Metal Lines Associated with the Lyα Forest from eBOSS Data

We investigate the metal species associated with the Lyα forest in eBOSS quasar spectra. Metal absorption lines are revealed in stacked spectra from cross-correlating the selected Lyα absorbers in the forest and the flux fluctuation field. Up to 13 metal species are identified as being associated with relatively strong Lyα absorbers (those with flux fluctuations − 1.0 < δ _Lyα < − 0.6 and with a neutral hydrogen column density of ∼ 10¹⁵⁻¹⁶ cm⁻²) over the absorber redshift range of 2 < z _abs < 4. The column densities of these species decrease toward higher redshift and for weaker Lyα absorbers. From modeling 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 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 ∼ 10⁴ K) from the CGM, the high-ionization species may reside in low-density or high-temperature gas in the IGM. The measurements provide inputs for modeling the metal contamination in the Lyα forest baryon acoustic oscillation measurements. Comparisons with metal absorptions in high-resolution quasar spectra and hydrodynamic galaxy formation simulations can further elucidate the physical conditions of these Lyα absorbers.