Characterizing cool, neutral gas, and ionized metals in the outskirts of low-z galaxy clusters

We present the first detection of cool, neutral gas in the outskirts of low-z galaxy clusters using a statistically significant sample of 3191 z ≈ 0.2 background quasar-foreground cluster pairs by cross-matching the Hubble Spectroscopic Legacy Archive quasar catalogue with optically- and SZ-selected cluster catalogues. The median cluster mass of our sample is ≈10^14.2 with a median impact parameter (ρ_cl) of ≈5 Mpc. We detect significant Lyα, marginal C iv, but no O vi absorption in the signal-to-noise ratio weighted mean stacked spectra with rest-frame equivalent widths of 0.096 ± 0.011, 0.032 ± 0.015, and <0.009 Å (3σ) for our sample. The Lyα REW shows a declining trend with increasing ρ_cl (ρ_cl/R₅₀₀), which is well explained by a power-law with a slope of -0.79 (-0.70). The covering fractions (CFs) measured for Lyα (21 per cent), C iv (10 per cent), and O vi (10 per cent) in cluster outskirts are significantly lower than in the circumgalatic medium (CGM). We also find that the CGM of galaxies that are closer to cluster centres or that are in massive clusters is considerably deficient in neutral gas. The low CF of the Lyα along with the non-detection of Lyα signal when the strong absorbers (10¹³ > cm^-2) are excluded, indicate the patchy distribution of cool gas in the outskirts. We argue that the cool gas in cluster outskirts in combination arises from the circumgalactic gas stripped from cluster galaxies and to large-scale filaments feeding the clusters with cool gas.