A MIKE + UVES survey of Sub-Damped Lyman-Alpha Systems at z<1.5

TL;DR

This study combines observations of sub-Damped Lyman-alpha systems with literature data to analyze their metallicity, ionization, and contribution to cosmic metal density, revealing that sub-DLAs are more metal-rich and evolve faster than classical DLAs.

Contribution

It provides the first comprehensive comparison of metallicity and abundance patterns between sub-DLAs and DLAs at z<1.5, highlighting the significance of sub-DLAs in cosmic metal budget and evolution.

Findings

Sub-DLAs have higher and more rapidly evolving metallicity than DLAs.

Ionization corrections are generally small, but H ionization can be significant (~90%).

Sub-DLAs contribute at least twice as much to the cosmic metal density as DLAs.

Abstract

We have combined the results from our recent observations of Damped and sub-Damped Lyman-alpha systems with the MIKE and UVES spectrographs on the Magellan Clay and VLT Kueyen telescopes with ones from the literature to determine the N(HI)-weighted mean metallicity of these systems based both on Fe, a depleted element in QSO absorbers and the local ISM, and Zn a relatively undepleted element. In each case, the N(HI)-weighted mean metallicity is higher and shows faster evolution in sub-DLAs than the classical DLA systems. Large grids of photoionisation models over the sub-DLA \nhI range with CLOUDY show that the ionisation corrections to the abundances are in general small, however the fraction of ionized H can be up to ~90 per cent. The individual spectra have been shifted to the rest frame of the absorber and averaged together to determine the average properties of these systems at z<1.5. We find that the average abundance pattern of the Sub-DLA systems is similar to the gas in the halo of the Milky Way, with an offset of ~0.3 dex in the overall metallicity. Both DLAs and Sub-DLAs show similar characteristics in their relative abundances patterns, although the DLAs have smaller <[Mn/Zn]> as well as higher <[Ti/Zn]> and <[Cr/Zn]>. We calculate the contribution of sub-DLAs to the metal budget of the Universe, and find that the sub-DLA systems at z<1.5 contain a comoving density of metals Omega_met (3.5-15.8)x10^{5} M_sun Mpc^{-3}, at least twice the comoving density of metals in the DLA systems. The sub-DLAs do however track global chemical evolution models much more closely than do the DLAs, perhaps indicating that they are a less dust biased metallicity indicator of galaxies at high redshifts than the DLA systems.