstract
Utilizing a new, high mass resolution hydrodynamic simulation of Lambda CDM cosmological model we compute the metallicity evolution of damped Lyman alpha systems (DLAs) and find a reasonable agreement with observations. In particular, the observed slow evolution of the DLA metallicity occurs naturally in the simulation due to the combined effects of physical and observational selection. The slow evolution is caused by the steady transformation, with increasing time, of the highest metallicity systems to galaxies, thus depleting this category, while all the lower metallicity systems show, individually, an increase in metallicity. Although the trend of DLAs metallicity with redshift is in good agreement with observations, it appears that the average metallicity of simulated DLAs is higher than observed by 0.3-0.5dex in the probed redshift range (z=0-5). Our study indicates that this difference may be attributed to observational selection effects due to dust obscuration. If we allow for a dust obscuration effect, our model reproduces the observed metallicity evolution in both amplitude and slope. DLAs are not a simple population but probe a range of different systems and the mix changes with redshift. The median luminosity of a DLA, L_{DLA}(z), in units of typical galaxy luminosity at that redshift, L^*(z), that is, (L_{DLA}/L^*)_z decreases from 1.1 to 0.5 as redshift declines from z=3 to z=0, but the absolute luminosity of the median DLA system increases in the same interval by a factor of five from 0.1L^*(z=0) to 0.5L^*(z=0). About 50% all metals in the gaseous phase is in DLAs from z=5 to z=1, making a rapid downturn at z<1 to ~20% by z=0. While not the primary focus of this study, we find that the model provides good matches to observations with respect to column density distribution and evolution of neutral gas content.Keywords
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