Neutral magnesium as a probe of high column density QSO absorbers.

QSOs, providing distant, luminous sources of radiation, provide a means to detect gas by absorption in the interstellar and intergalactic medium. Of particular interest to astronomers studying metal line systems in lines of sight to QSOs are the classes characterized by their neutral hydrogen column density known as Damped Lyman-a systems (DLAs) and Sub-damped Lyman-a systems (Sub-DLAs). DLAs and Sub-DLAs provide a means to probe the detailed abundance and dust content of the Universe at high redshift, revealing the evolving presence of elements heavier than hydrogen and helium. Here, we present a correlation between ionization and metallicity as indicated by the abundance ratios [Si II / Mg I] and [Zn II / HI], respectively, in such systems at redshift z~0.5-2.7. The abundances used were drawn from the literature where reported. However, where elemental abundances were omitted in the literature, we profile fit those spectra to obtain them. We believe that this correlation, if proven valid, could provide another tracer of evolution of elements throughout the history of the Universe