Metal Lines Associated with the Lyman-alpha Forest from eBOSS Data

TL;DR

This study identifies and analyzes multiple metal species associated with the Lyman-alpha forest in eBOSS quasar spectra, revealing their distribution, physical conditions, and implications for cosmic gas environments.

Contribution

It provides the first comprehensive identification of 13 metal species linked to Lyα absorbers across a broad redshift range using stacked spectra from eBOSS data.

Findings

13 metal species detected in Lyα absorbers

Column densities decrease with redshift and absorber strength

Different ionization states indicate diverse gas environments

Abstract

We investigate the metal species associated with the Ly$\alpha$ forest in the eBOSS quasar spectra. Metal absorption lines are revealed in the stacked spectra from cross-correlating the selected Ly$\alpha$ absorbers in the forest and the flux fluctuation field. Up to 13 metal species are identified associated with relatively strong Ly$\alpha$ absorbers (those with flux fluctuation $-1.0<\delta_{\rm Ly\alpha}<-0.6$ and with neutral hydrogen column density of ~ $10^{15-16}$ $\rm cm^{-2}$) over absorber redshift range of $2<z_{\rm abs}<4$. The column densities of these species decrease toward higher redshift and for weaker Ly$\alpha$ absorbers. From modelling the column densities of various species, we find that the column density pattern suggests contributions from multiple gas components both in the circumgalactic medium (CGM) and in the intergalactic medium (IGM). While the low-ionization species (e.g., C II, Si II, and Mg II) can be explained by high-density, cool gas ($T \sim 10^4$ K) from the CGM, the high-ionization species may reside in low-density or high-temperature gas in the IGM. The measurements provide inputs to model metal contamination in the Ly$\alpha$ forest baryon acoustic oscillations measurement. Comparison with metal absorptions in high-resolution quasar spectra and with hydrodynamic galaxy formation simulations can further elucidate the physical conditions of these Ly$\alpha$ absorbers.