2024MNRAS.532...32M

We study of the properties of a new class of circumgalactic medium absorbers identified in the Ly α forest: 'Strong, Blended Lyman-α' (or SBLA) absorption systems. We study SBLAs at 2.4 < z < 3.1 in SDSS-IV/eBOSS spectra by their strong extended Ly α absorption complexes covering 138 $\, \, {\rm km}\, {\rm s}^{-1}$ with an integrated $\log (N_{\rm H\, {\small I}}/\mathrm{cm}^{-2}) =16.04$$\substack{+0.05 \\ -0.06}$ and Doppler parameter b = 18.1$\substack{+0.7 \\ -0.4}$$\, \, {\rm km}\, {\rm s}^{-1}$. Clustering with the Ly α forest provides a large-scale structure bias of b = 2.34 ± 0.06 and halo mass estimate of $M_h \approx 10^{12}\, h^{-1}\, {\rm M_{\odot }}$ for our SBLA sample. We measure the ensemble mean column densities of 22 metal features in the SBLA composite spectrum and find that no single-population multiphase model for them is viable. We therefore explore the underlying SBLA population by forward modelling the SBLA absorption distribution. Based on covariance measurements and favoured populations we find that ≈25 per cent of our SBLAs have stronger metals. Using silicon only we find that our strong metal SBLAs trace gas with a log (n_H/cm^–3) > -2.40 for T = 10^3.5 K and show gas clumping on <210 parsec scales. We fit multiphase models to this strong subpopulation and find a low ionization phase with n_H = 1 cm^–3, T = 10^3.5 K, and [X/H] = 0.8, an intermediate ionization phase with log (n_H/cm^–3) = -3.05, T = 10^3.5 K and [X/H] = -0.8, and a poorly constrained higher ionization phase. We find that the low ionization phase favours cold, dense super-solar metallicity gas with a clumping scale of just 0.009 parsecs.